Patchwork Reset PCIe devices to stop ongoing DMA

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Submitter Takao Indoh
Date April 24, 2013, 4:58 a.m.
Message ID <1366779539-3584-1-git-send-email-indou.takao@jp.fujitsu.com>
Download mbox | patch
Permalink /patch/239041/
State Not Applicable
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Comments

Takao Indoh - April 24, 2013, 4:58 a.m.
This patch resets PCIe devices on boot to stop ongoing DMA. When
"pci=pcie_reset_devices" is specified, a hot reset is triggered on each
PCIe root port and downstream port to reset its downstream endpoint.

Problem:
This patch solves the problem that kdump can fail when intel_iommu=on is
specified. When intel_iommu=on is specified, many dma-remapping errors
occur in second kernel and it causes problems like driver error or PCI
SERR, at last kdump fails. This problem is caused as follows.
1) Devices are working on first kernel.
2) Switch to second kernel(kdump kernel). The devices are still working
   and its DMA continues during this switch.
3) iommu is initialized during second kernel boot and ongoing DMA causes
   dma-remapping errors.

Solution:
All DMA transactions have to be stopped before iommu is initialized. By
this patch devices are reset and in-flight DMA is stopped before
pci_iommu_init.

To invoke hot reset on an endpoint, its upstream link need to be reset.
reset_pcie_devices() is called from fs_initcall_sync, and it finds root
port/downstream port whose child is PCIe endpoint, and then reset link
between them. If the endpoint is VGA device, it is skipped because the
monitor blacks out if VGA controller is reset.

Actually this is v8 patch but quite different from v7 and it's been so
long since previous post, so I start over again.
Previous post:
[PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
https://lkml.org/lkml/2012/11/26/814

Signed-off-by: Takao Indoh <indou.takao@jp.fujitsu.com>
---
 Documentation/kernel-parameters.txt |    2 +
 drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
 2 files changed, 105 insertions(+), 0 deletions(-)
Don Dutile - April 24, 2013, 7:59 p.m.
On 04/24/2013 12:58 AM, Takao Indoh wrote:
> This patch resets PCIe devices on boot to stop ongoing DMA. When
> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
> PCIe root port and downstream port to reset its downstream endpoint.
>
> Problem:
> This patch solves the problem that kdump can fail when intel_iommu=on is
> specified. When intel_iommu=on is specified, many dma-remapping errors
> occur in second kernel and it causes problems like driver error or PCI
> SERR, at last kdump fails. This problem is caused as follows.
> 1) Devices are working on first kernel.
> 2) Switch to second kernel(kdump kernel). The devices are still working
>     and its DMA continues during this switch.
> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>     dma-remapping errors.
>
> Solution:
> All DMA transactions have to be stopped before iommu is initialized. By
> this patch devices are reset and in-flight DMA is stopped before
> pci_iommu_init.
>
> To invoke hot reset on an endpoint, its upstream link need to be reset.
> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
> port/downstream port whose child is PCIe endpoint, and then reset link
> between them. If the endpoint is VGA device, it is skipped because the
> monitor blacks out if VGA controller is reset.
>
Couple questions wrt VGA device:
(1) Many graphics devices are multi-function, one function being VGA;
     is the VGA always function 0, so this scan sees it first & doesn't
     do a reset on that PCIe link?  if the VGA is not function 0, won't
     this logic break (will reset b/c function 0 is non-VGA graphics) ?
(2) I'm hearing VGA will soon not be the a required console; this logic
     assumes it is, and why it isn't blanked.
     Q: Should the filter be based on a device having a device-class of display ?

> Actually this is v8 patch but quite different from v7 and it's been so
> long since previous post, so I start over again.
Thanks for this re-start.  I need to continue reviewing the rest.

Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
    dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
    That logic, along w/your previous patch to disable the IOMMU if iommu=off
    is set, would remove this (relatively slow) PCI init sequencing ?

> Previous post:
> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
> https://lkml.org/lkml/2012/11/26/814
>
> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
> ---
>   Documentation/kernel-parameters.txt |    2 +
>   drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>   2 files changed, 105 insertions(+), 0 deletions(-)
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 4609e81..2a31ade 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>   				any pair of devices, possibly at the cost of
>   				reduced performance.  This also guarantees
>   				that hot-added devices will work.
> +		pcie_reset_devices	Reset PCIe endpoint on boot by hot
> +				reset
>   		cbiosize=nn[KMG]	The fixed amount of bus space which is
>   				reserved for the CardBus bridge's IO window.
>   				The default value is 256 bytes.
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b099e00..42385c9 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>   }
>   EXPORT_SYMBOL(pci_fixup_cardbus);
>
> +/*
> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
> + * endpoint except VGA device.
> + */
> +static int __init need_reset(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	if (!pci_is_pcie(dev) || !dev->subordinate ||
> +	    list_empty(&dev->subordinate->devices) ||
> +	    ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
> +	     (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
> +		return 0;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
> +		if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
> +		    (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
> +		    ((child->class>>  16) == PCI_BASE_CLASS_DISPLAY))
> +			/* Don't reset switch, bridge, VGA device */
> +			return 0;
> +	}
> +
> +	return 1;
> +}
> +
> +static void __init save_config(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	if (!need_reset(dev))
> +		return;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
> +		dev_info(&child->dev, "save state\n");
> +		pci_save_state(child);
> +	}
> +}
> +
> +static void __init restore_config(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	if (!need_reset(dev))
> +		return;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
> +		dev_info(&child->dev, "restore state\n");
> +		pci_restore_state(child);
> +	}
> +}
> +
> +static void __init do_device_reset(struct pci_dev *dev)
> +{
> +	u16 ctrl;
> +
> +	if (!need_reset(dev))
> +		return;
> +
> +	dev_info(&dev->dev, "Reset Secondary bus\n");
> +
> +	/* Assert Secondary Bus Reset */
> +	pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
> +	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> +
> +	msleep(2);
> +
> +	/* De-assert Secondary Bus Reset */
> +	ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> +}
> +
> +static int __initdata pcie_reset_devices;
> +static int __init reset_pcie_devices(void)
> +{
> +	struct pci_dev *dev = NULL;
> +
> +	if (!pcie_reset_devices)
> +		return 0;
> +
> +	for_each_pci_dev(dev)
> +		save_config(dev);
> +
> +	for_each_pci_dev(dev)
> +		do_device_reset(dev);
> +
> +	msleep(1000);
> +
> +	for_each_pci_dev(dev)
> +		restore_config(dev);
> +
> +	return 0;
> +}
> +fs_initcall_sync(reset_pcie_devices);
> +
>   static int __init pci_setup(char *str)
>   {
>   	while (str) {
> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
>   				pcie_bus_config = PCIE_BUS_PEER2PEER;
>   			} else if (!strncmp(str, "pcie_scan_all", 13)) {
>   				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
> +			} else if (!strncmp(str, "pcie_reset_devices", 18)) {
> +				pcie_reset_devices = 1;
>   			} else {
>   				printk(KERN_ERR "PCI: Unknown option `%s'\n",
>   						str);

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Takao Indoh - April 25, 2013, 5:11 a.m.
(2013/04/25 4:59), Don Dutile wrote:
> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>> PCIe root port and downstream port to reset its downstream endpoint.
>>
>> Problem:
>> This patch solves the problem that kdump can fail when intel_iommu=on is
>> specified. When intel_iommu=on is specified, many dma-remapping errors
>> occur in second kernel and it causes problems like driver error or PCI
>> SERR, at last kdump fails. This problem is caused as follows.
>> 1) Devices are working on first kernel.
>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>     and its DMA continues during this switch.
>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>     dma-remapping errors.
>>
>> Solution:
>> All DMA transactions have to be stopped before iommu is initialized. By
>> this patch devices are reset and in-flight DMA is stopped before
>> pci_iommu_init.
>>
>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>> port/downstream port whose child is PCIe endpoint, and then reset link
>> between them. If the endpoint is VGA device, it is skipped because the
>> monitor blacks out if VGA controller is reset.
>>
> Couple questions wrt VGA device:
> (1) Many graphics devices are multi-function, one function being VGA;
>      is the VGA always function 0, so this scan sees it first & doesn't
>      do a reset on that PCIe link?  if the VGA is not function 0, won't
>      this logic break (will reset b/c function 0 is non-VGA graphics) ?

VGA is not reset irrespective of its function number. The logic of this
patch is:

for_each_pci_dev(dev) {
    if (dev is not PCIe)
       continue;
    if (dev is not root port/downstream port) ---(1)
       continue;
    list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
        if (child is upstream port or bridge or VGA) ---(2)
            continue;
    }
    do_reset_its_child(dev);
}

Therefore VGA itself is skipped by (1), and upstream device(root port or
downstream port) of VGA is also skipped by (2).


> (2) I'm hearing VGA will soon not be the a required console; this logic
>      assumes it is, and why it isn't blanked.
>      Q: Should the filter be based on a device having a device-class of display ?

I want to avoid the situation that user's monitor blacks out and user
cannot know what's going on. That's reason why I introduced the logic to
skip VGA. As far as I tested the logic based on device-class works well,
but I would appreciate it if there are better ways.

> 
>> Actually this is v8 patch but quite different from v7 and it's been so
>> long since previous post, so I start over again.
> Thanks for this re-start.  I need to continue reviewing the rest.

Thank you for your review!

> 
> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>     dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>     That logic, along w/your previous patch to disable the IOMMU if iommu=off
>     is set, would remove this (relatively slow) PCI init sequencing ?

To force iommu off, all ongoing DMA have to be stopped before that since
they are accessing the device address, not physical address. If we disable
iommu without stopping in-flihgt DMA, devices access invalid memory area
and it causes memory corruption or PCI-SERR due to DMA error.

So, whether we use iommu or not in second kernel, we have to stop DMA in
second kernel if iommu is used in first kernel.

Thanks,
Takao Indoh


> 
>> Previous post:
>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>> https://lkml.org/lkml/2012/11/26/814
>>
>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>> ---
>>   Documentation/kernel-parameters.txt |    2 +
>>   drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>   2 files changed, 105 insertions(+), 0 deletions(-)
>>
>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>> index 4609e81..2a31ade 100644
>> --- a/Documentation/kernel-parameters.txt
>> +++ b/Documentation/kernel-parameters.txt
>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>                   any pair of devices, possibly at the cost of
>>                   reduced performance.  This also guarantees
>>                   that hot-added devices will work.
>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>> +                reset
>>           cbiosize=nn[KMG]    The fixed amount of bus space which is
>>                   reserved for the CardBus bridge's IO window.
>>                   The default value is 256 bytes.
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index b099e00..42385c9 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>   }
>>   EXPORT_SYMBOL(pci_fixup_cardbus);
>>
>> +/*
>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>> + * endpoint except VGA device.
>> + */
>> +static int __init need_reset(struct pci_dev *dev)
>> +{
>> +    struct pci_bus *subordinate;
>> +    struct pci_dev *child;
>> +
>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>> +        list_empty(&dev->subordinate->devices) ||
>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>> +        return 0;
>> +
>> +    subordinate = dev->subordinate;
>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>> +            ((child->class>>  16) == PCI_BASE_CLASS_DISPLAY))
>> +            /* Don't reset switch, bridge, VGA device */
>> +            return 0;
>> +    }
>> +
>> +    return 1;
>> +}
>> +
>> +static void __init save_config(struct pci_dev *dev)
>> +{
>> +    struct pci_bus *subordinate;
>> +    struct pci_dev *child;
>> +
>> +    if (!need_reset(dev))
>> +        return;
>> +
>> +    subordinate = dev->subordinate;
>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>> +        dev_info(&child->dev, "save state\n");
>> +        pci_save_state(child);
>> +    }
>> +}
>> +
>> +static void __init restore_config(struct pci_dev *dev)
>> +{
>> +    struct pci_bus *subordinate;
>> +    struct pci_dev *child;
>> +
>> +    if (!need_reset(dev))
>> +        return;
>> +
>> +    subordinate = dev->subordinate;
>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>> +        dev_info(&child->dev, "restore state\n");
>> +        pci_restore_state(child);
>> +    }
>> +}
>> +
>> +static void __init do_device_reset(struct pci_dev *dev)
>> +{
>> +    u16 ctrl;
>> +
>> +    if (!need_reset(dev))
>> +        return;
>> +
>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>> +
>> +    /* Assert Secondary Bus Reset */
>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>> +
>> +    msleep(2);
>> +
>> +    /* De-assert Secondary Bus Reset */
>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>> +}
>> +
>> +static int __initdata pcie_reset_devices;
>> +static int __init reset_pcie_devices(void)
>> +{
>> +    struct pci_dev *dev = NULL;
>> +
>> +    if (!pcie_reset_devices)
>> +        return 0;
>> +
>> +    for_each_pci_dev(dev)
>> +        save_config(dev);
>> +
>> +    for_each_pci_dev(dev)
>> +        do_device_reset(dev);
>> +
>> +    msleep(1000);
>> +
>> +    for_each_pci_dev(dev)
>> +        restore_config(dev);
>> +
>> +    return 0;
>> +}
>> +fs_initcall_sync(reset_pcie_devices);
>> +
>>   static int __init pci_setup(char *str)
>>   {
>>       while (str) {
>> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
>>                   pcie_bus_config = PCIE_BUS_PEER2PEER;
>>               } else if (!strncmp(str, "pcie_scan_all", 13)) {
>>                   pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
>> +            } else if (!strncmp(str, "pcie_reset_devices", 18)) {
>> +                pcie_reset_devices = 1;
>>               } else {
>>                   printk(KERN_ERR "PCI: Unknown option `%s'\n",
>>                           str);
> 
> 
> 

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Don Dutile - April 25, 2013, 6:01 p.m.
On 04/25/2013 01:11 AM, Takao Indoh wrote:
> (2013/04/25 4:59), Don Dutile wrote:
>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>
>>> Problem:
>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>> occur in second kernel and it causes problems like driver error or PCI
>>> SERR, at last kdump fails. This problem is caused as follows.
>>> 1) Devices are working on first kernel.
>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>      and its DMA continues during this switch.
>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>      dma-remapping errors.
>>>
>>> Solution:
>>> All DMA transactions have to be stopped before iommu is initialized. By
>>> this patch devices are reset and in-flight DMA is stopped before
>>> pci_iommu_init.
>>>
>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>> between them. If the endpoint is VGA device, it is skipped because the
>>> monitor blacks out if VGA controller is reset.
>>>
>> Couple questions wrt VGA device:
>> (1) Many graphics devices are multi-function, one function being VGA;
>>       is the VGA always function 0, so this scan sees it first&  doesn't
>>       do a reset on that PCIe link?  if the VGA is not function 0, won't
>>       this logic break (will reset b/c function 0 is non-VGA graphics) ?
> 
> VGA is not reset irrespective of its function number. The logic of this
> patch is:
> 
> for_each_pci_dev(dev) {
>      if (dev is not PCIe)
>         continue;
>      if (dev is not root port/downstream port) ---(1)
>         continue;
>      list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>          if (child is upstream port or bridge or VGA) ---(2)
>              continue;
>      }
>      do_reset_its_child(dev);
> }
> 
> Therefore VGA itself is skipped by (1), and upstream device(root port or
> downstream port) of VGA is also skipped by (2).
> 
> 
>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>       assumes it is, and why it isn't blanked.
>>       Q: Should the filter be based on a device having a device-class of display ?
> 
> I want to avoid the situation that user's monitor blacks out and user
> cannot know what's going on. That's reason why I introduced the logic to
> skip VGA. As far as I tested the logic based on device-class works well,
sorry, I read your description, which said VGA, but your are filtering on display class,
which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
are probably one of the most aggressive DMA engines on a system.... and will grow as
asymmetric processing using GPUs gets architected into a device-agnostic manner.
So, this may work well for servers, which is the primary consumer/user of this feature,
and they typically have built-in graphics that are generally used in simple VGA mode,
so this may be sufficient for now.
 

> but I would appreciate it if there are better ways.
> 
You probably don't want to hear it but....
a) only turn off cmd-reg master enable bit
b) only do reset based on a list of devices known not to 
   obey their cmd-reg master enable bit, and only do reset to those devices.
But, given the testing you've done so far, this optional (need cmdline) feature,
let's start here.

>>
>>> Actually this is v8 patch but quite different from v7 and it's been so
>>> long since previous post, so I start over again.
>> Thanks for this re-start.  I need to continue reviewing the rest.
> 
> Thank you for your review!
> 
>>
>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>      dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>      That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>      is set, would remove this (relatively slow) PCI init sequencing ?
> 
> To force iommu off, all ongoing DMA have to be stopped before that since
> they are accessing the device address, not physical address. If we disable
> iommu without stopping in-flihgt DMA, devices access invalid memory area
> and it causes memory corruption or PCI-SERR due to DMA error.
Right, that's a 'duh' on my part.
I thought 'disable iommu' == 'block all dma' and it just turns it off &
let's the ongoing DMA run... 
Please ignore this question... sigh.

> 
> So, whether we use iommu or not in second kernel, we have to stop DMA in
> second kernel if iommu is used in first kernel.
> 
> Thanks,
> Takao Indoh
> 
> 
>>
>>> Previous post:
>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>> https://lkml.org/lkml/2012/11/26/814
>>>
>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>> ---
>>>    Documentation/kernel-parameters.txt |    2 +
>>>    drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>    2 files changed, 105 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>> index 4609e81..2a31ade 100644
>>> --- a/Documentation/kernel-parameters.txt
>>> +++ b/Documentation/kernel-parameters.txt
>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>                    any pair of devices, possibly at the cost of
>>>                    reduced performance.  This also guarantees
>>>                    that hot-added devices will work.
>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>> +                reset
>>>            cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>                    reserved for the CardBus bridge's IO window.
>>>                    The default value is 256 bytes.
>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>> index b099e00..42385c9 100644
>>> --- a/drivers/pci/pci.c
>>> +++ b/drivers/pci/pci.c
>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>    }
>>>    EXPORT_SYMBOL(pci_fixup_cardbus);
>>>
>>> +/*
>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>> + * endpoint except VGA device.
>>> + */
>>> +static int __init need_reset(struct pci_dev *dev)
>>> +{
>>> +    struct pci_bus *subordinate;
>>> +    struct pci_dev *child;
>>> +
>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>> +        list_empty(&dev->subordinate->devices) ||
>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>> +        return 0;
>>> +
>>> +    subordinate = dev->subordinate;
>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>> +            ((child->class>>   16) == PCI_BASE_CLASS_DISPLAY))
>>> +            /* Don't reset switch, bridge, VGA device */
>>> +            return 0;
>>> +    }
>>> +
>>> +    return 1;
>>> +}
>>> +
>>> +static void __init save_config(struct pci_dev *dev)
This should be renamed.  it implies it is saving the config of the pdev passed in,
when in reality, it is saving the config of all the devices attached to this pdev.
i.e., save_downstream_configs()

>>> +{
>>> +    struct pci_bus *subordinate;
>>> +    struct pci_dev *child;
>>> +
>>> +    if (!need_reset(dev))
>>> +        return;
>>> +
>>> +    subordinate = dev->subordinate;
>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>> +        dev_info(&child->dev, "save state\n");
>>> +        pci_save_state(child);
>>> +    }
>>> +}
>>> +
>>> +static void __init restore_config(struct pci_dev *dev)
inverse of above: restore_downstream_configs()
>>> +{
>>> +    struct pci_bus *subordinate;
>>> +    struct pci_dev *child;
>>> +
>>> +    if (!need_reset(dev))
>>> +        return;
>>> +
>>> +    subordinate = dev->subordinate;
>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>> +        dev_info(&child->dev, "restore state\n");
>>> +        pci_restore_state(child);
>>> +    }
>>> +}
>>> +
>>> +static void __init do_device_reset(struct pci_dev *dev)
do_downstream_device_reset() -- it's not resetting this pdev,
but the pdev's of the devices attached to it.

>>> +{
>>> +    u16 ctrl;
>>> +
>>> +    if (!need_reset(dev))
>>> +        return;
>>> +
>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>> +
>>> +    /* Assert Secondary Bus Reset */
>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>> +
>>> +    msleep(2);
>>> +
This works well for x86, which uses ioport registers to access 
these <256-offset registers, b/c the write() function can't return
until the write is actually completed, but for a non-x86 system,
with fully mmconf'd PCI space, a write() may still be a write & run
(sitting in CPU write-merge) buffer, so if you need a full 2ms,
you ought to do another read_config() to the device, to flush the write,
before starting the msleep(2) clock.

>>> +    /* De-assert Secondary Bus Reset */
>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>> +}
>>> +
>>> +static int __initdata pcie_reset_devices;
>>> +static int __init reset_pcie_devices(void)
this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>> +{
>>> +    struct pci_dev *dev = NULL;
>>> +
>>> +    if (!pcie_reset_devices)
>>> +        return 0;
>>> +
>>> +    for_each_pci_dev(dev)
>>> +        save_config(dev);
>>> +
>>> +    for_each_pci_dev(dev)
>>> +        do_device_reset(dev);
>>> +
>>> +    msleep(1000);
>>> +
>>> +    for_each_pci_dev(dev)
>>> +        restore_config(dev);
>>> +
My apologies if past thread covered this sequence...
Why three loops through all PCIe devices on the system? 
why not have the first for-each-pci-dev() loop filter devices
to be reset, and save_config those pdev's,
return a list of saved_pdev's;  feed that list into the do_device_reset();
then mpsleep(), then restore the list.
in fact, once you create a link list of pdev's to reset,
just loop that list doing save, then reset; rtn the list, do msleep(), 
then restore the config of pdevs in the list.
Otherwise, doing much more traversing than what's needed.
Doing a great deal more config-saving then needed right now as well
(saving all non-endpt devices that aren't reset).

>>> +    return 0;
>>> +}
>>> +fs_initcall_sync(reset_pcie_devices);
>>> +
>>>    static int __init pci_setup(char *str)
>>>    {
>>>        while (str) {
>>> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
>>>                    pcie_bus_config = PCIE_BUS_PEER2PEER;
>>>                } else if (!strncmp(str, "pcie_scan_all", 13)) {
>>>                    pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
>>> +            } else if (!strncmp(str, "pcie_reset_devices", 18)) {
pcie_reset_endpoint_devices
>>> +                pcie_reset_devices = 1;
pcie_reset_endpoint_devices
>>>                } else {
>>>                    printk(KERN_ERR "PCI: Unknown option `%s'\n",
>>>                            str);
>>
>>
>>
> 

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Takao Indoh - April 26, 2013, 3:10 a.m.
(2013/04/26 3:01), Don Dutile wrote:
> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>> (2013/04/25 4:59), Don Dutile wrote:
>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>
>>>> Problem:
>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>> occur in second kernel and it causes problems like driver error or PCI
>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>> 1) Devices are working on first kernel.
>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>       and its DMA continues during this switch.
>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>       dma-remapping errors.
>>>>
>>>> Solution:
>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>> this patch devices are reset and in-flight DMA is stopped before
>>>> pci_iommu_init.
>>>>
>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>> monitor blacks out if VGA controller is reset.
>>>>
>>> Couple questions wrt VGA device:
>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>        is the VGA always function 0, so this scan sees it first&  doesn't
>>>        do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>        this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>
>> VGA is not reset irrespective of its function number. The logic of this
>> patch is:
>>
>> for_each_pci_dev(dev) {
>>       if (dev is not PCIe)
>>          continue;
>>       if (dev is not root port/downstream port) ---(1)
>>          continue;
>>       list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>           if (child is upstream port or bridge or VGA) ---(2)
>>               continue;
>>       }
>>       do_reset_its_child(dev);
>> }
>>
>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>> downstream port) of VGA is also skipped by (2).
>>
>>
>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>        assumes it is, and why it isn't blanked.
>>>        Q: Should the filter be based on a device having a device-class of display ?
>>
>> I want to avoid the situation that user's monitor blacks out and user
>> cannot know what's going on. That's reason why I introduced the logic to
>> skip VGA. As far as I tested the logic based on device-class works well,
> sorry, I read your description, which said VGA, but your are filtering on display class,
> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
> are probably one of the most aggressive DMA engines on a system.... and will grow as
> asymmetric processing using GPUs gets architected into a device-agnostic manner.
> So, this may work well for servers, which is the primary consumer/user of this feature,
> and they typically have built-in graphics that are generally used in simple VGA mode,
> so this may be sufficient for now.

Ok, understood.


> 
>> but I would appreciate it if there are better ways.
>>
> You probably don't want to hear it but....
> a) only turn off cmd-reg master enable bit
> b) only do reset based on a list of devices known not to
>     obey their cmd-reg master enable bit, and only do reset to those devices.
> But, given the testing you've done so far, this optional (need cmdline) feature,
> let's start here.

Ok. Either way I think we need more testing.

 
>>>
>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>> long since previous post, so I start over again.
>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>
>> Thank you for your review!
>>
>>>
>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>       dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>       That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>       is set, would remove this (relatively slow) PCI init sequencing ?
>>
>> To force iommu off, all ongoing DMA have to be stopped before that since
>> they are accessing the device address, not physical address. If we disable
>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>> and it causes memory corruption or PCI-SERR due to DMA error.
> Right, that's a 'duh' on my part.
> I thought 'disable iommu' == 'block all dma' and it just turns it off &
> let's the ongoing DMA run...
> Please ignore this question... sigh.
> 
>>
>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>> second kernel if iommu is used in first kernel.
>>
>> Thanks,
>> Takao Indoh
>>
>>
>>>
>>>> Previous post:
>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>> https://lkml.org/lkml/2012/11/26/814
>>>>
>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>> ---
>>>>     Documentation/kernel-parameters.txt |    2 +
>>>>     drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>     2 files changed, 105 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>> index 4609e81..2a31ade 100644
>>>> --- a/Documentation/kernel-parameters.txt
>>>> +++ b/Documentation/kernel-parameters.txt
>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>                     any pair of devices, possibly at the cost of
>>>>                     reduced performance.  This also guarantees
>>>>                     that hot-added devices will work.
>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>> +                reset
>>>>             cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>                     reserved for the CardBus bridge's IO window.
>>>>                     The default value is 256 bytes.
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index b099e00..42385c9 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>     }
>>>>     EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>
>>>> +/*
>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>> + * endpoint except VGA device.
>>>> + */
>>>> +static int __init need_reset(struct pci_dev *dev)
>>>> +{
>>>> +    struct pci_bus *subordinate;
>>>> +    struct pci_dev *child;
>>>> +
>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>> +        list_empty(&dev->subordinate->devices) ||
>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>> +        return 0;
>>>> +
>>>> +    subordinate = dev->subordinate;
>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>> +            ((child->class>>   16) == PCI_BASE_CLASS_DISPLAY))
>>>> +            /* Don't reset switch, bridge, VGA device */
>>>> +            return 0;
>>>> +    }
>>>> +
>>>> +    return 1;
>>>> +}
>>>> +
>>>> +static void __init save_config(struct pci_dev *dev)
> This should be renamed.  it implies it is saving the config of the pdev passed in,
> when in reality, it is saving the config of all the devices attached to this pdev.
> i.e., save_downstream_configs()
> 
>>>> +{
>>>> +    struct pci_bus *subordinate;
>>>> +    struct pci_dev *child;
>>>> +
>>>> +    if (!need_reset(dev))
>>>> +        return;
>>>> +
>>>> +    subordinate = dev->subordinate;
>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +        dev_info(&child->dev, "save state\n");
>>>> +        pci_save_state(child);
>>>> +    }
>>>> +}
>>>> +
>>>> +static void __init restore_config(struct pci_dev *dev)
> inverse of above: restore_downstream_configs()
>>>> +{
>>>> +    struct pci_bus *subordinate;
>>>> +    struct pci_dev *child;
>>>> +
>>>> +    if (!need_reset(dev))
>>>> +        return;
>>>> +
>>>> +    subordinate = dev->subordinate;
>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +        dev_info(&child->dev, "restore state\n");
>>>> +        pci_restore_state(child);
>>>> +    }
>>>> +}
>>>> +
>>>> +static void __init do_device_reset(struct pci_dev *dev)
> do_downstream_device_reset() -- it's not resetting this pdev,
> but the pdev's of the devices attached to it.
> 
Right, I'll change them as you said.


>>>> +{
>>>> +    u16 ctrl;
>>>> +
>>>> +    if (!need_reset(dev))
>>>> +        return;
>>>> +
>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>> +
>>>> +    /* Assert Secondary Bus Reset */
>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>> +
>>>> +    msleep(2);
>>>> +
> This works well for x86, which uses ioport registers to access
> these <256-offset registers, b/c the write() function can't return
> until the write is actually completed, but for a non-x86 system,
> with fully mmconf'd PCI space, a write() may still be a write & run
> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
> you ought to do another read_config() to the device, to flush the write,
> before starting the msleep(2) clock.
> 
I didn't know that... I'll insert something like this before msleep.

pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &dummy);

BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
Probably need the same fix, right?


>>>> +    /* De-assert Secondary Bus Reset */
>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>> +}
>>>> +
>>>> +static int __initdata pcie_reset_devices;
>>>> +static int __init reset_pcie_devices(void)
> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>> +{
>>>> +    struct pci_dev *dev = NULL;
>>>> +
>>>> +    if (!pcie_reset_devices)
>>>> +        return 0;
>>>> +
>>>> +    for_each_pci_dev(dev)
>>>> +        save_config(dev);
>>>> +
>>>> +    for_each_pci_dev(dev)
>>>> +        do_device_reset(dev);
>>>> +
>>>> +    msleep(1000);
>>>> +
>>>> +    for_each_pci_dev(dev)
>>>> +        restore_config(dev);
>>>> +
> My apologies if past thread covered this sequence...
> Why three loops through all PCIe devices on the system?
> why not have the first for-each-pci-dev() loop filter devices
> to be reset, and save_config those pdev's,
> return a list of saved_pdev's;  feed that list into the do_device_reset();
> then mpsleep(), then restore the list.
> in fact, once you create a link list of pdev's to reset,
> just loop that list doing save, then reset; rtn the list, do msleep(),
> then restore the config of pdevs in the list.
> Otherwise, doing much more traversing than what's needed.
> Doing a great deal more config-saving then needed right now as well
> (saving all non-endpt devices that aren't reset).
> 

Creating list is nice idea, thanks. I'll do.

I'll have vacation next week, so I'll post v2 patch the week after
next.

Thanks,
Takao Indoh

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Don Dutile - April 26, 2013, 1:42 p.m.
On 04/25/2013 11:10 PM, Takao Indoh wrote:
> (2013/04/26 3:01), Don Dutile wrote:
>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>> (2013/04/25 4:59), Don Dutile wrote:
>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>
>>>>> Problem:
>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>> 1) Devices are working on first kernel.
>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>        and its DMA continues during this switch.
>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>        dma-remapping errors.
>>>>>
>>>>> Solution:
>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>> pci_iommu_init.
>>>>>
>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>> monitor blacks out if VGA controller is reset.
>>>>>
>>>> Couple questions wrt VGA device:
>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>         is the VGA always function 0, so this scan sees it first&   doesn't
>>>>         do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>         this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>
>>> VGA is not reset irrespective of its function number. The logic of this
>>> patch is:
>>>
>>> for_each_pci_dev(dev) {
>>>        if (dev is not PCIe)
>>>           continue;
>>>        if (dev is not root port/downstream port) ---(1)
>>>           continue;
>>>        list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>            if (child is upstream port or bridge or VGA) ---(2)
>>>                continue;
>>>        }
>>>        do_reset_its_child(dev);
>>> }
>>>
>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>> downstream port) of VGA is also skipped by (2).
>>>
>>>
>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>         assumes it is, and why it isn't blanked.
>>>>         Q: Should the filter be based on a device having a device-class of display ?
>>>
>>> I want to avoid the situation that user's monitor blacks out and user
>>> cannot know what's going on. That's reason why I introduced the logic to
>>> skip VGA. As far as I tested the logic based on device-class works well,
>> sorry, I read your description, which said VGA, but your are filtering on display class,
>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>> So, this may work well for servers, which is the primary consumer/user of this feature,
>> and they typically have built-in graphics that are generally used in simple VGA mode,
>> so this may be sufficient for now.
> 
> Ok, understood.
> 
> 
>>
>>> but I would appreciate it if there are better ways.
>>>
>> You probably don't want to hear it but....
>> a) only turn off cmd-reg master enable bit
>> b) only do reset based on a list of devices known not to
>>      obey their cmd-reg master enable bit, and only do reset to those devices.
>> But, given the testing you've done so far, this optional (need cmdline) feature,
>> let's start here.
> 
> Ok. Either way I think we need more testing.
> 
> 
>>>>
>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>> long since previous post, so I start over again.
>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>
>>> Thank you for your review!
>>>
>>>>
>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>        dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>        That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>        is set, would remove this (relatively slow) PCI init sequencing ?
>>>
>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>> they are accessing the device address, not physical address. If we disable
>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>> and it causes memory corruption or PCI-SERR due to DMA error.
>> Right, that's a 'duh' on my part.
>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>> let's the ongoing DMA run...
>> Please ignore this question... sigh.
>>
>>>
>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>> second kernel if iommu is used in first kernel.
>>>
>>> Thanks,
>>> Takao Indoh
>>>
>>>
>>>>
>>>>> Previous post:
>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>
>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>> ---
>>>>>      Documentation/kernel-parameters.txt |    2 +
>>>>>      drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>      2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>> index 4609e81..2a31ade 100644
>>>>> --- a/Documentation/kernel-parameters.txt
>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>                      any pair of devices, possibly at the cost of
>>>>>                      reduced performance.  This also guarantees
>>>>>                      that hot-added devices will work.
>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>> +                reset
>>>>>              cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>                      reserved for the CardBus bridge's IO window.
>>>>>                      The default value is 256 bytes.
>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>> index b099e00..42385c9 100644
>>>>> --- a/drivers/pci/pci.c
>>>>> +++ b/drivers/pci/pci.c
>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>      }
>>>>>      EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>
>>>>> +/*
>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>> + * endpoint except VGA device.
>>>>> + */
>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>> +{
>>>>> +    struct pci_bus *subordinate;
>>>>> +    struct pci_dev *child;
>>>>> +
>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>> +        return 0;
>>>>> +
>>>>> +    subordinate = dev->subordinate;
>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>> +            ((child->class>>    16) == PCI_BASE_CLASS_DISPLAY))
>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>> +            return 0;
>>>>> +    }
>>>>> +
>>>>> +    return 1;
>>>>> +}
>>>>> +
>>>>> +static void __init save_config(struct pci_dev *dev)
>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>> when in reality, it is saving the config of all the devices attached to this pdev.
>> i.e., save_downstream_configs()
>>
>>>>> +{
>>>>> +    struct pci_bus *subordinate;
>>>>> +    struct pci_dev *child;
>>>>> +
>>>>> +    if (!need_reset(dev))
>>>>> +        return;
>>>>> +
>>>>> +    subordinate = dev->subordinate;
>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>> +        dev_info(&child->dev, "save state\n");
>>>>> +        pci_save_state(child);
>>>>> +    }
>>>>> +}
>>>>> +
>>>>> +static void __init restore_config(struct pci_dev *dev)
>> inverse of above: restore_downstream_configs()
>>>>> +{
>>>>> +    struct pci_bus *subordinate;
>>>>> +    struct pci_dev *child;
>>>>> +
>>>>> +    if (!need_reset(dev))
>>>>> +        return;
>>>>> +
>>>>> +    subordinate = dev->subordinate;
>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>> +        pci_restore_state(child);
>>>>> +    }
>>>>> +}
>>>>> +
>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>> do_downstream_device_reset() -- it's not resetting this pdev,
>> but the pdev's of the devices attached to it.
>>
> Right, I'll change them as you said.
> 
> 
>>>>> +{
>>>>> +    u16 ctrl;
>>>>> +
>>>>> +    if (!need_reset(dev))
>>>>> +        return;
>>>>> +
>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>> +
>>>>> +    /* Assert Secondary Bus Reset */
>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>> +
>>>>> +    msleep(2);
>>>>> +
>> This works well for x86, which uses ioport registers to access
>> these<256-offset registers, b/c the write() function can't return
>> until the write is actually completed, but for a non-x86 system,
>> with fully mmconf'd PCI space, a write() may still be a write&  run
>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>> you ought to do another read_config() to the device, to flush the write,
>> before starting the msleep(2) clock.
>>
> I didn't know that... I'll insert something like this before msleep.
> 
> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
> 
> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
> Probably need the same fix, right?
> 
sounds like it.
Interested to hear from someone in non-x86-land how
they ensure pci cfg writes aren't buffered in their cpus;
maybe handled via special uncached regions on other arches.

> 
>>>>> +    /* De-assert Secondary Bus Reset */
>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>> +}
>>>>> +
>>>>> +static int __initdata pcie_reset_devices;
>>>>> +static int __init reset_pcie_devices(void)
>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>> +{
>>>>> +    struct pci_dev *dev = NULL;
>>>>> +
>>>>> +    if (!pcie_reset_devices)
>>>>> +        return 0;
>>>>> +
>>>>> +    for_each_pci_dev(dev)
>>>>> +        save_config(dev);
>>>>> +
>>>>> +    for_each_pci_dev(dev)
>>>>> +        do_device_reset(dev);
>>>>> +
>>>>> +    msleep(1000);
>>>>> +
>>>>> +    for_each_pci_dev(dev)
>>>>> +        restore_config(dev);
>>>>> +
>> My apologies if past thread covered this sequence...
>> Why three loops through all PCIe devices on the system?
>> why not have the first for-each-pci-dev() loop filter devices
>> to be reset, and save_config those pdev's,
>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>> then mpsleep(), then restore the list.
>> in fact, once you create a link list of pdev's to reset,
>> just loop that list doing save, then reset; rtn the list, do msleep(),
>> then restore the config of pdevs in the list.
>> Otherwise, doing much more traversing than what's needed.
>> Doing a great deal more config-saving then needed right now as well
>> (saving all non-endpt devices that aren't reset).
>>
> 
> Creating list is nice idea, thanks. I'll do.
> 
> I'll have vacation next week, so I'll post v2 patch the week after
> next.
> 
> Thanks,
> Takao Indoh
> 
Have a good vacation! Thanks for your willingness
to respin again & make this improvement.

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Sumner, William - April 30, 2013, 2:54 p.m.
I have installed your original patch set (from last November) and tested with three platforms, each with a different IO configuration.  On the first platform crashdumps were consistently successful.  On the second and third platforms, the reset of one specific PCI device on each platform (a different device type on each platform) immediately hung the crash kernel. This was consistent across multiple tries.  Temporarily modifying the patch to skip resetting the one problem-device on each platform allowed each platform to create a dump successfully.

I am now working with our IO team to determine the exact nature of the hang and to see if the hang is unique to the specific cards or the specific platforms.

Also I am starting to look at an alternate possibility:

The PCIe spec (my copy is version 2.0) in section 6.6.2 (Function-Level Reset) describes reset use-cases for "a partitioned environment where hardware is migrated from one partition to another" and for "system software is taking down the software stack for a Function and then rebuilding that stack".

These use-cases seem very similar to transitioning into the crash kernel.  The "Implementation Note" at the end of that section describes an algorithm for "Avoiding Data Corruption From Stale Completions" which looks like it might be applicable to stopping ongoing DMA.  I am hopeful that adding some of the steps from this algorithm to one of the already proposed patches would avoid the hang that I saw on two platforms.

Bill Sumner

-----Original Message-----
From: kexec [mailto:kexec-bounces@lists.infradead.org] On Behalf Of Don Dutile
Sent: Friday, April 26, 2013 8:43 AM
To: Takao Indoh
Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com
Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA

On 04/25/2013 11:10 PM, Takao Indoh wrote:
> (2013/04/26 3:01), Don Dutile wrote:
>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>> (2013/04/25 4:59), Don Dutile wrote:
>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>
>>>>> Problem:
>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>> 1) Devices are working on first kernel.
>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>        and its DMA continues during this switch.
>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>        dma-remapping errors.
>>>>>
>>>>> Solution:
>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>> pci_iommu_init.
>>>>>
>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>> monitor blacks out if VGA controller is reset.
>>>>>
>>>> Couple questions wrt VGA device:
>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>         is the VGA always function 0, so this scan sees it first&   doesn't
>>>>         do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>         this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>
>>> VGA is not reset irrespective of its function number. The logic of this
>>> patch is:
>>>
>>> for_each_pci_dev(dev) {
>>>        if (dev is not PCIe)
>>>           continue;
>>>        if (dev is not root port/downstream port) ---(1)
>>>           continue;
>>>        list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>            if (child is upstream port or bridge or VGA) ---(2)
>>>                continue;
>>>        }
>>>        do_reset_its_child(dev);
>>> }
>>>
>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>> downstream port) of VGA is also skipped by (2).
>>>
>>>
>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>         assumes it is, and why it isn't blanked.
>>>>         Q: Should the filter be based on a device having a device-class of display ?
>>>
>>> I want to avoid the situation that user's monitor blacks out and user
>>> cannot know what's going on. That's reason why I introduced the logic to
>>> skip VGA. As far as I tested the logic based on device-class works well,
>> sorry, I read your description, which said VGA, but your are filtering on display class,
>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>> So, this may work well for servers, which is the primary consumer/user of this feature,
>> and they typically have built-in graphics that are generally used in simple VGA mode,
>> so this may be sufficient for now.
>
> Ok, understood.
>
>
>>
>>> but I would appreciate it if there are better ways.
>>>
>> You probably don't want to hear it but....
>> a) only turn off cmd-reg master enable bit
>> b) only do reset based on a list of devices known not to
>>      obey their cmd-reg master enable bit, and only do reset to those devices.
>> But, given the testing you've done so far, this optional (need cmdline) feature,
>> let's start here.
>
> Ok. Either way I think we need more testing.
>
>
>>>>
>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>> long since previous post, so I start over again.
>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>
>>> Thank you for your review!
>>>
>>>>
>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>        dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>        That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>        is set, would remove this (relatively slow) PCI init sequencing ?
>>>
>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>> they are accessing the device address, not physical address. If we disable
>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>> and it causes memory corruption or PCI-SERR due to DMA error.
>> Right, that's a 'duh' on my part.
>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>> let's the ongoing DMA run...
>> Please ignore this question... sigh.
>>
>>>
>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>> second kernel if iommu is used in first kernel.
>>>
>>> Thanks,
>>> Takao Indoh
>>>
>>>
>>>>
>>>>> Previous post:
>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>
>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>> ---
>>>>>      Documentation/kernel-parameters.txt |    2 +
>>>>>      drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>      2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>> index 4609e81..2a31ade 100644
>>>>> --- a/Documentation/kernel-parameters.txt
>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>                      any pair of devices, possibly at the cost of
>>>>>                      reduced performance.  This also guarantees
>>>>>                      that hot-added devices will work.
>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>> +                reset
>>>>>              cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>                      reserved for the CardBus bridge's IO window.
>>>>>                      The default value is 256 bytes.
>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>> index b099e00..42385c9 100644
>>>>> --- a/drivers/pci/pci.c
>>>>> +++ b/drivers/pci/pci.c
>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>      }
>>>>>      EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>
>>>>> +/*
>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>> + * endpoint except VGA device.
>>>>> + */
>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>> +{
>>>>> +    struct pci_bus *subordinate;
>>>>> +    struct pci_dev *child;
>>>>> +
>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>> +        return 0;
>>>>> +
>>>>> +    subordinate = dev->subordinate;
>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>> +            ((child->class>>    16) == PCI_BASE_CLASS_DISPLAY))
>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>> +            return 0;
>>>>> +    }
>>>>> +
>>>>> +    return 1;
>>>>> +}
>>>>> +
>>>>> +static void __init save_config(struct pci_dev *dev)
>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>> when in reality, it is saving the config of all the devices attached to this pdev.
>> i.e., save_downstream_configs()
>>
>>>>> +{
>>>>> +    struct pci_bus *subordinate;
>>>>> +    struct pci_dev *child;
>>>>> +
>>>>> +    if (!need_reset(dev))
>>>>> +        return;
>>>>> +
>>>>> +    subordinate = dev->subordinate;
>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>> +        dev_info(&child->dev, "save state\n");
>>>>> +        pci_save_state(child);
>>>>> +    }
>>>>> +}
>>>>> +
>>>>> +static void __init restore_config(struct pci_dev *dev)
>> inverse of above: restore_downstream_configs()
>>>>> +{
>>>>> +    struct pci_bus *subordinate;
>>>>> +    struct pci_dev *child;
>>>>> +
>>>>> +    if (!need_reset(dev))
>>>>> +        return;
>>>>> +
>>>>> +    subordinate = dev->subordinate;
>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>> +        pci_restore_state(child);
>>>>> +    }
>>>>> +}
>>>>> +
>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>> do_downstream_device_reset() -- it's not resetting this pdev,
>> but the pdev's of the devices attached to it.
>>
> Right, I'll change them as you said.
>
>
>>>>> +{
>>>>> +    u16 ctrl;
>>>>> +
>>>>> +    if (!need_reset(dev))
>>>>> +        return;
>>>>> +
>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>> +
>>>>> +    /* Assert Secondary Bus Reset */
>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>> +
>>>>> +    msleep(2);
>>>>> +
>> This works well for x86, which uses ioport registers to access
>> these<256-offset registers, b/c the write() function can't return
>> until the write is actually completed, but for a non-x86 system,
>> with fully mmconf'd PCI space, a write() may still be a write&  run
>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>> you ought to do another read_config() to the device, to flush the write,
>> before starting the msleep(2) clock.
>>
> I didn't know that... I'll insert something like this before msleep.
>
> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
>
> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
> Probably need the same fix, right?
>
sounds like it.
Interested to hear from someone in non-x86-land how
they ensure pci cfg writes aren't buffered in their cpus;
maybe handled via special uncached regions on other arches.

>
>>>>> +    /* De-assert Secondary Bus Reset */
>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>> +}
>>>>> +
>>>>> +static int __initdata pcie_reset_devices;
>>>>> +static int __init reset_pcie_devices(void)
>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>> +{
>>>>> +    struct pci_dev *dev = NULL;
>>>>> +
>>>>> +    if (!pcie_reset_devices)
>>>>> +        return 0;
>>>>> +
>>>>> +    for_each_pci_dev(dev)
>>>>> +        save_config(dev);
>>>>> +
>>>>> +    for_each_pci_dev(dev)
>>>>> +        do_device_reset(dev);
>>>>> +
>>>>> +    msleep(1000);
>>>>> +
>>>>> +    for_each_pci_dev(dev)
>>>>> +        restore_config(dev);
>>>>> +
>> My apologies if past thread covered this sequence...
>> Why three loops through all PCIe devices on the system?
>> why not have the first for-each-pci-dev() loop filter devices
>> to be reset, and save_config those pdev's,
>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>> then mpsleep(), then restore the list.
>> in fact, once you create a link list of pdev's to reset,
>> just loop that list doing save, then reset; rtn the list, do msleep(),
>> then restore the config of pdevs in the list.
>> Otherwise, doing much more traversing than what's needed.
>> Doing a great deal more config-saving then needed right now as well
>> (saving all non-endpt devices that aren't reset).
>>
>
> Creating list is nice idea, thanks. I'll do.
>
> I'll have vacation next week, so I'll post v2 patch the week after
> next.
>
> Thanks,
> Takao Indoh
>
Have a good vacation! Thanks for your willingness
to respin again & make this improvement.
Don Dutile - April 30, 2013, 3:30 p.m.
On 04/30/2013 10:54 AM, Sumner, William wrote:
> I have installed your original patch set (from last November) and tested with three platforms, each with a different IO configuration.  On the first platform crashdumps were consistently successful.  On the second and third platforms, the reset of one specific PCI device on each platform (a different device type on each platform) immediately hung the crash kernel. This was consistent across multiple tries.  Temporarily modifying the patch to skip resetting the one problem-device on each platform allowed each platform to create a dump successfully.
>
> I am now working with our IO team to determine the exact nature of the hang and to see if the hang is unique to the specific cards or the specific platforms.
>
> Also I am starting to look at an alternate possibility:
>
> The PCIe spec (my copy is version 2.0) in section 6.6.2 (Function-Level Reset) describes reset use-cases for "a partitioned environment where hardware is migrated from one partition to another" and for "system software is taking down the software stack for a Function and then rebuilding that stack".
>
Same section in PCIe spec v3.0.
The first paragraph after the 3 (square) bullets in 6.6.2 states: "Implemenation of FLR is optional (not required), but is strongly recommended.

This optional feature complicates the reset code, and may be the
reason you are seeing hangs.  do an lspci -xxx dump to see if the
devices you need to skip on reset have FLR capability (find device cap register, offset 0x4 in PCIe Cap struct, bit 28).

The only thing the device-endpoint-reset() doesn't do is monitor/check the
device's Device Status register (offset 0xA in PCI Cap struct) Transaction Pending bit.
The spec says that could take seconds to clear... typically milliseconds.
If typical, the reset mpsleep() handles that range.  Scanning the endpt-pdev list
to see if any Transaction Pending bits are set could be added, and then additional
1->2 second delay if still set added... potentially giving up after 2 secs.

> These use-cases seem very similar to transitioning into the crash kernel.  The "Implementation Note" at the end of that section describes an algorithm for "Avoiding Data Corruption From Stale Completions" which looks like it might be applicable to stopping ongoing DMA.  I am hopeful that adding some of the steps from this algorithm to one of the already proposed patches would avoid the hang that I saw on two platforms.
>
> Bill Sumner
>
> -----Original Message-----
> From: kexec [mailto:kexec-bounces@lists.infradead.org] On Behalf Of Don Dutile
> Sent: Friday, April 26, 2013 8:43 AM
> To: Takao Indoh
> Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com
> Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA
>
> On 04/25/2013 11:10 PM, Takao Indoh wrote:
>> (2013/04/26 3:01), Don Dutile wrote:
>>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>>> (2013/04/25 4:59), Don Dutile wrote:
>>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>>
>>>>>> Problem:
>>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>>> 1) Devices are working on first kernel.
>>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>>         and its DMA continues during this switch.
>>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>>         dma-remapping errors.
>>>>>>
>>>>>> Solution:
>>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>>> pci_iommu_init.
>>>>>>
>>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>>> monitor blacks out if VGA controller is reset.
>>>>>>
>>>>> Couple questions wrt VGA device:
>>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>>          is the VGA always function 0, so this scan sees it first&    doesn't
>>>>>          do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>>          this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>>
>>>> VGA is not reset irrespective of its function number. The logic of this
>>>> patch is:
>>>>
>>>> for_each_pci_dev(dev) {
>>>>         if (dev is not PCIe)
>>>>            continue;
>>>>         if (dev is not root port/downstream port) ---(1)
>>>>            continue;
>>>>         list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>>             if (child is upstream port or bridge or VGA) ---(2)
>>>>                 continue;
>>>>         }
>>>>         do_reset_its_child(dev);
>>>> }
>>>>
>>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>>> downstream port) of VGA is also skipped by (2).
>>>>
>>>>
>>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>>          assumes it is, and why it isn't blanked.
>>>>>          Q: Should the filter be based on a device having a device-class of display ?
>>>>
>>>> I want to avoid the situation that user's monitor blacks out and user
>>>> cannot know what's going on. That's reason why I introduced the logic to
>>>> skip VGA. As far as I tested the logic based on device-class works well,
>>> sorry, I read your description, which said VGA, but your are filtering on display class,
>>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>>> So, this may work well for servers, which is the primary consumer/user of this feature,
>>> and they typically have built-in graphics that are generally used in simple VGA mode,
>>> so this may be sufficient for now.
>>
>> Ok, understood.
>>
>>
>>>
>>>> but I would appreciate it if there are better ways.
>>>>
>>> You probably don't want to hear it but....
>>> a) only turn off cmd-reg master enable bit
>>> b) only do reset based on a list of devices known not to
>>>       obey their cmd-reg master enable bit, and only do reset to those devices.
>>> But, given the testing you've done so far, this optional (need cmdline) feature,
>>> let's start here.
>>
>> Ok. Either way I think we need more testing.
>>
>>
>>>>>
>>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>>> long since previous post, so I start over again.
>>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>>
>>>> Thank you for your review!
>>>>
>>>>>
>>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>>         dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>>         That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>>         is set, would remove this (relatively slow) PCI init sequencing ?
>>>>
>>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>>> they are accessing the device address, not physical address. If we disable
>>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>>> and it causes memory corruption or PCI-SERR due to DMA error.
>>> Right, that's a 'duh' on my part.
>>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>>> let's the ongoing DMA run...
>>> Please ignore this question... sigh.
>>>
>>>>
>>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>>> second kernel if iommu is used in first kernel.
>>>>
>>>> Thanks,
>>>> Takao Indoh
>>>>
>>>>
>>>>>
>>>>>> Previous post:
>>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>>
>>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>>> ---
>>>>>>       Documentation/kernel-parameters.txt |    2 +
>>>>>>       drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>>       2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>>> index 4609e81..2a31ade 100644
>>>>>> --- a/Documentation/kernel-parameters.txt
>>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>>                       any pair of devices, possibly at the cost of
>>>>>>                       reduced performance.  This also guarantees
>>>>>>                       that hot-added devices will work.
>>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>>> +                reset
>>>>>>               cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>>                       reserved for the CardBus bridge's IO window.
>>>>>>                       The default value is 256 bytes.
>>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>>> index b099e00..42385c9 100644
>>>>>> --- a/drivers/pci/pci.c
>>>>>> +++ b/drivers/pci/pci.c
>>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>>       }
>>>>>>       EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>>
>>>>>> +/*
>>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>>> + * endpoint except VGA device.
>>>>>> + */
>>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>>> +            ((child->class>>     16) == PCI_BASE_CLASS_DISPLAY))
>>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>>> +            return 0;
>>>>>> +    }
>>>>>> +
>>>>>> +    return 1;
>>>>>> +}
>>>>>> +
>>>>>> +static void __init save_config(struct pci_dev *dev)
>>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>>> when in reality, it is saving the config of all the devices attached to this pdev.
>>> i.e., save_downstream_configs()
>>>
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        dev_info(&child->dev, "save state\n");
>>>>>> +        pci_save_state(child);
>>>>>> +    }
>>>>>> +}
>>>>>> +
>>>>>> +static void __init restore_config(struct pci_dev *dev)
>>> inverse of above: restore_downstream_configs()
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>>> +        pci_restore_state(child);
>>>>>> +    }
>>>>>> +}
>>>>>> +
>>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>>> do_downstream_device_reset() -- it's not resetting this pdev,
>>> but the pdev's of the devices attached to it.
>>>
>> Right, I'll change them as you said.
>>
>>
>>>>>> +{
>>>>>> +    u16 ctrl;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>>> +
>>>>>> +    /* Assert Secondary Bus Reset */
>>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>> +
>>>>>> +    msleep(2);
>>>>>> +
>>> This works well for x86, which uses ioport registers to access
>>> these<256-offset registers, b/c the write() function can't return
>>> until the write is actually completed, but for a non-x86 system,
>>> with fully mmconf'd PCI space, a write() may still be a write&   run
>>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>>> you ought to do another read_config() to the device, to flush the write,
>>> before starting the msleep(2) clock.
>>>
>> I didn't know that... I'll insert something like this before msleep.
>>
>> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
>>
>> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
>> Probably need the same fix, right?
>>
> sounds like it.
> Interested to hear from someone in non-x86-land how
> they ensure pci cfg writes aren't buffered in their cpus;
> maybe handled via special uncached regions on other arches.
>
>>
>>>>>> +    /* De-assert Secondary Bus Reset */
>>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>> +}
>>>>>> +
>>>>>> +static int __initdata pcie_reset_devices;
>>>>>> +static int __init reset_pcie_devices(void)
>>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>>> +{
>>>>>> +    struct pci_dev *dev = NULL;
>>>>>> +
>>>>>> +    if (!pcie_reset_devices)
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        save_config(dev);
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        do_device_reset(dev);
>>>>>> +
>>>>>> +    msleep(1000);
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        restore_config(dev);
>>>>>> +
>>> My apologies if past thread covered this sequence...
>>> Why three loops through all PCIe devices on the system?
>>> why not have the first for-each-pci-dev() loop filter devices
>>> to be reset, and save_config those pdev's,
>>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>>> then mpsleep(), then restore the list.
>>> in fact, once you create a link list of pdev's to reset,
>>> just loop that list doing save, then reset; rtn the list, do msleep(),
>>> then restore the config of pdevs in the list.
>>> Otherwise, doing much more traversing than what's needed.
>>> Doing a great deal more config-saving then needed right now as well
>>> (saving all non-endpt devices that aren't reset).
>>>
>>
>> Creating list is nice idea, thanks. I'll do.
>>
>> I'll have vacation next week, so I'll post v2 patch the week after
>> next.
>>
>> Thanks,
>> Takao Indoh
>>
> Have a good vacation! Thanks for your willingness
> to respin again&  make this improvement.
>
>
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Takao Indoh - May 7, 2013, 7:09 a.m.
Sorry for the delayed response.

(2013/04/30 23:54), Sumner, William wrote:
> I have installed your original patch set (from last November) and tested with three platforms, each with a different IO configuration.  On the first platform crashdumps were consistently successful.  On the second and third platforms, the reset of one specific PCI device on each platform (a different device type on each platform) immediately hung the crash kernel. This was consistent across multiple tries.  Temporarily modifying the patch to skip resetting the one problem-device on each platform allowed each platform to create a dump successfully.
> 
> I am now working with our IO team to determine the exact nature of the hang and to see if the hang is unique to the specific cards or the specific platforms.
> 
> Also I am starting to look at an alternate possibility:
> 
> The PCIe spec (my copy is version 2.0) in section 6.6.2 (Function-Level Reset) describes reset use-cases for "a partitioned environment where hardware is migrated from one partition to another" and for "system software is taking down the software stack for a Function and then rebuilding that stack".
> 
> These use-cases seem very similar to transitioning into the crash kernel.  The "Implementation Note" at the end of that section describes an algorithm for "Avoiding Data Corruption From Stale Completions" which looks like it might be applicable to stopping ongoing DMA.  I am hopeful that adding some of the steps from this algorithm to one of the already proposed patches would avoid the hang that I saw on two platforms.

It seems that the algorithm you mentioned requires four steps.

1. Clear Command register
2. Wait a few milliseconds (Or polling Transactions Pending bit)
3. Do FLR
4. Wait 100 ms

My patch does not do step 1 and 2. So, I would appreciate it if you
could add the following into save_config() in my latest patch and
confirm if kernel still hangs up or not.

        subordinate = dev->subordinate;
        list_for_each_entry(child, &subordinate->devices, bus_list) {
                dev_info(&child->dev, "save state\n");
                pci_save_state(child);
+               pci_write_config_word(child, PCI_COMMAND, 0);
+               msleep(1000);
        }

Thanks,
Takao Indoh


> 
> Bill Sumner
> 
> -----Original Message-----
> From: kexec [mailto:kexec-bounces@lists.infradead.org] On Behalf Of Don Dutile
> Sent: Friday, April 26, 2013 8:43 AM
> To: Takao Indoh
> Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com
> Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA
> 
> On 04/25/2013 11:10 PM, Takao Indoh wrote:
>> (2013/04/26 3:01), Don Dutile wrote:
>>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>>> (2013/04/25 4:59), Don Dutile wrote:
>>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>>
>>>>>> Problem:
>>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>>> 1) Devices are working on first kernel.
>>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>>         and its DMA continues during this switch.
>>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>>         dma-remapping errors.
>>>>>>
>>>>>> Solution:
>>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>>> pci_iommu_init.
>>>>>>
>>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>>> monitor blacks out if VGA controller is reset.
>>>>>>
>>>>> Couple questions wrt VGA device:
>>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>>          is the VGA always function 0, so this scan sees it first&   doesn't
>>>>>          do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>>          this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>>
>>>> VGA is not reset irrespective of its function number. The logic of this
>>>> patch is:
>>>>
>>>> for_each_pci_dev(dev) {
>>>>         if (dev is not PCIe)
>>>>            continue;
>>>>         if (dev is not root port/downstream port) ---(1)
>>>>            continue;
>>>>         list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>>             if (child is upstream port or bridge or VGA) ---(2)
>>>>                 continue;
>>>>         }
>>>>         do_reset_its_child(dev);
>>>> }
>>>>
>>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>>> downstream port) of VGA is also skipped by (2).
>>>>
>>>>
>>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>>          assumes it is, and why it isn't blanked.
>>>>>          Q: Should the filter be based on a device having a device-class of display ?
>>>>
>>>> I want to avoid the situation that user's monitor blacks out and user
>>>> cannot know what's going on. That's reason why I introduced the logic to
>>>> skip VGA. As far as I tested the logic based on device-class works well,
>>> sorry, I read your description, which said VGA, but your are filtering on display class,
>>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>>> So, this may work well for servers, which is the primary consumer/user of this feature,
>>> and they typically have built-in graphics that are generally used in simple VGA mode,
>>> so this may be sufficient for now.
>>
>> Ok, understood.
>>
>>
>>>
>>>> but I would appreciate it if there are better ways.
>>>>
>>> You probably don't want to hear it but....
>>> a) only turn off cmd-reg master enable bit
>>> b) only do reset based on a list of devices known not to
>>>       obey their cmd-reg master enable bit, and only do reset to those devices.
>>> But, given the testing you've done so far, this optional (need cmdline) feature,
>>> let's start here.
>>
>> Ok. Either way I think we need more testing.
>>
>>
>>>>>
>>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>>> long since previous post, so I start over again.
>>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>>
>>>> Thank you for your review!
>>>>
>>>>>
>>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>>         dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>>         That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>>         is set, would remove this (relatively slow) PCI init sequencing ?
>>>>
>>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>>> they are accessing the device address, not physical address. If we disable
>>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>>> and it causes memory corruption or PCI-SERR due to DMA error.
>>> Right, that's a 'duh' on my part.
>>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>>> let's the ongoing DMA run...
>>> Please ignore this question... sigh.
>>>
>>>>
>>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>>> second kernel if iommu is used in first kernel.
>>>>
>>>> Thanks,
>>>> Takao Indoh
>>>>
>>>>
>>>>>
>>>>>> Previous post:
>>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>>
>>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>>> ---
>>>>>>       Documentation/kernel-parameters.txt |    2 +
>>>>>>       drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>>       2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>>> index 4609e81..2a31ade 100644
>>>>>> --- a/Documentation/kernel-parameters.txt
>>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>>                       any pair of devices, possibly at the cost of
>>>>>>                       reduced performance.  This also guarantees
>>>>>>                       that hot-added devices will work.
>>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>>> +                reset
>>>>>>               cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>>                       reserved for the CardBus bridge's IO window.
>>>>>>                       The default value is 256 bytes.
>>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>>> index b099e00..42385c9 100644
>>>>>> --- a/drivers/pci/pci.c
>>>>>> +++ b/drivers/pci/pci.c
>>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>>       }
>>>>>>       EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>>
>>>>>> +/*
>>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>>> + * endpoint except VGA device.
>>>>>> + */
>>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>>> +            ((child->class>>    16) == PCI_BASE_CLASS_DISPLAY))
>>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>>> +            return 0;
>>>>>> +    }
>>>>>> +
>>>>>> +    return 1;
>>>>>> +}
>>>>>> +
>>>>>> +static void __init save_config(struct pci_dev *dev)
>>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>>> when in reality, it is saving the config of all the devices attached to this pdev.
>>> i.e., save_downstream_configs()
>>>
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        dev_info(&child->dev, "save state\n");
>>>>>> +        pci_save_state(child);
>>>>>> +    }
>>>>>> +}
>>>>>> +
>>>>>> +static void __init restore_config(struct pci_dev *dev)
>>> inverse of above: restore_downstream_configs()
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>>> +        pci_restore_state(child);
>>>>>> +    }
>>>>>> +}
>>>>>> +
>>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>>> do_downstream_device_reset() -- it's not resetting this pdev,
>>> but the pdev's of the devices attached to it.
>>>
>> Right, I'll change them as you said.
>>
>>
>>>>>> +{
>>>>>> +    u16 ctrl;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>>> +
>>>>>> +    /* Assert Secondary Bus Reset */
>>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>> +
>>>>>> +    msleep(2);
>>>>>> +
>>> This works well for x86, which uses ioport registers to access
>>> these<256-offset registers, b/c the write() function can't return
>>> until the write is actually completed, but for a non-x86 system,
>>> with fully mmconf'd PCI space, a write() may still be a write&  run
>>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>>> you ought to do another read_config() to the device, to flush the write,
>>> before starting the msleep(2) clock.
>>>
>> I didn't know that... I'll insert something like this before msleep.
>>
>> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
>>
>> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
>> Probably need the same fix, right?
>>
> sounds like it.
> Interested to hear from someone in non-x86-land how
> they ensure pci cfg writes aren't buffered in their cpus;
> maybe handled via special uncached regions on other arches.
> 
>>
>>>>>> +    /* De-assert Secondary Bus Reset */
>>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>> +}
>>>>>> +
>>>>>> +static int __initdata pcie_reset_devices;
>>>>>> +static int __init reset_pcie_devices(void)
>>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>>> +{
>>>>>> +    struct pci_dev *dev = NULL;
>>>>>> +
>>>>>> +    if (!pcie_reset_devices)
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        save_config(dev);
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        do_device_reset(dev);
>>>>>> +
>>>>>> +    msleep(1000);
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        restore_config(dev);
>>>>>> +
>>> My apologies if past thread covered this sequence...
>>> Why three loops through all PCIe devices on the system?
>>> why not have the first for-each-pci-dev() loop filter devices
>>> to be reset, and save_config those pdev's,
>>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>>> then mpsleep(), then restore the list.
>>> in fact, once you create a link list of pdev's to reset,
>>> just loop that list doing save, then reset; rtn the list, do msleep(),
>>> then restore the config of pdevs in the list.
>>> Otherwise, doing much more traversing than what's needed.
>>> Doing a great deal more config-saving then needed right now as well
>>> (saving all non-endpt devices that aren't reset).
>>>
>>
>> Creating list is nice idea, thanks. I'll do.
>>
>> I'll have vacation next week, so I'll post v2 patch the week after
>> next.
>>
>> Thanks,
>> Takao Indoh
>>
> Have a good vacation! Thanks for your willingness
> to respin again & make this improvement.
> 
> 
> _______________________________________________
> kexec mailing list
> kexec@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/kexec
> 
> 


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Don Dutile - May 7, 2013, 12:50 p.m.
On 05/07/2013 03:09 AM, Takao Indoh wrote:
> Sorry for the delayed response.
> 
> (2013/04/30 23:54), Sumner, William wrote:
>> I have installed your original patch set (from last November) and tested with three platforms, each with a different IO configuration.  On the first platform crashdumps were consistently successful.  On the second and third platforms, the reset of one specific PCI device on each platform (a different device type on each platform) immediately hung the crash kernel. This was consistent across multiple tries.  Temporarily modifying the patch to skip resetting the one problem-device on each platform allowed each platform to create a dump successfully.
>>
>> I am now working with our IO team to determine the exact nature of the hang and to see if the hang is unique to the specific cards or the specific platforms.
>>
>> Also I am starting to look at an alternate possibility:
>>
>> The PCIe spec (my copy is version 2.0) in section 6.6.2 (Function-Level Reset) describes reset use-cases for "a partitioned environment where hardware is migrated from one partition to another" and for "system software is taking down the software stack for a Function and then rebuilding that stack".
>>
>> These use-cases seem very similar to transitioning into the crash kernel.  The "Implementation Note" at the end of that section describes an algorithm for "Avoiding Data Corruption From Stale Completions" which looks like it might be applicable to stopping ongoing DMA.  I am hopeful that adding some of the steps from this algorithm to one of the already proposed patches would avoid the hang that I saw on two platforms.
> 
> It seems that the algorithm you mentioned requires four steps.
> 
> 1. Clear Command register
> 2. Wait a few milliseconds (Or polling Transactions Pending bit)
> 3. Do FLR
> 4. Wait 100 ms
> 
> My patch does not do step 1 and 2. So, I would appreciate it if you
> could add the following into save_config() in my latest patch and
> confirm if kernel still hangs up or not.
> 
>          subordinate = dev->subordinate;
>          list_for_each_entry(child,&subordinate->devices, bus_list) {
>                  dev_info(&child->dev, "save state\n");
>                  pci_save_state(child);
> +               pci_write_config_word(child, PCI_COMMAND, 0);
> +               msleep(1000);
As Linus pointed out in an earlier patch, msleep() after each device
is s-l-o-w and unnecessary; should do a single msleep(1000) after
*all* the command registers are written.  That way, the added delay is 1sec,
not 1sec*ndevs.
q: is this turning off command register for only PCI(e) endpoints?
-- shouldn't have to turn off command register in bridges/switches.

>          }
> 
> Thanks,
> Takao Indoh
> 
> 
>>
>> Bill Sumner
>>
>> -----Original Message-----
>> From: kexec [mailto:kexec-bounces@lists.infradead.org] On Behalf Of Don Dutile
>> Sent: Friday, April 26, 2013 8:43 AM
>> To: Takao Indoh
>> Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com
>> Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA
>>
>> On 04/25/2013 11:10 PM, Takao Indoh wrote:
>>> (2013/04/26 3:01), Don Dutile wrote:
>>>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>>>> (2013/04/25 4:59), Don Dutile wrote:
>>>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>>>
>>>>>>> Problem:
>>>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>>>> 1) Devices are working on first kernel.
>>>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>>>          and its DMA continues during this switch.
>>>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>>>          dma-remapping errors.
>>>>>>>
>>>>>>> Solution:
>>>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>>>> pci_iommu_init.
>>>>>>>
>>>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>>>> monitor blacks out if VGA controller is reset.
>>>>>>>
>>>>>> Couple questions wrt VGA device:
>>>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>>>           is the VGA always function 0, so this scan sees it first&    doesn't
>>>>>>           do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>>>           this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>>>
>>>>> VGA is not reset irrespective of its function number. The logic of this
>>>>> patch is:
>>>>>
>>>>> for_each_pci_dev(dev) {
>>>>>          if (dev is not PCIe)
>>>>>             continue;
>>>>>          if (dev is not root port/downstream port) ---(1)
>>>>>             continue;
>>>>>          list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>>>              if (child is upstream port or bridge or VGA) ---(2)
>>>>>                  continue;
>>>>>          }
>>>>>          do_reset_its_child(dev);
>>>>> }
>>>>>
>>>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>>>> downstream port) of VGA is also skipped by (2).
>>>>>
>>>>>
>>>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>>>           assumes it is, and why it isn't blanked.
>>>>>>           Q: Should the filter be based on a device having a device-class of display ?
>>>>>
>>>>> I want to avoid the situation that user's monitor blacks out and user
>>>>> cannot know what's going on. That's reason why I introduced the logic to
>>>>> skip VGA. As far as I tested the logic based on device-class works well,
>>>> sorry, I read your description, which said VGA, but your are filtering on display class,
>>>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>>>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>>>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>>>> So, this may work well for servers, which is the primary consumer/user of this feature,
>>>> and they typically have built-in graphics that are generally used in simple VGA mode,
>>>> so this may be sufficient for now.
>>>
>>> Ok, understood.
>>>
>>>
>>>>
>>>>> but I would appreciate it if there are better ways.
>>>>>
>>>> You probably don't want to hear it but....
>>>> a) only turn off cmd-reg master enable bit
>>>> b) only do reset based on a list of devices known not to
>>>>        obey their cmd-reg master enable bit, and only do reset to those devices.
>>>> But, given the testing you've done so far, this optional (need cmdline) feature,
>>>> let's start here.
>>>
>>> Ok. Either way I think we need more testing.
>>>
>>>
>>>>>>
>>>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>>>> long since previous post, so I start over again.
>>>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>>>
>>>>> Thank you for your review!
>>>>>
>>>>>>
>>>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>>>          dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>>>          That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>>>          is set, would remove this (relatively slow) PCI init sequencing ?
>>>>>
>>>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>>>> they are accessing the device address, not physical address. If we disable
>>>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>>>> and it causes memory corruption or PCI-SERR due to DMA error.
>>>> Right, that's a 'duh' on my part.
>>>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>>>> let's the ongoing DMA run...
>>>> Please ignore this question... sigh.
>>>>
>>>>>
>>>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>>>> second kernel if iommu is used in first kernel.
>>>>>
>>>>> Thanks,
>>>>> Takao Indoh
>>>>>
>>>>>
>>>>>>
>>>>>>> Previous post:
>>>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>>>
>>>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>>>> ---
>>>>>>>        Documentation/kernel-parameters.txt |    2 +
>>>>>>>        drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>>>        2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>>>
>>>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>>>> index 4609e81..2a31ade 100644
>>>>>>> --- a/Documentation/kernel-parameters.txt
>>>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>>>                        any pair of devices, possibly at the cost of
>>>>>>>                        reduced performance.  This also guarantees
>>>>>>>                        that hot-added devices will work.
>>>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>>>> +                reset
>>>>>>>                cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>>>                        reserved for the CardBus bridge's IO window.
>>>>>>>                        The default value is 256 bytes.
>>>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>>>> index b099e00..42385c9 100644
>>>>>>> --- a/drivers/pci/pci.c
>>>>>>> +++ b/drivers/pci/pci.c
>>>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>>>        }
>>>>>>>        EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>>>
>>>>>>> +/*
>>>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>>>> + * endpoint except VGA device.
>>>>>>> + */
>>>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>>>> +{
>>>>>>> +    struct pci_bus *subordinate;
>>>>>>> +    struct pci_dev *child;
>>>>>>> +
>>>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>>>> +        return 0;
>>>>>>> +
>>>>>>> +    subordinate = dev->subordinate;
>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>>>> +            ((child->class>>     16) == PCI_BASE_CLASS_DISPLAY))
>>>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>>>> +            return 0;
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    return 1;
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void __init save_config(struct pci_dev *dev)
>>>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>>>> when in reality, it is saving the config of all the devices attached to this pdev.
>>>> i.e., save_downstream_configs()
>>>>
>>>>>>> +{
>>>>>>> +    struct pci_bus *subordinate;
>>>>>>> +    struct pci_dev *child;
>>>>>>> +
>>>>>>> +    if (!need_reset(dev))
>>>>>>> +        return;
>>>>>>> +
>>>>>>> +    subordinate = dev->subordinate;
>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>> +        dev_info(&child->dev, "save state\n");
>>>>>>> +        pci_save_state(child);
>>>>>>> +    }
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void __init restore_config(struct pci_dev *dev)
>>>> inverse of above: restore_downstream_configs()
>>>>>>> +{
>>>>>>> +    struct pci_bus *subordinate;
>>>>>>> +    struct pci_dev *child;
>>>>>>> +
>>>>>>> +    if (!need_reset(dev))
>>>>>>> +        return;
>>>>>>> +
>>>>>>> +    subordinate = dev->subordinate;
>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>>>> +        pci_restore_state(child);
>>>>>>> +    }
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>>>> do_downstream_device_reset() -- it's not resetting this pdev,
>>>> but the pdev's of the devices attached to it.
>>>>
>>> Right, I'll change them as you said.
>>>
>>>
>>>>>>> +{
>>>>>>> +    u16 ctrl;
>>>>>>> +
>>>>>>> +    if (!need_reset(dev))
>>>>>>> +        return;
>>>>>>> +
>>>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>>>> +
>>>>>>> +    /* Assert Secondary Bus Reset */
>>>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>>> +
>>>>>>> +    msleep(2);
>>>>>>> +
>>>> This works well for x86, which uses ioport registers to access
>>>> these<256-offset registers, b/c the write() function can't return
>>>> until the write is actually completed, but for a non-x86 system,
>>>> with fully mmconf'd PCI space, a write() may still be a write&   run
>>>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>>>> you ought to do another read_config() to the device, to flush the write,
>>>> before starting the msleep(2) clock.
>>>>
>>> I didn't know that... I'll insert something like this before msleep.
>>>
>>> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
>>>
>>> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
>>> Probably need the same fix, right?
>>>
>> sounds like it.
>> Interested to hear from someone in non-x86-land how
>> they ensure pci cfg writes aren't buffered in their cpus;
>> maybe handled via special uncached regions on other arches.
>>
>>>
>>>>>>> +    /* De-assert Secondary Bus Reset */
>>>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>>> +}
>>>>>>> +
>>>>>>> +static int __initdata pcie_reset_devices;
>>>>>>> +static int __init reset_pcie_devices(void)
>>>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>>>> +{
>>>>>>> +    struct pci_dev *dev = NULL;
>>>>>>> +
>>>>>>> +    if (!pcie_reset_devices)
>>>>>>> +        return 0;
>>>>>>> +
>>>>>>> +    for_each_pci_dev(dev)
>>>>>>> +        save_config(dev);
>>>>>>> +
>>>>>>> +    for_each_pci_dev(dev)
>>>>>>> +        do_device_reset(dev);
>>>>>>> +
>>>>>>> +    msleep(1000);
>>>>>>> +
>>>>>>> +    for_each_pci_dev(dev)
>>>>>>> +        restore_config(dev);
>>>>>>> +
>>>> My apologies if past thread covered this sequence...
>>>> Why three loops through all PCIe devices on the system?
>>>> why not have the first for-each-pci-dev() loop filter devices
>>>> to be reset, and save_config those pdev's,
>>>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>>>> then mpsleep(), then restore the list.
>>>> in fact, once you create a link list of pdev's to reset,
>>>> just loop that list doing save, then reset; rtn the list, do msleep(),
>>>> then restore the config of pdevs in the list.
>>>> Otherwise, doing much more traversing than what's needed.
>>>> Doing a great deal more config-saving then needed right now as well
>>>> (saving all non-endpt devices that aren't reset).
>>>>
>>>
>>> Creating list is nice idea, thanks. I'll do.
>>>
>>> I'll have vacation next week, so I'll post v2 patch the week after
>>> next.
>>>
>>> Thanks,
>>> Takao Indoh
>>>
>> Have a good vacation! Thanks for your willingness
>> to respin again&  make this improvement.
>>
>>
>> _______________________________________________
>> kexec mailing list
>> kexec@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/kexec
>>
>>
> 
> 

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Alex Williamson - May 7, 2013, 4:39 p.m.
On Wed, 2013-04-24 at 13:58 +0900, Takao Indoh wrote:
> This patch resets PCIe devices on boot to stop ongoing DMA. When
> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
> PCIe root port and downstream port to reset its downstream endpoint.
> 
> Problem:
> This patch solves the problem that kdump can fail when intel_iommu=on is
> specified. When intel_iommu=on is specified, many dma-remapping errors
> occur in second kernel and it causes problems like driver error or PCI
> SERR, at last kdump fails. This problem is caused as follows.
> 1) Devices are working on first kernel.
> 2) Switch to second kernel(kdump kernel). The devices are still working
>    and its DMA continues during this switch.
> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>    dma-remapping errors.
> 
> Solution:
> All DMA transactions have to be stopped before iommu is initialized. By
> this patch devices are reset and in-flight DMA is stopped before
> pci_iommu_init.
> 
> To invoke hot reset on an endpoint, its upstream link need to be reset.
> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
> port/downstream port whose child is PCIe endpoint, and then reset link
> between them. If the endpoint is VGA device, it is skipped because the
> monitor blacks out if VGA controller is reset.
> 
> Actually this is v8 patch but quite different from v7 and it's been so
> long since previous post, so I start over again.
> Previous post:
> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
> https://lkml.org/lkml/2012/11/26/814
> 
> Signed-off-by: Takao Indoh <indou.takao@jp.fujitsu.com>
> ---
>  Documentation/kernel-parameters.txt |    2 +
>  drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>  2 files changed, 105 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 4609e81..2a31ade 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  				any pair of devices, possibly at the cost of
>  				reduced performance.  This also guarantees
>  				that hot-added devices will work.
> +		pcie_reset_devices	Reset PCIe endpoint on boot by hot
> +				reset
>  		cbiosize=nn[KMG]	The fixed amount of bus space which is
>  				reserved for the CardBus bridge's IO window.
>  				The default value is 256 bytes.
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b099e00..42385c9 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>  }
>  EXPORT_SYMBOL(pci_fixup_cardbus);
>  
> +/*
> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
> + * endpoint except VGA device.
> + */
> +static int __init need_reset(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	if (!pci_is_pcie(dev) || !dev->subordinate ||
> +	    list_empty(&dev->subordinate->devices) ||
> +	    ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT) &&
> +	     (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
> +		return 0;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child, &subordinate->devices, bus_list) {
> +		if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
> +		    (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
> +		    ((child->class >> 16) == PCI_BASE_CLASS_DISPLAY))
> +			/* Don't reset switch, bridge, VGA device */
> +			return 0;
> +	}
> +
> +	return 1;
> +}
> +
> +static void __init save_config(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	if (!need_reset(dev))
> +		return;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child, &subordinate->devices, bus_list) {
> +		dev_info(&child->dev, "save state\n");
> +		pci_save_state(child);
> +	}
> +}
> +
> +static void __init restore_config(struct pci_dev *dev)
> +{
> +	struct pci_bus *subordinate;
> +	struct pci_dev *child;
> +
> +	if (!need_reset(dev))
> +		return;
> +
> +	subordinate = dev->subordinate;
> +	list_for_each_entry(child, &subordinate->devices, bus_list) {
> +		dev_info(&child->dev, "restore state\n");
> +		pci_restore_state(child);
> +	}
> +}
> +
> +static void __init do_device_reset(struct pci_dev *dev)
> +{
> +	u16 ctrl;
> +
> +	if (!need_reset(dev))
> +		return;
> +
> +	dev_info(&dev->dev, "Reset Secondary bus\n");
> +
> +	/* Assert Secondary Bus Reset */
> +	pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &ctrl);
> +	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> +
> +	msleep(2);
> +
> +	/* De-assert Secondary Bus Reset */
> +	ctrl &= ~PCI_BRIDGE_CTL_BUS_RESET;
> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> +}
> +
> +static int __initdata pcie_reset_devices;
> +static int __init reset_pcie_devices(void)
> +{
> +	struct pci_dev *dev = NULL;
> +
> +	if (!pcie_reset_devices)
> +		return 0;
> +
> +	for_each_pci_dev(dev)
> +		save_config(dev);
> +
> +	for_each_pci_dev(dev)
> +		do_device_reset(dev);

So we do a reset on each root port or downstream port, but the PCI to
PCI bridge spec (1.2) states:

        11.1.1. Secondary Reset Signal
                The secondary reset signal, RST#, is a logical OR of the
                primary interface RST# signal and
                the state of the Secondary Bus Reset bit of the Bridge
                Control register (see Section 3.2.5.18).

I read that to mean that in a legacy topology, doing a reset on the top
level bridge triggers a reset down the entire hierarchy.  How does this
apply to a PCIe topology?  Can we just do a reset on the top level root
ports?  If so, that would also imply that a reset propagates down
through PCIe-to-PCI bridges, so legacy PCI devices may be reset and the
option name is misleading.

Even so, you still have root complex endpoints, which are not getting
reset through this, so it's really not a complete solution.  Thanks,

Alex

> +
> +	msleep(1000);
> +
> +	for_each_pci_dev(dev)
> +		restore_config(dev);
> +
> +	return 0;
> +}
> +fs_initcall_sync(reset_pcie_devices);
> +
>  static int __init pci_setup(char *str)
>  {
>  	while (str) {
> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
>  				pcie_bus_config = PCIE_BUS_PEER2PEER;
>  			} else if (!strncmp(str, "pcie_scan_all", 13)) {
>  				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
> +			} else if (!strncmp(str, "pcie_reset_devices", 18)) {
> +				pcie_reset_devices = 1;
>  			} else {
>  				printk(KERN_ERR "PCI: Unknown option `%s'\n",
>  						str);



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Don Dutile - May 7, 2013, 8:10 p.m.
On 05/07/2013 12:39 PM, Alex Williamson wrote:
> On Wed, 2013-04-24 at 13:58 +0900, Takao Indoh wrote:
>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>> PCIe root port and downstream port to reset its downstream endpoint.
>>
>> Problem:
>> This patch solves the problem that kdump can fail when intel_iommu=on is
>> specified. When intel_iommu=on is specified, many dma-remapping errors
>> occur in second kernel and it causes problems like driver error or PCI
>> SERR, at last kdump fails. This problem is caused as follows.
>> 1) Devices are working on first kernel.
>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>     and its DMA continues during this switch.
>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>     dma-remapping errors.
>>
>> Solution:
>> All DMA transactions have to be stopped before iommu is initialized. By
>> this patch devices are reset and in-flight DMA is stopped before
>> pci_iommu_init.
>>
>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>> port/downstream port whose child is PCIe endpoint, and then reset link
>> between them. If the endpoint is VGA device, it is skipped because the
>> monitor blacks out if VGA controller is reset.
>>
>> Actually this is v8 patch but quite different from v7 and it's been so
>> long since previous post, so I start over again.
>> Previous post:
>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>> https://lkml.org/lkml/2012/11/26/814
>>
>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>> ---
>>   Documentation/kernel-parameters.txt |    2 +
>>   drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>   2 files changed, 105 insertions(+), 0 deletions(-)
>>
>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>> index 4609e81..2a31ade 100644
>> --- a/Documentation/kernel-parameters.txt
>> +++ b/Documentation/kernel-parameters.txt
>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>   				any pair of devices, possibly at the cost of
>>   				reduced performance.  This also guarantees
>>   				that hot-added devices will work.
>> +		pcie_reset_devices	Reset PCIe endpoint on boot by hot
>> +				reset
>>   		cbiosize=nn[KMG]	The fixed amount of bus space which is
>>   				reserved for the CardBus bridge's IO window.
>>   				The default value is 256 bytes.
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index b099e00..42385c9 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>   }
>>   EXPORT_SYMBOL(pci_fixup_cardbus);
>>
>> +/*
>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>> + * endpoint except VGA device.
>> + */
>> +static int __init need_reset(struct pci_dev *dev)
>> +{
>> +	struct pci_bus *subordinate;
>> +	struct pci_dev *child;
>> +
>> +	if (!pci_is_pcie(dev) || !dev->subordinate ||
>> +	    list_empty(&dev->subordinate->devices) ||
>> +	    ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>> +	     (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>> +		return 0;
>> +
>> +	subordinate = dev->subordinate;
>> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
>> +		if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>> +		    (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>> +		    ((child->class>>  16) == PCI_BASE_CLASS_DISPLAY))
>> +			/* Don't reset switch, bridge, VGA device */
>> +			return 0;
>> +	}
>> +
>> +	return 1;
>> +}
>> +
>> +static void __init save_config(struct pci_dev *dev)
>> +{
>> +	struct pci_bus *subordinate;
>> +	struct pci_dev *child;
>> +
>> +	if (!need_reset(dev))
>> +		return;
>> +
>> +	subordinate = dev->subordinate;
>> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
>> +		dev_info(&child->dev, "save state\n");
>> +		pci_save_state(child);
>> +	}
>> +}
>> +
>> +static void __init restore_config(struct pci_dev *dev)
>> +{
>> +	struct pci_bus *subordinate;
>> +	struct pci_dev *child;
>> +
>> +	if (!need_reset(dev))
>> +		return;
>> +
>> +	subordinate = dev->subordinate;
>> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
>> +		dev_info(&child->dev, "restore state\n");
>> +		pci_restore_state(child);
>> +	}
>> +}
>> +
>> +static void __init do_device_reset(struct pci_dev *dev)
>> +{
>> +	u16 ctrl;
>> +
>> +	if (!need_reset(dev))
>> +		return;
>> +
>> +	dev_info(&dev->dev, "Reset Secondary bus\n");
>> +
>> +	/* Assert Secondary Bus Reset */
>> +	pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>> +	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>> +
>> +	msleep(2);
>> +
>> +	/* De-assert Secondary Bus Reset */
>> +	ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>> +}
>> +
>> +static int __initdata pcie_reset_devices;
>> +static int __init reset_pcie_devices(void)
>> +{
>> +	struct pci_dev *dev = NULL;
>> +
>> +	if (!pcie_reset_devices)
>> +		return 0;
>> +
>> +	for_each_pci_dev(dev)
>> +		save_config(dev);
>> +
>> +	for_each_pci_dev(dev)
>> +		do_device_reset(dev);
>
> So we do a reset on each root port or downstream port, but the PCI to
> PCI bridge spec (1.2) states:
>
>          11.1.1. Secondary Reset Signal
>                  The secondary reset signal, RST#, is a logical OR of the
>                  primary interface RST# signal and
>                  the state of the Secondary Bus Reset bit of the Bridge
>                  Control register (see Section 3.2.5.18).
>
> I read that to mean that in a legacy topology, doing a reset on the top
> level bridge triggers a reset down the entire hierarchy.  How does this
> apply to a PCIe topology?  Can we just do a reset on the top level root
> ports?  If so, that would also imply that a reset propagates down
On PCIe, a reset does not propagate down the tree (from upstream link to
downstream link of a bridge/switch).
> through PCIe-to-PCI bridges, so legacy PCI devices may be reset and the
> option name is misleading.
>
In earlier reply, I stated that the functions ought to be renamed to
reflect their intentions: endpoint reset.
It is not a reset-all-pci-devices interface, as it implies

> Even so, you still have root complex endpoints, which are not getting
> reset through this, so it's really not a complete solution.  Thanks,
>
> Alex
>
>> +
>> +	msleep(1000);
>> +
>> +	for_each_pci_dev(dev)
>> +		restore_config(dev);
>> +
>> +	return 0;
>> +}
>> +fs_initcall_sync(reset_pcie_devices);
>> +
>>   static int __init pci_setup(char *str)
>>   {
>>   	while (str) {
>> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
>>   				pcie_bus_config = PCIE_BUS_PEER2PEER;
>>   			} else if (!strncmp(str, "pcie_scan_all", 13)) {
>>   				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
>> +			} else if (!strncmp(str, "pcie_reset_devices", 18)) {
>> +				pcie_reset_devices = 1;
>>   			} else {
>>   				printk(KERN_ERR "PCI: Unknown option `%s'\n",
>>   						str);
>
>
>
> _______________________________________________
> iommu mailing list
> iommu@lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu

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Alex Williamson - May 7, 2013, 10:04 p.m.
On Tue, 2013-05-07 at 16:10 -0400, Don Dutile wrote:
> On 05/07/2013 12:39 PM, Alex Williamson wrote:
> > On Wed, 2013-04-24 at 13:58 +0900, Takao Indoh wrote:
> >> This patch resets PCIe devices on boot to stop ongoing DMA. When
> >> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
> >> PCIe root port and downstream port to reset its downstream endpoint.
> >>
> >> Problem:
> >> This patch solves the problem that kdump can fail when intel_iommu=on is
> >> specified. When intel_iommu=on is specified, many dma-remapping errors
> >> occur in second kernel and it causes problems like driver error or PCI
> >> SERR, at last kdump fails. This problem is caused as follows.
> >> 1) Devices are working on first kernel.
> >> 2) Switch to second kernel(kdump kernel). The devices are still working
> >>     and its DMA continues during this switch.
> >> 3) iommu is initialized during second kernel boot and ongoing DMA causes
> >>     dma-remapping errors.
> >>
> >> Solution:
> >> All DMA transactions have to be stopped before iommu is initialized. By
> >> this patch devices are reset and in-flight DMA is stopped before
> >> pci_iommu_init.
> >>
> >> To invoke hot reset on an endpoint, its upstream link need to be reset.
> >> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
> >> port/downstream port whose child is PCIe endpoint, and then reset link
> >> between them. If the endpoint is VGA device, it is skipped because the
> >> monitor blacks out if VGA controller is reset.
> >>
> >> Actually this is v8 patch but quite different from v7 and it's been so
> >> long since previous post, so I start over again.
> >> Previous post:
> >> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
> >> https://lkml.org/lkml/2012/11/26/814
> >>
> >> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
> >> ---
> >>   Documentation/kernel-parameters.txt |    2 +
> >>   drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
> >>   2 files changed, 105 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> >> index 4609e81..2a31ade 100644
> >> --- a/Documentation/kernel-parameters.txt
> >> +++ b/Documentation/kernel-parameters.txt
> >> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >>   				any pair of devices, possibly at the cost of
> >>   				reduced performance.  This also guarantees
> >>   				that hot-added devices will work.
> >> +		pcie_reset_devices	Reset PCIe endpoint on boot by hot
> >> +				reset
> >>   		cbiosize=nn[KMG]	The fixed amount of bus space which is
> >>   				reserved for the CardBus bridge's IO window.
> >>   				The default value is 256 bytes.
> >> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> >> index b099e00..42385c9 100644
> >> --- a/drivers/pci/pci.c
> >> +++ b/drivers/pci/pci.c
> >> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
> >>   }
> >>   EXPORT_SYMBOL(pci_fixup_cardbus);
> >>
> >> +/*
> >> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
> >> + * endpoint except VGA device.
> >> + */
> >> +static int __init need_reset(struct pci_dev *dev)
> >> +{
> >> +	struct pci_bus *subordinate;
> >> +	struct pci_dev *child;
> >> +
> >> +	if (!pci_is_pcie(dev) || !dev->subordinate ||
> >> +	    list_empty(&dev->subordinate->devices) ||
> >> +	    ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
> >> +	     (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
> >> +		return 0;
> >> +
> >> +	subordinate = dev->subordinate;
> >> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
> >> +		if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
> >> +		    (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
> >> +		    ((child->class>>  16) == PCI_BASE_CLASS_DISPLAY))
> >> +			/* Don't reset switch, bridge, VGA device */
> >> +			return 0;
> >> +	}
> >> +
> >> +	return 1;
> >> +}
> >> +
> >> +static void __init save_config(struct pci_dev *dev)
> >> +{
> >> +	struct pci_bus *subordinate;
> >> +	struct pci_dev *child;
> >> +
> >> +	if (!need_reset(dev))
> >> +		return;
> >> +
> >> +	subordinate = dev->subordinate;
> >> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
> >> +		dev_info(&child->dev, "save state\n");
> >> +		pci_save_state(child);
> >> +	}
> >> +}
> >> +
> >> +static void __init restore_config(struct pci_dev *dev)
> >> +{
> >> +	struct pci_bus *subordinate;
> >> +	struct pci_dev *child;
> >> +
> >> +	if (!need_reset(dev))
> >> +		return;
> >> +
> >> +	subordinate = dev->subordinate;
> >> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
> >> +		dev_info(&child->dev, "restore state\n");
> >> +		pci_restore_state(child);
> >> +	}
> >> +}
> >> +
> >> +static void __init do_device_reset(struct pci_dev *dev)
> >> +{
> >> +	u16 ctrl;
> >> +
> >> +	if (!need_reset(dev))
> >> +		return;
> >> +
> >> +	dev_info(&dev->dev, "Reset Secondary bus\n");
> >> +
> >> +	/* Assert Secondary Bus Reset */
> >> +	pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
> >> +	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
> >> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> >> +
> >> +	msleep(2);
> >> +
> >> +	/* De-assert Secondary Bus Reset */
> >> +	ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
> >> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> >> +}
> >> +
> >> +static int __initdata pcie_reset_devices;
> >> +static int __init reset_pcie_devices(void)
> >> +{
> >> +	struct pci_dev *dev = NULL;
> >> +
> >> +	if (!pcie_reset_devices)
> >> +		return 0;
> >> +
> >> +	for_each_pci_dev(dev)
> >> +		save_config(dev);
> >> +
> >> +	for_each_pci_dev(dev)
> >> +		do_device_reset(dev);
> >
> > So we do a reset on each root port or downstream port, but the PCI to
> > PCI bridge spec (1.2) states:
> >
> >          11.1.1. Secondary Reset Signal
> >                  The secondary reset signal, RST#, is a logical OR of the
> >                  primary interface RST# signal and
> >                  the state of the Secondary Bus Reset bit of the Bridge
> >                  Control register (see Section 3.2.5.18).
> >
> > I read that to mean that in a legacy topology, doing a reset on the top
> > level bridge triggers a reset down the entire hierarchy.  How does this
> > apply to a PCIe topology?  Can we just do a reset on the top level root
> > ports?  If so, that would also imply that a reset propagates down
> On PCIe, a reset does not propagate down the tree (from upstream link to
> downstream link of a bridge/switch).

I don't think this is actually true.  Section 6.6.1 of the spec defines
3 types of conventional resets, cold, warm and hot.  I believe the one
we're interested in is hot, which is an in-band mechanism for
propagating a conventional reset across a link.  Section 4.2.5.11
defines hot reset:

        Hot Reset uses bit 0 (Hot Reset) in the Training Control field
        (see Table 4-5 and Table 4-6) within the TS1 and TS2 Ordered
        Sets.
        
        A Link can enter Hot Reset if directed by a higher Layer. A Link
        can also reach the Hot Reset state by receiving two consecutive
        TS1 Ordered Sets with the Hot Reset bit asserted (see Section
        4.2.6.11).

Section 4.2.6.11 then goes on as:

      * Lanes that were not directed by a higher Layer to initiate Hot
        Reset (i.e., received two consecutive TS1 Ordered Sets with the
        Hot Reset bit asserted on any configured Lanes):
              * If any Lane of an Upstream Port of a Switch receives two
                consecutive TS1 Ordered Sets with the Hot Reset bit
                asserted, all configured Downstream Ports must
                transition to Hot Reset as soon as possible.
              * All Lanes in the configured Link transmit TS1 Ordered
                Sets with the Hot Reset bit asserted and the configured
                Link and Lane numbers.

Also included is the footnote:

        Note: Generally, Lanes of a Downstream or optional crosslink
        Port will be directed to Hot Reset, and Lanes of an Upstream or
        optional crosslink Port will enter Hot Reset by receiving two
        consecutive TS1 Ordered Sets with the Hot Reset bit asserted on
        any configured Lanes...

So I think that means that if a hot reset is received by an upstream
port, it's propagated to the downstream ports and on to the downstream
links.  The latter point is where there's potential for interpretation.

Going back to sections 6.6.1, we can generate a hot reset via:

      * For any Root or Switch Downstream Port, setting the Secondary
        Bus Reset bit of the Bridge Control register associated with the
        Port must cause a hot reset to be sent.
      * For a Switch, the following must cause a hot reset to be sent on
        all Downstream Ports:
              * Setting the Secondary Bus Reset bit of the Bridge
                Control register associated with the    Upstream Port
              * Receiving a hot reset on the Upstream Port

Sounds like it propagates to me.  But, re-reading this patch, I think
the goal is to attempt a bus reset on the most downstream
root/downstream port, but it's pretty confusing.

I also have a need to add a bus reset interface, in my case though the
end goal is specifically to reset display class devices to get them into
a usable state.  I just want to provide the kernel interfaces though,
it's up to the drivers how to use them.  Thanks,

Alex

> > through PCIe-to-PCI bridges, so legacy PCI devices may be reset and the
> > option name is misleading.
> >
> In earlier reply, I stated that the functions ought to be renamed to
> reflect their intentions: endpoint reset.
> It is not a reset-all-pci-devices interface, as it implies
> 
> > Even so, you still have root complex endpoints, which are not getting
> > reset through this, so it's really not a complete solution.  Thanks,
> >
> > Alex
> >
> >> +
> >> +	msleep(1000);
> >> +
> >> +	for_each_pci_dev(dev)
> >> +		restore_config(dev);
> >> +
> >> +	return 0;
> >> +}
> >> +fs_initcall_sync(reset_pcie_devices);
> >> +
> >>   static int __init pci_setup(char *str)
> >>   {
> >>   	while (str) {
> >> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
> >>   				pcie_bus_config = PCIE_BUS_PEER2PEER;
> >>   			} else if (!strncmp(str, "pcie_scan_all", 13)) {
> >>   				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
> >> +			} else if (!strncmp(str, "pcie_reset_devices", 18)) {
> >> +				pcie_reset_devices = 1;
> >>   			} else {
> >>   				printk(KERN_ERR "PCI: Unknown option `%s'\n",
> >>   						str);
> >
> >
> >
> > _______________________________________________
> > iommu mailing list
> > iommu@lists.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/iommu
> 



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Takao Indoh - May 8, 2013, 8:32 a.m.
(2013/05/07 21:50), Don Dutile wrote:
> On 05/07/2013 03:09 AM, Takao Indoh wrote:
>> Sorry for the delayed response.
>>
>> (2013/04/30 23:54), Sumner, William wrote:
>>> I have installed your original patch set (from last November) and tested with three platforms, each with a different IO configuration.  On the first platform crashdumps were consistently successful.  On the second and third platforms, the reset of one specific PCI device on each platform (a different device type on each platform) immediately hung the crash kernel. This was consistent across multiple tries.  Temporarily modifying the patch to skip resetting the one problem-device on each platform allowed each platform to create a dump successfully.
>>>
>>> I am now working with our IO team to determine the exact nature of the hang and to see if the hang is unique to the specific cards or the specific platforms.
>>>
>>> Also I am starting to look at an alternate possibility:
>>>
>>> The PCIe spec (my copy is version 2.0) in section 6.6.2 (Function-Level Reset) describes reset use-cases for "a partitioned environment where hardware is migrated from one partition to another" and for "system software is taking down the software stack for a Function and then rebuilding that stack".
>>>
>>> These use-cases seem very similar to transitioning into the crash kernel.  The "Implementation Note" at the end of that section describes an algorithm for "Avoiding Data Corruption From Stale Completions" which looks like it might be applicable to stopping ongoing DMA.  I am hopeful that adding some of the steps from this algorithm to one of the already proposed patches would avoid the hang that I saw on two platforms.
>>
>> It seems that the algorithm you mentioned requires four steps.
>>
>> 1. Clear Command register
>> 2. Wait a few milliseconds (Or polling Transactions Pending bit)
>> 3. Do FLR
>> 4. Wait 100 ms
>>
>> My patch does not do step 1 and 2. So, I would appreciate it if you
>> could add the following into save_config() in my latest patch and
>> confirm if kernel still hangs up or not.
>>
>>           subordinate = dev->subordinate;
>>           list_for_each_entry(child,&subordinate->devices, bus_list) {
>>                   dev_info(&child->dev, "save state\n");
>>                   pci_save_state(child);
>> +               pci_write_config_word(child, PCI_COMMAND, 0);
>> +               msleep(1000);
> As Linus pointed out in an earlier patch, msleep() after each device
> is s-l-o-w and unnecessary; should do a single msleep(1000) after
> *all* the command registers are written.  That way, the added delay is 1sec,
> not 1sec*ndevs.

Yeah, of course. I added these two lines just to confirm if clearing
command register resolves Bill's problem. They are tentative things.

> q: is this turning off command register for only PCI(e) endpoints?
> -- shouldn't have to turn off command register in bridges/switches.

Yes, only clearing register of endpoints because kdump fails due to
following error when turning off command register of switch as well.

Fusion MPT SAS Host driver 3.04.20
mptbase: ioc0: Initiating bringup
mptbase: ioc0: WARNING - Unexpected doorbell active!
mptbase: ioc0: ERROR - Failed to come READY after reset! IocState=f0000000
mptbase: ioc0: WARNING - ResetHistory bit failed to clear!
mptbase: ioc0: ERROR - Diagnostic reset FAILED! (ffffffffh)
mptbase: ioc0: WARNING - NOT READY WARNING!
mptbase: ioc0: ERROR - didn't initialize properly! (-1)
mptsas: probe of 0000:50:00.0 failed with error -1

Thanks,
Takao Indoh

> 
>>           }
>>
>> Thanks,
>> Takao Indoh
>>
>>
>>>
>>> Bill Sumner
>>>
>>> -----Original Message-----
>>> From: kexec [mailto:kexec-bounces@lists.infradead.org] On Behalf Of Don Dutile
>>> Sent: Friday, April 26, 2013 8:43 AM
>>> To: Takao Indoh
>>> Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com
>>> Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA
>>>
>>> On 04/25/2013 11:10 PM, Takao Indoh wrote:
>>>> (2013/04/26 3:01), Don Dutile wrote:
>>>>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>>>>> (2013/04/25 4:59), Don Dutile wrote:
>>>>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>>>>
>>>>>>>> Problem:
>>>>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>>>>> 1) Devices are working on first kernel.
>>>>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>>>>           and its DMA continues during this switch.
>>>>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>>>>           dma-remapping errors.
>>>>>>>>
>>>>>>>> Solution:
>>>>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>>>>> pci_iommu_init.
>>>>>>>>
>>>>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>>>>> monitor blacks out if VGA controller is reset.
>>>>>>>>
>>>>>>> Couple questions wrt VGA device:
>>>>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>>>>            is the VGA always function 0, so this scan sees it first&    doesn't
>>>>>>>            do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>>>>            this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>>>>
>>>>>> VGA is not reset irrespective of its function number. The logic of this
>>>>>> patch is:
>>>>>>
>>>>>> for_each_pci_dev(dev) {
>>>>>>           if (dev is not PCIe)
>>>>>>              continue;
>>>>>>           if (dev is not root port/downstream port) ---(1)
>>>>>>              continue;
>>>>>>           list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>>>>               if (child is upstream port or bridge or VGA) ---(2)
>>>>>>                   continue;
>>>>>>           }
>>>>>>           do_reset_its_child(dev);
>>>>>> }
>>>>>>
>>>>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>>>>> downstream port) of VGA is also skipped by (2).
>>>>>>
>>>>>>
>>>>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>>>>            assumes it is, and why it isn't blanked.
>>>>>>>            Q: Should the filter be based on a device having a device-class of display ?
>>>>>>
>>>>>> I want to avoid the situation that user's monitor blacks out and user
>>>>>> cannot know what's going on. That's reason why I introduced the logic to
>>>>>> skip VGA. As far as I tested the logic based on device-class works well,
>>>>> sorry, I read your description, which said VGA, but your are filtering on display class,
>>>>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>>>>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>>>>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>>>>> So, this may work well for servers, which is the primary consumer/user of this feature,
>>>>> and they typically have built-in graphics that are generally used in simple VGA mode,
>>>>> so this may be sufficient for now.
>>>>
>>>> Ok, understood.
>>>>
>>>>
>>>>>
>>>>>> but I would appreciate it if there are better ways.
>>>>>>
>>>>> You probably don't want to hear it but....
>>>>> a) only turn off cmd-reg master enable bit
>>>>> b) only do reset based on a list of devices known not to
>>>>>         obey their cmd-reg master enable bit, and only do reset to those devices.
>>>>> But, given the testing you've done so far, this optional (need cmdline) feature,
>>>>> let's start here.
>>>>
>>>> Ok. Either way I think we need more testing.
>>>>
>>>>
>>>>>>>
>>>>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>>>>> long since previous post, so I start over again.
>>>>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>>>>
>>>>>> Thank you for your review!
>>>>>>
>>>>>>>
>>>>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>>>>           dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>>>>           That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>>>>           is set, would remove this (relatively slow) PCI init sequencing ?
>>>>>>
>>>>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>>>>> they are accessing the device address, not physical address. If we disable
>>>>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>>>>> and it causes memory corruption or PCI-SERR due to DMA error.
>>>>> Right, that's a 'duh' on my part.
>>>>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>>>>> let's the ongoing DMA run...
>>>>> Please ignore this question... sigh.
>>>>>
>>>>>>
>>>>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>>>>> second kernel if iommu is used in first kernel.
>>>>>>
>>>>>> Thanks,
>>>>>> Takao Indoh
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>> Previous post:
>>>>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>>>>
>>>>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>>>>> ---
>>>>>>>>         Documentation/kernel-parameters.txt |    2 +
>>>>>>>>         drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>>>>         2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>>>>> index 4609e81..2a31ade 100644
>>>>>>>> --- a/Documentation/kernel-parameters.txt
>>>>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>>>>                         any pair of devices, possibly at the cost of
>>>>>>>>                         reduced performance.  This also guarantees
>>>>>>>>                         that hot-added devices will work.
>>>>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>>>>> +                reset
>>>>>>>>                 cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>>>>                         reserved for the CardBus bridge's IO window.
>>>>>>>>                         The default value is 256 bytes.
>>>>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>>>>> index b099e00..42385c9 100644
>>>>>>>> --- a/drivers/pci/pci.c
>>>>>>>> +++ b/drivers/pci/pci.c
>>>>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>>>>         }
>>>>>>>>         EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>>>>
>>>>>>>> +/*
>>>>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>>>>> + * endpoint except VGA device.
>>>>>>>> + */
>>>>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>>>>> +{
>>>>>>>> +    struct pci_bus *subordinate;
>>>>>>>> +    struct pci_dev *child;
>>>>>>>> +
>>>>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>>>>> +        return 0;
>>>>>>>> +
>>>>>>>> +    subordinate = dev->subordinate;
>>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>>>>> +            ((child->class>>     16) == PCI_BASE_CLASS_DISPLAY))
>>>>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>>>>> +            return 0;
>>>>>>>> +    }
>>>>>>>> +
>>>>>>>> +    return 1;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static void __init save_config(struct pci_dev *dev)
>>>>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>>>>> when in reality, it is saving the config of all the devices attached to this pdev.
>>>>> i.e., save_downstream_configs()
>>>>>
>>>>>>>> +{
>>>>>>>> +    struct pci_bus *subordinate;
>>>>>>>> +    struct pci_dev *child;
>>>>>>>> +
>>>>>>>> +    if (!need_reset(dev))
>>>>>>>> +        return;
>>>>>>>> +
>>>>>>>> +    subordinate = dev->subordinate;
>>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>>> +        dev_info(&child->dev, "save state\n");
>>>>>>>> +        pci_save_state(child);
>>>>>>>> +    }
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static void __init restore_config(struct pci_dev *dev)
>>>>> inverse of above: restore_downstream_configs()
>>>>>>>> +{
>>>>>>>> +    struct pci_bus *subordinate;
>>>>>>>> +    struct pci_dev *child;
>>>>>>>> +
>>>>>>>> +    if (!need_reset(dev))
>>>>>>>> +        return;
>>>>>>>> +
>>>>>>>> +    subordinate = dev->subordinate;
>>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>>>>> +        pci_restore_state(child);
>>>>>>>> +    }
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>>>>> do_downstream_device_reset() -- it's not resetting this pdev,
>>>>> but the pdev's of the devices attached to it.
>>>>>
>>>> Right, I'll change them as you said.
>>>>
>>>>
>>>>>>>> +{
>>>>>>>> +    u16 ctrl;
>>>>>>>> +
>>>>>>>> +    if (!need_reset(dev))
>>>>>>>> +        return;
>>>>>>>> +
>>>>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>>>>> +
>>>>>>>> +    /* Assert Secondary Bus Reset */
>>>>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>>>> +
>>>>>>>> +    msleep(2);
>>>>>>>> +
>>>>> This works well for x86, which uses ioport registers to access
>>>>> these<256-offset registers, b/c the write() function can't return
>>>>> until the write is actually completed, but for a non-x86 system,
>>>>> with fully mmconf'd PCI space, a write() may still be a write&   run
>>>>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>>>>> you ought to do another read_config() to the device, to flush the write,
>>>>> before starting the msleep(2) clock.
>>>>>
>>>> I didn't know that... I'll insert something like this before msleep.
>>>>
>>>> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
>>>>
>>>> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
>>>> Probably need the same fix, right?
>>>>
>>> sounds like it.
>>> Interested to hear from someone in non-x86-land how
>>> they ensure pci cfg writes aren't buffered in their cpus;
>>> maybe handled via special uncached regions on other arches.
>>>
>>>>
>>>>>>>> +    /* De-assert Secondary Bus Reset */
>>>>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static int __initdata pcie_reset_devices;
>>>>>>>> +static int __init reset_pcie_devices(void)
>>>>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>>>>> +{
>>>>>>>> +    struct pci_dev *dev = NULL;
>>>>>>>> +
>>>>>>>> +    if (!pcie_reset_devices)
>>>>>>>> +        return 0;
>>>>>>>> +
>>>>>>>> +    for_each_pci_dev(dev)
>>>>>>>> +        save_config(dev);
>>>>>>>> +
>>>>>>>> +    for_each_pci_dev(dev)
>>>>>>>> +        do_device_reset(dev);
>>>>>>>> +
>>>>>>>> +    msleep(1000);
>>>>>>>> +
>>>>>>>> +    for_each_pci_dev(dev)
>>>>>>>> +        restore_config(dev);
>>>>>>>> +
>>>>> My apologies if past thread covered this sequence...
>>>>> Why three loops through all PCIe devices on the system?
>>>>> why not have the first for-each-pci-dev() loop filter devices
>>>>> to be reset, and save_config those pdev's,
>>>>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>>>>> then mpsleep(), then restore the list.
>>>>> in fact, once you create a link list of pdev's to reset,
>>>>> just loop that list doing save, then reset; rtn the list, do msleep(),
>>>>> then restore the config of pdevs in the list.
>>>>> Otherwise, doing much more traversing than what's needed.
>>>>> Doing a great deal more config-saving then needed right now as well
>>>>> (saving all non-endpt devices that aren't reset).
>>>>>
>>>>
>>>> Creating list is nice idea, thanks. I'll do.
>>>>
>>>> I'll have vacation next week, so I'll post v2 patch the week after
>>>> next.
>>>>
>>>> Thanks,
>>>> Takao Indoh
>>>>
>>> Have a good vacation! Thanks for your willingness
>>> to respin again&  make this improvement.
>>>
>>>
>>> _______________________________________________
>>> kexec mailing list
>>> kexec@lists.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/kexec
>>>
>>>
>>
>>
> 
> 
> 


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Takao Indoh - May 8, 2013, 8:34 a.m.
(2013/05/08 7:04), Alex Williamson wrote:
> On Tue, 2013-05-07 at 16:10 -0400, Don Dutile wrote:
>> On 05/07/2013 12:39 PM, Alex Williamson wrote:
>>> On Wed, 2013-04-24 at 13:58 +0900, Takao Indoh wrote:
>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>
>>>> Problem:
>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>> occur in second kernel and it causes problems like driver error or PCI
>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>> 1) Devices are working on first kernel.
>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>      and its DMA continues during this switch.
>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>      dma-remapping errors.
>>>>
>>>> Solution:
>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>> this patch devices are reset and in-flight DMA is stopped before
>>>> pci_iommu_init.
>>>>
>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>> monitor blacks out if VGA controller is reset.
>>>>
>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>> long since previous post, so I start over again.
>>>> Previous post:
>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>> https://lkml.org/lkml/2012/11/26/814
>>>>
>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>> ---
>>>>    Documentation/kernel-parameters.txt |    2 +
>>>>    drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>    2 files changed, 105 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>> index 4609e81..2a31ade 100644
>>>> --- a/Documentation/kernel-parameters.txt
>>>> +++ b/Documentation/kernel-parameters.txt
>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>    				any pair of devices, possibly at the cost of
>>>>    				reduced performance.  This also guarantees
>>>>    				that hot-added devices will work.
>>>> +		pcie_reset_devices	Reset PCIe endpoint on boot by hot
>>>> +				reset
>>>>    		cbiosize=nn[KMG]	The fixed amount of bus space which is
>>>>    				reserved for the CardBus bridge's IO window.
>>>>    				The default value is 256 bytes.
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index b099e00..42385c9 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>    }
>>>>    EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>
>>>> +/*
>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>> + * endpoint except VGA device.
>>>> + */
>>>> +static int __init need_reset(struct pci_dev *dev)
>>>> +{
>>>> +	struct pci_bus *subordinate;
>>>> +	struct pci_dev *child;
>>>> +
>>>> +	if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>> +	    list_empty(&dev->subordinate->devices) ||
>>>> +	    ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>> +	     (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>> +		return 0;
>>>> +
>>>> +	subordinate = dev->subordinate;
>>>> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +		if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>> +		    (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>> +		    ((child->class>>  16) == PCI_BASE_CLASS_DISPLAY))
>>>> +			/* Don't reset switch, bridge, VGA device */
>>>> +			return 0;
>>>> +	}
>>>> +
>>>> +	return 1;
>>>> +}
>>>> +
>>>> +static void __init save_config(struct pci_dev *dev)
>>>> +{
>>>> +	struct pci_bus *subordinate;
>>>> +	struct pci_dev *child;
>>>> +
>>>> +	if (!need_reset(dev))
>>>> +		return;
>>>> +
>>>> +	subordinate = dev->subordinate;
>>>> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +		dev_info(&child->dev, "save state\n");
>>>> +		pci_save_state(child);
>>>> +	}
>>>> +}
>>>> +
>>>> +static void __init restore_config(struct pci_dev *dev)
>>>> +{
>>>> +	struct pci_bus *subordinate;
>>>> +	struct pci_dev *child;
>>>> +
>>>> +	if (!need_reset(dev))
>>>> +		return;
>>>> +
>>>> +	subordinate = dev->subordinate;
>>>> +	list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +		dev_info(&child->dev, "restore state\n");
>>>> +		pci_restore_state(child);
>>>> +	}
>>>> +}
>>>> +
>>>> +static void __init do_device_reset(struct pci_dev *dev)
>>>> +{
>>>> +	u16 ctrl;
>>>> +
>>>> +	if (!need_reset(dev))
>>>> +		return;
>>>> +
>>>> +	dev_info(&dev->dev, "Reset Secondary bus\n");
>>>> +
>>>> +	/* Assert Secondary Bus Reset */
>>>> +	pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>> +	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>> +
>>>> +	msleep(2);
>>>> +
>>>> +	/* De-assert Secondary Bus Reset */
>>>> +	ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>> +	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>> +}
>>>> +
>>>> +static int __initdata pcie_reset_devices;
>>>> +static int __init reset_pcie_devices(void)
>>>> +{
>>>> +	struct pci_dev *dev = NULL;
>>>> +
>>>> +	if (!pcie_reset_devices)
>>>> +		return 0;
>>>> +
>>>> +	for_each_pci_dev(dev)
>>>> +		save_config(dev);
>>>> +
>>>> +	for_each_pci_dev(dev)
>>>> +		do_device_reset(dev);
>>>
>>> So we do a reset on each root port or downstream port, but the PCI to
>>> PCI bridge spec (1.2) states:
>>>
>>>           11.1.1. Secondary Reset Signal
>>>                   The secondary reset signal, RST#, is a logical OR of the
>>>                   primary interface RST# signal and
>>>                   the state of the Secondary Bus Reset bit of the Bridge
>>>                   Control register (see Section 3.2.5.18).
>>>
>>> I read that to mean that in a legacy topology, doing a reset on the top
>>> level bridge triggers a reset down the entire hierarchy.  How does this
>>> apply to a PCIe topology?  Can we just do a reset on the top level root
>>> ports?  If so, that would also imply that a reset propagates down
>> On PCIe, a reset does not propagate down the tree (from upstream link to
>> downstream link of a bridge/switch).
> 
> I don't think this is actually true.  Section 6.6.1 of the spec defines
> 3 types of conventional resets, cold, warm and hot.  I believe the one
> we're interested in is hot, which is an in-band mechanism for
> propagating a conventional reset across a link.  Section 4.2.5.11
> defines hot reset:
> 
>          Hot Reset uses bit 0 (Hot Reset) in the Training Control field
>          (see Table 4-5 and Table 4-6) within the TS1 and TS2 Ordered
>          Sets.
>          
>          A Link can enter Hot Reset if directed by a higher Layer. A Link
>          can also reach the Hot Reset state by receiving two consecutive
>          TS1 Ordered Sets with the Hot Reset bit asserted (see Section
>          4.2.6.11).
> 
> Section 4.2.6.11 then goes on as:
> 
>        * Lanes that were not directed by a higher Layer to initiate Hot
>          Reset (i.e., received two consecutive TS1 Ordered Sets with the
>          Hot Reset bit asserted on any configured Lanes):
>                * If any Lane of an Upstream Port of a Switch receives two
>                  consecutive TS1 Ordered Sets with the Hot Reset bit
>                  asserted, all configured Downstream Ports must
>                  transition to Hot Reset as soon as possible.
>                * All Lanes in the configured Link transmit TS1 Ordered
>                  Sets with the Hot Reset bit asserted and the configured
>                  Link and Lane numbers.
> 
> Also included is the footnote:
> 
>          Note: Generally, Lanes of a Downstream or optional crosslink
>          Port will be directed to Hot Reset, and Lanes of an Upstream or
>          optional crosslink Port will enter Hot Reset by receiving two
>          consecutive TS1 Ordered Sets with the Hot Reset bit asserted on
>          any configured Lanes...
> 
> So I think that means that if a hot reset is received by an upstream
> port, it's propagated to the downstream ports and on to the downstream
> links.  The latter point is where there's potential for interpretation.
> 
> Going back to sections 6.6.1, we can generate a hot reset via:
> 
>        * For any Root or Switch Downstream Port, setting the Secondary
>          Bus Reset bit of the Bridge Control register associated with the
>          Port must cause a hot reset to be sent.
>        * For a Switch, the following must cause a hot reset to be sent on
>          all Downstream Ports:
>                * Setting the Secondary Bus Reset bit of the Bridge
>                  Control register associated with the    Upstream Port
>                * Receiving a hot reset on the Upstream Port
> 
> Sounds like it propagates to me.  But, re-reading this patch, I think
> the goal is to attempt a bus reset on the most downstream
> root/downstream port, but it's pretty confusing.

Yes, I also think a hot reset is propagated to all downstream link. This
patch does bus reset only on root port/downstream port whose children
are endpoints since I need to skip display class devices to avoid
monitor blacks out. 

Thanks,
Takao Indoh

> 
> I also have a need to add a bus reset interface, in my case though the
> end goal is specifically to reset display class devices to get them into
> a usable state.  I just want to provide the kernel interfaces though,
> it's up to the drivers how to use them.  Thanks,
> 
> Alex
> 
>>> through PCIe-to-PCI bridges, so legacy PCI devices may be reset and the
>>> option name is misleading.
>>>
>> In earlier reply, I stated that the functions ought to be renamed to
>> reflect their intentions: endpoint reset.
>> It is not a reset-all-pci-devices interface, as it implies
>>
>>> Even so, you still have root complex endpoints, which are not getting
>>> reset through this, so it's really not a complete solution.  Thanks,
>>>
>>> Alex
>>>
>>>> +
>>>> +	msleep(1000);
>>>> +
>>>> +	for_each_pci_dev(dev)
>>>> +		restore_config(dev);
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +fs_initcall_sync(reset_pcie_devices);
>>>> +
>>>>    static int __init pci_setup(char *str)
>>>>    {
>>>>    	while (str) {
>>>> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
>>>>    				pcie_bus_config = PCIE_BUS_PEER2PEER;
>>>>    			} else if (!strncmp(str, "pcie_scan_all", 13)) {
>>>>    				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
>>>> +			} else if (!strncmp(str, "pcie_reset_devices", 18)) {
>>>> +				pcie_reset_devices = 1;
>>>>    			} else {
>>>>    				printk(KERN_ERR "PCI: Unknown option `%s'\n",
>>>>    						str);
>>>
>>>
>>>
>>> _______________________________________________
>>> iommu mailing list
>>> iommu@lists.linux-foundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/iommu
>>
> 
> 
> 
> 
> 

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Takao Indoh - May 8, 2013, 8:38 a.m.
(2013/04/26 3:01), Don Dutile wrote:
> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>> (2013/04/25 4:59), Don Dutile wrote:
>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>
>>>> Problem:
>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>> occur in second kernel and it causes problems like driver error or PCI
>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>> 1) Devices are working on first kernel.
>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>       and its DMA continues during this switch.
>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>       dma-remapping errors.
>>>>
>>>> Solution:
>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>> this patch devices are reset and in-flight DMA is stopped before
>>>> pci_iommu_init.
>>>>
>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>> monitor blacks out if VGA controller is reset.
>>>>
>>> Couple questions wrt VGA device:
>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>        is the VGA always function 0, so this scan sees it first&  doesn't
>>>        do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>        this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>
>> VGA is not reset irrespective of its function number. The logic of this
>> patch is:
>>
>> for_each_pci_dev(dev) {
>>       if (dev is not PCIe)
>>          continue;
>>       if (dev is not root port/downstream port) ---(1)
>>          continue;
>>       list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>           if (child is upstream port or bridge or VGA) ---(2)
>>               continue;
>>       }
>>       do_reset_its_child(dev);
>> }
>>
>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>> downstream port) of VGA is also skipped by (2).
>>
>>
>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>        assumes it is, and why it isn't blanked.
>>>        Q: Should the filter be based on a device having a device-class of display ?
>>
>> I want to avoid the situation that user's monitor blacks out and user
>> cannot know what's going on. That's reason why I introduced the logic to
>> skip VGA. As far as I tested the logic based on device-class works well,
> sorry, I read your description, which said VGA, but your are filtering on display class,
> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
> are probably one of the most aggressive DMA engines on a system.... and will grow as
> asymmetric processing using GPUs gets architected into a device-agnostic manner.
> So, this may work well for servers, which is the primary consumer/user of this feature,
> and they typically have built-in graphics that are generally used in simple VGA mode,
> so this may be sufficient for now.
>   
> 
>> but I would appreciate it if there are better ways.
>>
> You probably don't want to hear it but....
> a) only turn off cmd-reg master enable bit
> b) only do reset based on a list of devices known not to
>     obey their cmd-reg master enable bit, and only do reset to those devices.
> But, given the testing you've done so far, this optional (need cmdline) feature,
> let's start here.
> 
>>>
>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>> long since previous post, so I start over again.
>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>
>> Thank you for your review!
>>
>>>
>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>       dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>       That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>       is set, would remove this (relatively slow) PCI init sequencing ?
>>
>> To force iommu off, all ongoing DMA have to be stopped before that since
>> they are accessing the device address, not physical address. If we disable
>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>> and it causes memory corruption or PCI-SERR due to DMA error.
> Right, that's a 'duh' on my part.
> I thought 'disable iommu' == 'block all dma' and it just turns it off &
> let's the ongoing DMA run...
> Please ignore this question... sigh.
> 
>>
>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>> second kernel if iommu is used in first kernel.
>>
>> Thanks,
>> Takao Indoh
>>
>>
>>>
>>>> Previous post:
>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>> https://lkml.org/lkml/2012/11/26/814
>>>>
>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>> ---
>>>>     Documentation/kernel-parameters.txt |    2 +
>>>>     drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>     2 files changed, 105 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>> index 4609e81..2a31ade 100644
>>>> --- a/Documentation/kernel-parameters.txt
>>>> +++ b/Documentation/kernel-parameters.txt
>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>                     any pair of devices, possibly at the cost of
>>>>                     reduced performance.  This also guarantees
>>>>                     that hot-added devices will work.
>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>> +                reset
>>>>             cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>                     reserved for the CardBus bridge's IO window.
>>>>                     The default value is 256 bytes.
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index b099e00..42385c9 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>     }
>>>>     EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>
>>>> +/*
>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>> + * endpoint except VGA device.
>>>> + */
>>>> +static int __init need_reset(struct pci_dev *dev)
>>>> +{
>>>> +    struct pci_bus *subordinate;
>>>> +    struct pci_dev *child;
>>>> +
>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>> +        list_empty(&dev->subordinate->devices) ||
>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>> +        return 0;
>>>> +
>>>> +    subordinate = dev->subordinate;
>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>> +            ((child->class>>   16) == PCI_BASE_CLASS_DISPLAY))
>>>> +            /* Don't reset switch, bridge, VGA device */
>>>> +            return 0;
>>>> +    }
>>>> +
>>>> +    return 1;
>>>> +}
>>>> +
>>>> +static void __init save_config(struct pci_dev *dev)
> This should be renamed.  it implies it is saving the config of the pdev passed in,
> when in reality, it is saving the config of all the devices attached to this pdev.
> i.e., save_downstream_configs()
> 
>>>> +{
>>>> +    struct pci_bus *subordinate;
>>>> +    struct pci_dev *child;
>>>> +
>>>> +    if (!need_reset(dev))
>>>> +        return;
>>>> +
>>>> +    subordinate = dev->subordinate;
>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +        dev_info(&child->dev, "save state\n");
>>>> +        pci_save_state(child);
>>>> +    }
>>>> +}
>>>> +
>>>> +static void __init restore_config(struct pci_dev *dev)
> inverse of above: restore_downstream_configs()
>>>> +{
>>>> +    struct pci_bus *subordinate;
>>>> +    struct pci_dev *child;
>>>> +
>>>> +    if (!need_reset(dev))
>>>> +        return;
>>>> +
>>>> +    subordinate = dev->subordinate;
>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>> +        dev_info(&child->dev, "restore state\n");
>>>> +        pci_restore_state(child);
>>>> +    }
>>>> +}
>>>> +
>>>> +static void __init do_device_reset(struct pci_dev *dev)
> do_downstream_device_reset() -- it's not resetting this pdev,
> but the pdev's of the devices attached to it.
> 
>>>> +{
>>>> +    u16 ctrl;
>>>> +
>>>> +    if (!need_reset(dev))
>>>> +        return;
>>>> +
>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>> +
>>>> +    /* Assert Secondary Bus Reset */
>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>> +
>>>> +    msleep(2);
>>>> +
> This works well for x86, which uses ioport registers to access
> these <256-offset registers, b/c the write() function can't return
> until the write is actually completed, but for a non-x86 system,
> with fully mmconf'd PCI space, a write() may still be a write & run
> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
> you ought to do another read_config() to the device, to flush the write,
> before starting the msleep(2) clock.
> 
>>>> +    /* De-assert Secondary Bus Reset */
>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>> +}
>>>> +
>>>> +static int __initdata pcie_reset_devices;
>>>> +static int __init reset_pcie_devices(void)
> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>> +{
>>>> +    struct pci_dev *dev = NULL;
>>>> +
>>>> +    if (!pcie_reset_devices)
>>>> +        return 0;
>>>> +
>>>> +    for_each_pci_dev(dev)
>>>> +        save_config(dev);
>>>> +
>>>> +    for_each_pci_dev(dev)
>>>> +        do_device_reset(dev);
>>>> +
>>>> +    msleep(1000);
>>>> +
>>>> +    for_each_pci_dev(dev)
>>>> +        restore_config(dev);
>>>> +
> My apologies if past thread covered this sequence...
> Why three loops through all PCIe devices on the system?
> why not have the first for-each-pci-dev() loop filter devices
> to be reset, and save_config those pdev's,
> return a list of saved_pdev's;  feed that list into the do_device_reset();
> then mpsleep(), then restore the list.
> in fact, once you create a link list of pdev's to reset,
> just loop that list doing save, then reset; rtn the list, do msleep(),
> then restore the config of pdevs in the list.
> Otherwise, doing much more traversing than what's needed.
> Doing a great deal more config-saving then needed right now as well
> (saving all non-endpt devices that aren't reset).

One question, do you mean we should have two lists? For example,

LIST_HEAD(pci_reset_list); /* pdev list to reset */
LIST_HEAD(pci_saved_list); /* pdev list to save/restore config */

Or simply making one list and just loop it like this:

    LIST_HEAD(pdev_list);

    for_each_pci_dev(pdev)
        if (need_reset(pdev))
            /* Add pdev to pdev_list */

    list_for_each_entry(pdev, &pdev_list)
        save_downstream_configs(pdev);

    list_for_each_entry(pdev, &pdev_list)
        do_downstream_device_reset(pdev);

    list_for_each_entry(pdev, &pdev_list)
        restore_downstream_configs(pdev);

Thanks,
Takao Indoh

> 
>>>> +    return 0;
>>>> +}
>>>> +fs_initcall_sync(reset_pcie_devices);
>>>> +
>>>>     static int __init pci_setup(char *str)
>>>>     {
>>>>         while (str) {
>>>> @@ -3920,6 +4021,8 @@ static int __init pci_setup(char *str)
>>>>                     pcie_bus_config = PCIE_BUS_PEER2PEER;
>>>>                 } else if (!strncmp(str, "pcie_scan_all", 13)) {
>>>>                     pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
>>>> +            } else if (!strncmp(str, "pcie_reset_devices", 18)) {
> pcie_reset_endpoint_devices
>>>> +                pcie_reset_devices = 1;
> pcie_reset_endpoint_devices
>>>>                 } else {
>>>>                     printk(KERN_ERR "PCI: Unknown option `%s'\n",
>>>>                             str);
>>>
>>>
>>>
>>
> 
> 
> 

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Sumner, William - May 15, 2013, 4:59 p.m.
Thank you for sending the 2-line addition to your patch that helps me test the suggestions in the PCIe standard.

I started with a new Linux top-of-tree as of last week, added the patch from late April, added these new lines of code, then tested on one of the platforms that showed the problem before.  

This addition seems to improve operation on my platform.  Previously the lock-up occurred immediately when the problem PCI card was reset.  With these two lines added, the patch code completed: all resets occurred, all status was restored to PCI cards, and the crash kernel began loading modules.  The kernel then stopped after loading a few modules -- around 5 or 6 modules.  This was consistent over a few tries.  Using the same kernel, eliminating only the "pci=pcie_reset_devices" from the crash kernel's command line, a dump succeeds.  I am still investigating why the crash kernel stops during module loading.

Reading other reset code in Linux: pci_reset_function() writes PCI_COMMAND_INTX_DISABLE into the command register instead of writing 0.  This clears all bits and sets the INTX_DISABLE bit.  So I tried that in the patch and the results were the same as in the paragraph above.

Two questions:
1. Since the two added lines prevent the PCIe device from initiating new bus activity including DMA IO, and also prevent the PCIe device from sending interrupts; is this not sufficient (without resetting the PCIe device) to solve the original problem of leftover ongoing DMA IO when the crash kernel resets the hardware IOMMU ?

1a. My guess is that the answer to question 1 is "No" because some PCIe card may not check the bus master flag just before each bus transaction -- its engineers having interpreted the standard differently than I do.  However, this seems like a question worth asking of the community.  


2. Linux has a set of functions to reset PCIe devices beginning at " int pci_reset_function(struct pci_dev *dev) " and calling down to "__pci_dev_reset"  which tries several different methods to reset the PCI device.  Would any of these be useful additions to the proposed patch ?  

Bill Sumner 



-----Original Message-----
From: Takao Indoh [mailto:indou.takao@jp.fujitsu.com] 
Sent: Tuesday, May 07, 2013 2:10 AM
To: Sumner, William
Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com; ddutile@redhat.com
Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA

Sorry for the delayed response.

(2013/04/30 23:54), Sumner, William wrote:
> I have installed your original patch set (from last November) and tested with three platforms, each with a different IO configuration.  On the first platform crashdumps were consistently successful.  On the second and third platforms, the reset of one specific PCI device on each platform (a different device type on each platform) immediately hung the crash kernel. This was consistent across multiple tries.  Temporarily modifying the patch to skip resetting the one problem-device on each platform allowed each platform to create a dump successfully.
> 
> I am now working with our IO team to determine the exact nature of the hang and to see if the hang is unique to the specific cards or the specific platforms.
> 
> Also I am starting to look at an alternate possibility:
> 
> The PCIe spec (my copy is version 2.0) in section 6.6.2 (Function-Level Reset) describes reset use-cases for "a partitioned environment where hardware is migrated from one partition to another" and for "system software is taking down the software stack for a Function and then rebuilding that stack".
> 
> These use-cases seem very similar to transitioning into the crash kernel.  The "Implementation Note" at the end of that section describes an algorithm for "Avoiding Data Corruption From Stale Completions" which looks like it might be applicable to stopping ongoing DMA.  I am hopeful that adding some of the steps from this algorithm to one of the already proposed patches would avoid the hang that I saw on two platforms.

It seems that the algorithm you mentioned requires four steps.

1. Clear Command register
2. Wait a few milliseconds (Or polling Transactions Pending bit)
3. Do FLR
4. Wait 100 ms

My patch does not do step 1 and 2. So, I would appreciate it if you
could add the following into save_config() in my latest patch and
confirm if kernel still hangs up or not.

        subordinate = dev->subordinate;
        list_for_each_entry(child, &subordinate->devices, bus_list) {
                dev_info(&child->dev, "save state\n");
                pci_save_state(child);
+               pci_write_config_word(child, PCI_COMMAND, 0);
+               msleep(1000);
        }

Thanks,
Takao Indoh


> 
> Bill Sumner
> 
> -----Original Message-----
> From: kexec [mailto:kexec-bounces@lists.infradead.org] On Behalf Of Don Dutile
> Sent: Friday, April 26, 2013 8:43 AM
> To: Takao Indoh
> Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com
> Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA
> 
> On 04/25/2013 11:10 PM, Takao Indoh wrote:
>> (2013/04/26 3:01), Don Dutile wrote:
>>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>>> (2013/04/25 4:59), Don Dutile wrote:
>>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>>
>>>>>> Problem:
>>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>>> 1) Devices are working on first kernel.
>>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>>         and its DMA continues during this switch.
>>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>>         dma-remapping errors.
>>>>>>
>>>>>> Solution:
>>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>>> pci_iommu_init.
>>>>>>
>>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>>> monitor blacks out if VGA controller is reset.
>>>>>>
>>>>> Couple questions wrt VGA device:
>>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>>          is the VGA always function 0, so this scan sees it first&   doesn't
>>>>>          do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>>          this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>>
>>>> VGA is not reset irrespective of its function number. The logic of this
>>>> patch is:
>>>>
>>>> for_each_pci_dev(dev) {
>>>>         if (dev is not PCIe)
>>>>            continue;
>>>>         if (dev is not root port/downstream port) ---(1)
>>>>            continue;
>>>>         list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>>             if (child is upstream port or bridge or VGA) ---(2)
>>>>                 continue;
>>>>         }
>>>>         do_reset_its_child(dev);
>>>> }
>>>>
>>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>>> downstream port) of VGA is also skipped by (2).
>>>>
>>>>
>>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>>          assumes it is, and why it isn't blanked.
>>>>>          Q: Should the filter be based on a device having a device-class of display ?
>>>>
>>>> I want to avoid the situation that user's monitor blacks out and user
>>>> cannot know what's going on. That's reason why I introduced the logic to
>>>> skip VGA. As far as I tested the logic based on device-class works well,
>>> sorry, I read your description, which said VGA, but your are filtering on display class,
>>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>>> So, this may work well for servers, which is the primary consumer/user of this feature,
>>> and they typically have built-in graphics that are generally used in simple VGA mode,
>>> so this may be sufficient for now.
>>
>> Ok, understood.
>>
>>
>>>
>>>> but I would appreciate it if there are better ways.
>>>>
>>> You probably don't want to hear it but....
>>> a) only turn off cmd-reg master enable bit
>>> b) only do reset based on a list of devices known not to
>>>       obey their cmd-reg master enable bit, and only do reset to those devices.
>>> But, given the testing you've done so far, this optional (need cmdline) feature,
>>> let's start here.
>>
>> Ok. Either way I think we need more testing.
>>
>>
>>>>>
>>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>>> long since previous post, so I start over again.
>>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>>
>>>> Thank you for your review!
>>>>
>>>>>
>>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>>         dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>>         That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>>         is set, would remove this (relatively slow) PCI init sequencing ?
>>>>
>>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>>> they are accessing the device address, not physical address. If we disable
>>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>>> and it causes memory corruption or PCI-SERR due to DMA error.
>>> Right, that's a 'duh' on my part.
>>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>>> let's the ongoing DMA run...
>>> Please ignore this question... sigh.
>>>
>>>>
>>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>>> second kernel if iommu is used in first kernel.
>>>>
>>>> Thanks,
>>>> Takao Indoh
>>>>
>>>>
>>>>>
>>>>>> Previous post:
>>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>>
>>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>>> ---
>>>>>>       Documentation/kernel-parameters.txt |    2 +
>>>>>>       drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>>       2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>>> index 4609e81..2a31ade 100644
>>>>>> --- a/Documentation/kernel-parameters.txt
>>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>>                       any pair of devices, possibly at the cost of
>>>>>>                       reduced performance.  This also guarantees
>>>>>>                       that hot-added devices will work.
>>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>>> +                reset
>>>>>>               cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>>                       reserved for the CardBus bridge's IO window.
>>>>>>                       The default value is 256 bytes.
>>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>>> index b099e00..42385c9 100644
>>>>>> --- a/drivers/pci/pci.c
>>>>>> +++ b/drivers/pci/pci.c
>>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>>       }
>>>>>>       EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>>
>>>>>> +/*
>>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>>> + * endpoint except VGA device.
>>>>>> + */
>>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>>> +            ((child->class>>    16) == PCI_BASE_CLASS_DISPLAY))
>>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>>> +            return 0;
>>>>>> +    }
>>>>>> +
>>>>>> +    return 1;
>>>>>> +}
>>>>>> +
>>>>>> +static void __init save_config(struct pci_dev *dev)
>>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>>> when in reality, it is saving the config of all the devices attached to this pdev.
>>> i.e., save_downstream_configs()
>>>
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        dev_info(&child->dev, "save state\n");
>>>>>> +        pci_save_state(child);
>>>>>> +    }
>>>>>> +}
>>>>>> +
>>>>>> +static void __init restore_config(struct pci_dev *dev)
>>> inverse of above: restore_downstream_configs()
>>>>>> +{
>>>>>> +    struct pci_bus *subordinate;
>>>>>> +    struct pci_dev *child;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    subordinate = dev->subordinate;
>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>>> +        pci_restore_state(child);
>>>>>> +    }
>>>>>> +}
>>>>>> +
>>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>>> do_downstream_device_reset() -- it's not resetting this pdev,
>>> but the pdev's of the devices attached to it.
>>>
>> Right, I'll change them as you said.
>>
>>
>>>>>> +{
>>>>>> +    u16 ctrl;
>>>>>> +
>>>>>> +    if (!need_reset(dev))
>>>>>> +        return;
>>>>>> +
>>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>>> +
>>>>>> +    /* Assert Secondary Bus Reset */
>>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>> +
>>>>>> +    msleep(2);
>>>>>> +
>>> This works well for x86, which uses ioport registers to access
>>> these<256-offset registers, b/c the write() function can't return
>>> until the write is actually completed, but for a non-x86 system,
>>> with fully mmconf'd PCI space, a write() may still be a write&  run
>>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>>> you ought to do another read_config() to the device, to flush the write,
>>> before starting the msleep(2) clock.
>>>
>> I didn't know that... I'll insert something like this before msleep.
>>
>> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
>>
>> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
>> Probably need the same fix, right?
>>
> sounds like it.
> Interested to hear from someone in non-x86-land how
> they ensure pci cfg writes aren't buffered in their cpus;
> maybe handled via special uncached regions on other arches.
> 
>>
>>>>>> +    /* De-assert Secondary Bus Reset */
>>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>> +}
>>>>>> +
>>>>>> +static int __initdata pcie_reset_devices;
>>>>>> +static int __init reset_pcie_devices(void)
>>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>>> +{
>>>>>> +    struct pci_dev *dev = NULL;
>>>>>> +
>>>>>> +    if (!pcie_reset_devices)
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        save_config(dev);
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        do_device_reset(dev);
>>>>>> +
>>>>>> +    msleep(1000);
>>>>>> +
>>>>>> +    for_each_pci_dev(dev)
>>>>>> +        restore_config(dev);
>>>>>> +
>>> My apologies if past thread covered this sequence...
>>> Why three loops through all PCIe devices on the system?
>>> why not have the first for-each-pci-dev() loop filter devices
>>> to be reset, and save_config those pdev's,
>>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>>> then mpsleep(), then restore the list.
>>> in fact, once you create a link list of pdev's to reset,
>>> just loop that list doing save, then reset; rtn the list, do msleep(),
>>> then restore the config of pdevs in the list.
>>> Otherwise, doing much more traversing than what's needed.
>>> Doing a great deal more config-saving then needed right now as well
>>> (saving all non-endpt devices that aren't reset).
>>>
>>
>> Creating list is nice idea, thanks. I'll do.
>>
>> I'll have vacation next week, so I'll post v2 patch the week after
>> next.
>>
>> Thanks,
>> Takao Indoh
>>
> Have a good vacation! Thanks for your willingness
> to respin again & make this improvement.
> 
> 
> _______________________________________________
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> kexec@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/kexec
> 
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Takao Indoh - May 16, 2013, 5:31 a.m.
(2013/05/16 1:59), Sumner, William wrote:
> Thank you for sending the 2-line addition to your patch that helps me test the suggestions in the PCIe standard.
> 
> I started with a new Linux top-of-tree as of last week, added the patch from late April, added these new lines of code, then tested on one of the platforms that showed the problem before.
> 
> This addition seems to improve operation on my platform.  Previously the lock-up occurred immediately when the problem PCI card was reset.  With these two lines added, the patch code completed: all resets occurred, all status was restored to PCI cards, and the crash kernel began loading modules.  The kernel then stopped after loading a few modules -- around 5 or 6 modules.  This was consistent over a few tries.  Using the same kernel, eliminating only the "pci=pcie_reset_devices" from the crash kernel's command line, a dump succeeds.  I am still investigating why the crash kernel stops during module loading.
> 
> Reading other reset code in Linux: pci_reset_function() writes PCI_COMMAND_INTX_DISABLE into the command register instead of writing 0.  This clears all bits and sets the INTX_DISABLE bit.  So I tried that in the patch and the results were the same as in the paragraph above.

Thank you for testing. Hmm, clearing command register is effective but
something else seems to be also needed.

> Two questions:
> 1. Since the two added lines prevent the PCIe device from initiating new bus activity including DMA IO, and also prevent the PCIe device from sending interrupts; is this not sufficient (without resetting the PCIe device) to solve the original problem of leftover ongoing DMA IO when the crash kernel resets the hardware IOMMU ?
> 
> 1a. My guess is that the answer to question 1 is "No" because some PCIe card may not check the bus master flag just before each bus transaction -- its engineers having interpreted the standard differently than I do.  However, this seems like a question worth asking of the community.

From my experience, my answer is also "No".  On certain raid card test,
DMA remapping fault or PCI SERR are still detected after just clearing
bus master bit, though it may be a problem of its driver or hardware.


> 2. Linux has a set of functions to reset PCIe devices beginning at " int pci_reset_function(struct pci_dev *dev) " and calling down to "__pci_dev_reset"  which tries several different methods to reset the PCI device.  Would any of these be useful additions to the proposed patch ?

Good point. When I started this work, at first I made a patch to reset
devices like __pci_dev_reset. As a result, it did not work for a certain
raid card. In this case, pcie_flr() is called in __pci_dev_reset() and
it returned true, but DMAR error occurred after its driver was loaded.

One of ideas is adding boot parameter so user can choose appropriate
method: just clearing bus master, FLR, hot-reset, etc.

Thanks,
Takao Indoh


> 
> Bill Sumner
> 
> 
> 
> -----Original Message-----
> From: Takao Indoh [mailto:indou.takao@jp.fujitsu.com]
> Sent: Tuesday, May 07, 2013 2:10 AM
> To: Sumner, William
> Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com; ddutile@redhat.com
> Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA
> 
> Sorry for the delayed response.
> 
> (2013/04/30 23:54), Sumner, William wrote:
>> I have installed your original patch set (from last November) and tested with three platforms, each with a different IO configuration.  On the first platform crashdumps were consistently successful.  On the second and third platforms, the reset of one specific PCI device on each platform (a different device type on each platform) immediately hung the crash kernel. This was consistent across multiple tries.  Temporarily modifying the patch to skip resetting the one problem-device on each platform allowed each platform to create a dump successfully.
>>
>> I am now working with our IO team to determine the exact nature of the hang and to see if the hang is unique to the specific cards or the specific platforms.
>>
>> Also I am starting to look at an alternate possibility:
>>
>> The PCIe spec (my copy is version 2.0) in section 6.6.2 (Function-Level Reset) describes reset use-cases for "a partitioned environment where hardware is migrated from one partition to another" and for "system software is taking down the software stack for a Function and then rebuilding that stack".
>>
>> These use-cases seem very similar to transitioning into the crash kernel.  The "Implementation Note" at the end of that section describes an algorithm for "Avoiding Data Corruption From Stale Completions" which looks like it might be applicable to stopping ongoing DMA.  I am hopeful that adding some of the steps from this algorithm to one of the already proposed patches would avoid the hang that I saw on two platforms.
> 
> It seems that the algorithm you mentioned requires four steps.
> 
> 1. Clear Command register
> 2. Wait a few milliseconds (Or polling Transactions Pending bit)
> 3. Do FLR
> 4. Wait 100 ms
> 
> My patch does not do step 1 and 2. So, I would appreciate it if you
> could add the following into save_config() in my latest patch and
> confirm if kernel still hangs up or not.
> 
>          subordinate = dev->subordinate;
>          list_for_each_entry(child, &subordinate->devices, bus_list) {
>                  dev_info(&child->dev, "save state\n");
>                  pci_save_state(child);
> +               pci_write_config_word(child, PCI_COMMAND, 0);
> +               msleep(1000);
>          }
> 
> Thanks,
> Takao Indoh
> 
> 
>>
>> Bill Sumner
>>
>> -----Original Message-----
>> From: kexec [mailto:kexec-bounces@lists.infradead.org] On Behalf Of Don Dutile
>> Sent: Friday, April 26, 2013 8:43 AM
>> To: Takao Indoh
>> Cc: linux-pci@vger.kernel.org; kexec@lists.infradead.org; linux-kernel@vger.kernel.org; tindoh@gmail.com; iommu@lists.linux-foundation.org; bhelgaas@google.com
>> Subject: Re: [PATCH] Reset PCIe devices to stop ongoing DMA
>>
>> On 04/25/2013 11:10 PM, Takao Indoh wrote:
>>> (2013/04/26 3:01), Don Dutile wrote:
>>>> On 04/25/2013 01:11 AM, Takao Indoh wrote:
>>>>> (2013/04/25 4:59), Don Dutile wrote:
>>>>>> On 04/24/2013 12:58 AM, Takao Indoh wrote:
>>>>>>> This patch resets PCIe devices on boot to stop ongoing DMA. When
>>>>>>> "pci=pcie_reset_devices" is specified, a hot reset is triggered on each
>>>>>>> PCIe root port and downstream port to reset its downstream endpoint.
>>>>>>>
>>>>>>> Problem:
>>>>>>> This patch solves the problem that kdump can fail when intel_iommu=on is
>>>>>>> specified. When intel_iommu=on is specified, many dma-remapping errors
>>>>>>> occur in second kernel and it causes problems like driver error or PCI
>>>>>>> SERR, at last kdump fails. This problem is caused as follows.
>>>>>>> 1) Devices are working on first kernel.
>>>>>>> 2) Switch to second kernel(kdump kernel). The devices are still working
>>>>>>>          and its DMA continues during this switch.
>>>>>>> 3) iommu is initialized during second kernel boot and ongoing DMA causes
>>>>>>>          dma-remapping errors.
>>>>>>>
>>>>>>> Solution:
>>>>>>> All DMA transactions have to be stopped before iommu is initialized. By
>>>>>>> this patch devices are reset and in-flight DMA is stopped before
>>>>>>> pci_iommu_init.
>>>>>>>
>>>>>>> To invoke hot reset on an endpoint, its upstream link need to be reset.
>>>>>>> reset_pcie_devices() is called from fs_initcall_sync, and it finds root
>>>>>>> port/downstream port whose child is PCIe endpoint, and then reset link
>>>>>>> between them. If the endpoint is VGA device, it is skipped because the
>>>>>>> monitor blacks out if VGA controller is reset.
>>>>>>>
>>>>>> Couple questions wrt VGA device:
>>>>>> (1) Many graphics devices are multi-function, one function being VGA;
>>>>>>           is the VGA always function 0, so this scan sees it first&   doesn't
>>>>>>           do a reset on that PCIe link?  if the VGA is not function 0, won't
>>>>>>           this logic break (will reset b/c function 0 is non-VGA graphics) ?
>>>>>
>>>>> VGA is not reset irrespective of its function number. The logic of this
>>>>> patch is:
>>>>>
>>>>> for_each_pci_dev(dev) {
>>>>>          if (dev is not PCIe)
>>>>>             continue;
>>>>>          if (dev is not root port/downstream port) ---(1)
>>>>>             continue;
>>>>>          list_for_each_entry(child,&dev->subordinate->devices, bus_list) {
>>>>>              if (child is upstream port or bridge or VGA) ---(2)
>>>>>                  continue;
>>>>>          }
>>>>>          do_reset_its_child(dev);
>>>>> }
>>>>>
>>>>> Therefore VGA itself is skipped by (1), and upstream device(root port or
>>>>> downstream port) of VGA is also skipped by (2).
>>>>>
>>>>>
>>>>>> (2) I'm hearing VGA will soon not be the a required console; this logic
>>>>>>           assumes it is, and why it isn't blanked.
>>>>>>           Q: Should the filter be based on a device having a device-class of display ?
>>>>>
>>>>> I want to avoid the situation that user's monitor blacks out and user
>>>>> cannot know what's going on. That's reason why I introduced the logic to
>>>>> skip VGA. As far as I tested the logic based on device-class works well,
>>>> sorry, I read your description, which said VGA, but your are filtering on display class,
>>>> which includes non-VGA as well. So, all set ... but large, (x16) non-VGA display devices
>>>> are probably one of the most aggressive DMA engines on a system.... and will grow as
>>>> asymmetric processing using GPUs gets architected into a device-agnostic manner.
>>>> So, this may work well for servers, which is the primary consumer/user of this feature,
>>>> and they typically have built-in graphics that are generally used in simple VGA mode,
>>>> so this may be sufficient for now.
>>>
>>> Ok, understood.
>>>
>>>
>>>>
>>>>> but I would appreciate it if there are better ways.
>>>>>
>>>> You probably don't want to hear it but....
>>>> a) only turn off cmd-reg master enable bit
>>>> b) only do reset based on a list of devices known not to
>>>>        obey their cmd-reg master enable bit, and only do reset to those devices.
>>>> But, given the testing you've done so far, this optional (need cmdline) feature,
>>>> let's start here.
>>>
>>> Ok. Either way I think we need more testing.
>>>
>>>
>>>>>>
>>>>>>> Actually this is v8 patch but quite different from v7 and it's been so
>>>>>>> long since previous post, so I start over again.
>>>>>> Thanks for this re-start.  I need to continue reviewing the rest.
>>>>>
>>>>> Thank you for your review!
>>>>>
>>>>>>
>>>>>> Q: Why not force IOMMU off when re-booting a kexec kernel to perform a crash
>>>>>>          dump?  After the crash dump, the system is rebooting to previous (iommu=on) setting.
>>>>>>          That logic, along w/your previous patch to disable the IOMMU if iommu=off
>>>>>>          is set, would remove this (relatively slow) PCI init sequencing ?
>>>>>
>>>>> To force iommu off, all ongoing DMA have to be stopped before that since
>>>>> they are accessing the device address, not physical address. If we disable
>>>>> iommu without stopping in-flihgt DMA, devices access invalid memory area
>>>>> and it causes memory corruption or PCI-SERR due to DMA error.
>>>> Right, that's a 'duh' on my part.
>>>> I thought 'disable iommu' == 'block all dma' and it just turns it off&
>>>> let's the ongoing DMA run...
>>>> Please ignore this question... sigh.
>>>>
>>>>>
>>>>> So, whether we use iommu or not in second kernel, we have to stop DMA in
>>>>> second kernel if iommu is used in first kernel.
>>>>>
>>>>> Thanks,
>>>>> Takao Indoh
>>>>>
>>>>>
>>>>>>
>>>>>>> Previous post:
>>>>>>> [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
>>>>>>> https://lkml.org/lkml/2012/11/26/814
>>>>>>>
>>>>>>> Signed-off-by: Takao Indoh<indou.takao@jp.fujitsu.com>
>>>>>>> ---
>>>>>>>        Documentation/kernel-parameters.txt |    2 +
>>>>>>>        drivers/pci/pci.c                   |  103 +++++++++++++++++++++++++++++++++++
>>>>>>>        2 files changed, 105 insertions(+), 0 deletions(-)
>>>>>>>
>>>>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>>>>> index 4609e81..2a31ade 100644
>>>>>>> --- a/Documentation/kernel-parameters.txt
>>>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>>>> @@ -2250,6 +2250,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>>>>                        any pair of devices, possibly at the cost of
>>>>>>>                        reduced performance.  This also guarantees
>>>>>>>                        that hot-added devices will work.
>>>>>>> +        pcie_reset_devices    Reset PCIe endpoint on boot by hot
>>>>>>> +                reset
>>>>>>>                cbiosize=nn[KMG]    The fixed amount of bus space which is
>>>>>>>                        reserved for the CardBus bridge's IO window.
>>>>>>>                        The default value is 256 bytes.
>>>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>>>> index b099e00..42385c9 100644
>>>>>>> --- a/drivers/pci/pci.c
>>>>>>> +++ b/drivers/pci/pci.c
>>>>>>> @@ -3878,6 +3878,107 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>>>>        }
>>>>>>>        EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>>>>
>>>>>>> +/*
>>>>>>> + * Return true if dev is PCIe root port or downstream port whose child is PCIe
>>>>>>> + * endpoint except VGA device.
>>>>>>> + */
>>>>>>> +static int __init need_reset(struct pci_dev *dev)
>>>>>>> +{
>>>>>>> +    struct pci_bus *subordinate;
>>>>>>> +    struct pci_dev *child;
>>>>>>> +
>>>>>>> +    if (!pci_is_pcie(dev) || !dev->subordinate ||
>>>>>>> +        list_empty(&dev->subordinate->devices) ||
>>>>>>> +        ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)&&
>>>>>>> +         (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
>>>>>>> +        return 0;
>>>>>>> +
>>>>>>> +    subordinate = dev->subordinate;
>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>> +        if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>>>>> +            (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
>>>>>>> +            ((child->class>>    16) == PCI_BASE_CLASS_DISPLAY))
>>>>>>> +            /* Don't reset switch, bridge, VGA device */
>>>>>>> +            return 0;
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    return 1;
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void __init save_config(struct pci_dev *dev)
>>>> This should be renamed.  it implies it is saving the config of the pdev passed in,
>>>> when in reality, it is saving the config of all the devices attached to this pdev.
>>>> i.e., save_downstream_configs()
>>>>
>>>>>>> +{
>>>>>>> +    struct pci_bus *subordinate;
>>>>>>> +    struct pci_dev *child;
>>>>>>> +
>>>>>>> +    if (!need_reset(dev))
>>>>>>> +        return;
>>>>>>> +
>>>>>>> +    subordinate = dev->subordinate;
>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>> +        dev_info(&child->dev, "save state\n");
>>>>>>> +        pci_save_state(child);
>>>>>>> +    }
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void __init restore_config(struct pci_dev *dev)
>>>> inverse of above: restore_downstream_configs()
>>>>>>> +{
>>>>>>> +    struct pci_bus *subordinate;
>>>>>>> +    struct pci_dev *child;
>>>>>>> +
>>>>>>> +    if (!need_reset(dev))
>>>>>>> +        return;
>>>>>>> +
>>>>>>> +    subordinate = dev->subordinate;
>>>>>>> +    list_for_each_entry(child,&subordinate->devices, bus_list) {
>>>>>>> +        dev_info(&child->dev, "restore state\n");
>>>>>>> +        pci_restore_state(child);
>>>>>>> +    }
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void __init do_device_reset(struct pci_dev *dev)
>>>> do_downstream_device_reset() -- it's not resetting this pdev,
>>>> but the pdev's of the devices attached to it.
>>>>
>>> Right, I'll change them as you said.
>>>
>>>
>>>>>>> +{
>>>>>>> +    u16 ctrl;
>>>>>>> +
>>>>>>> +    if (!need_reset(dev))
>>>>>>> +        return;
>>>>>>> +
>>>>>>> +    dev_info(&dev->dev, "Reset Secondary bus\n");
>>>>>>> +
>>>>>>> +    /* Assert Secondary Bus Reset */
>>>>>>> +    pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>>>>>>> +    ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>>> +
>>>>>>> +    msleep(2);
>>>>>>> +
>>>> This works well for x86, which uses ioport registers to access
>>>> these<256-offset registers, b/c the write() function can't return
>>>> until the write is actually completed, but for a non-x86 system,
>>>> with fully mmconf'd PCI space, a write() may still be a write&  run
>>>> (sitting in CPU write-merge) buffer, so if you need a full 2ms,
>>>> you ought to do another read_config() to the device, to flush the write,
>>>> before starting the msleep(2) clock.
>>>>
>>> I didn't know that... I'll insert something like this before msleep.
>>>
>>> pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&dummy);
>>>
>>> BTW, I referred aer_do_secondary_bus_reset() when I wrote this code.
>>> Probably need the same fix, right?
>>>
>> sounds like it.
>> Interested to hear from someone in non-x86-land how
>> they ensure pci cfg writes aren't buffered in their cpus;
>> maybe handled via special uncached regions on other arches.
>>
>>>
>>>>>>> +    /* De-assert Secondary Bus Reset */
>>>>>>> +    ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>>>>>>> +    pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
>>>>>>> +}
>>>>>>> +
>>>>>>> +static int __initdata pcie_reset_devices;
>>>>>>> +static int __init reset_pcie_devices(void)
>>>> this should be reset_pcie_endpoints() ... it's not resetting all pcie devices
>>>>>>> +{
>>>>>>> +    struct pci_dev *dev = NULL;
>>>>>>> +
>>>>>>> +    if (!pcie_reset_devices)
>>>>>>> +        return 0;
>>>>>>> +
>>>>>>> +    for_each_pci_dev(dev)
>>>>>>> +        save_config(dev);
>>>>>>> +
>>>>>>> +    for_each_pci_dev(dev)
>>>>>>> +        do_device_reset(dev);
>>>>>>> +
>>>>>>> +    msleep(1000);
>>>>>>> +
>>>>>>> +    for_each_pci_dev(dev)
>>>>>>> +        restore_config(dev);
>>>>>>> +
>>>> My apologies if past thread covered this sequence...
>>>> Why three loops through all PCIe devices on the system?
>>>> why not have the first for-each-pci-dev() loop filter devices
>>>> to be reset, and save_config those pdev's,
>>>> return a list of saved_pdev's;  feed that list into the do_device_reset();
>>>> then mpsleep(), then restore the list.
>>>> in fact, once you create a link list of pdev's to reset,
>>>> just loop that list doing save, then reset; rtn the list, do msleep(),
>>>> then restore the config of pdevs in the list.
>>>> Otherwise, doing much more traversing than what's needed.
>>>> Doing a great deal more config-saving then needed right now as well
>>>> (saving all non-endpt devices that aren't reset).
>>>>
>>>
>>> Creating list is nice idea, thanks. I'll do.
>>>
>>> I'll have vacation next week, so I'll post v2 patch the week after
>>> next.
>>>
>>> Thanks,
>>> Takao Indoh
>>>
>> Have a good vacation! Thanks for your willingness
>> to respin again & make this improvement.
>>
>>
>> _______________________________________________
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>> kexec@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/kexec
>>
>>
> 
> 
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Patch

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4609e81..2a31ade 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2250,6 +2250,8 @@  bytes respectively. Such letter suffixes can also be entirely omitted.
 				any pair of devices, possibly at the cost of
 				reduced performance.  This also guarantees
 				that hot-added devices will work.
+		pcie_reset_devices	Reset PCIe endpoint on boot by hot
+				reset
 		cbiosize=nn[KMG]	The fixed amount of bus space which is
 				reserved for the CardBus bridge's IO window.
 				The default value is 256 bytes.
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b099e00..42385c9 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3878,6 +3878,107 @@  void __weak pci_fixup_cardbus(struct pci_bus *bus)
 }
 EXPORT_SYMBOL(pci_fixup_cardbus);
 
+/*
+ * Return true if dev is PCIe root port or downstream port whose child is PCIe
+ * endpoint except VGA device.
+ */
+static int __init need_reset(struct pci_dev *dev)
+{
+	struct pci_bus *subordinate;
+	struct pci_dev *child;
+
+	if (!pci_is_pcie(dev) || !dev->subordinate ||
+	    list_empty(&dev->subordinate->devices) ||
+	    ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT) &&
+	     (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM)))
+		return 0;
+
+	subordinate = dev->subordinate;
+	list_for_each_entry(child, &subordinate->devices, bus_list) {
+		if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
+		    (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE) ||
+		    ((child->class >> 16) == PCI_BASE_CLASS_DISPLAY))
+			/* Don't reset switch, bridge, VGA device */
+			return 0;
+	}
+
+	return 1;
+}
+
+static void __init save_config(struct pci_dev *dev)
+{
+	struct pci_bus *subordinate;
+	struct pci_dev *child;
+
+	if (!need_reset(dev))
+		return;
+
+	subordinate = dev->subordinate;
+	list_for_each_entry(child, &subordinate->devices, bus_list) {
+		dev_info(&child->dev, "save state\n");
+		pci_save_state(child);
+	}
+}
+
+static void __init restore_config(struct pci_dev *dev)
+{
+	struct pci_bus *subordinate;
+	struct pci_dev *child;
+
+	if (!need_reset(dev))
+		return;
+
+	subordinate = dev->subordinate;
+	list_for_each_entry(child, &subordinate->devices, bus_list) {
+		dev_info(&child->dev, "restore state\n");
+		pci_restore_state(child);
+	}
+}
+
+static void __init do_device_reset(struct pci_dev *dev)
+{
+	u16 ctrl;
+
+	if (!need_reset(dev))
+		return;
+
+	dev_info(&dev->dev, "Reset Secondary bus\n");
+
+	/* Assert Secondary Bus Reset */
+	pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &ctrl);
+	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
+	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
+
+	msleep(2);
+
+	/* De-assert Secondary Bus Reset */
+	ctrl &= ~PCI_BRIDGE_CTL_BUS_RESET;
+	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
+}
+
+static int __initdata pcie_reset_devices;
+static int __init reset_pcie_devices(void)
+{
+	struct pci_dev *dev = NULL;
+
+	if (!pcie_reset_devices)
+		return 0;
+
+	for_each_pci_dev(dev)
+		save_config(dev);
+
+	for_each_pci_dev(dev)
+		do_device_reset(dev);
+
+	msleep(1000);
+
+	for_each_pci_dev(dev)
+		restore_config(dev);
+
+	return 0;
+}
+fs_initcall_sync(reset_pcie_devices);
+
 static int __init pci_setup(char *str)
 {
 	while (str) {
@@ -3920,6 +4021,8 @@  static int __init pci_setup(char *str)
 				pcie_bus_config = PCIE_BUS_PEER2PEER;
 			} else if (!strncmp(str, "pcie_scan_all", 13)) {
 				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
+			} else if (!strncmp(str, "pcie_reset_devices", 18)) {
+				pcie_reset_devices = 1;
 			} else {
 				printk(KERN_ERR "PCI: Unknown option `%s'\n",
 						str);