Message ID | 20140614212126.15202.66060.stgit@bhelgaas-glaptop.roam.corp.google.com |
---|---|
State | Accepted |
Headers | show |
Sorry, I'm digging up and old patch here and this RFC was the only copy I could find in my mailbox. On Sat, 2014-06-14 at 15:21 -0600, Bjorn Helgaas wrote: > Previously we issued a hotplug command and waited for it to complete. But > there's no need to wait until we're ready to issue the *next* command. The > next command will probably be much later, so the first one may have already > completed and we may not have to actually wait at all. I'm seeing a regression as a result of this patch. Consider the following function: int pciehp_reset_slot(struct slot *slot, int probe) { ... pcie_write_cmd(ctrl, 0, ctrl_mask); ... pci_reset_bridge_secondary_bus(ctrl->pcie->port); Our command write is clearing bits that control whether the slot has presence detection and link layer state change enabled. These are the things that we're trying to clear to prevent a secondary bus reset from turning into a surprise hotplug. If we don't wait for the command to complete, what state are when in as we initiate the secondary bus reset? On my system, it's like we never issued the write command at all, the previous behavior where the hotplug controller detects the link down as a surprise hotplug returns. So I'm afraid we do need to wait until we're ready to issue the *next* command, because it's our only indication that the current command has completed. I thought maybe we just need a separate flush command for cases like this, but then I started looking at the cases where we use this function: pciehp_set_attention_status pciehp_green_led_on pciehp_green_led_off pciehp_green_led_blink pciehp_power_on_slot pciehp_power_off_slot pcie_enable_notification pcie_disable_notification The slot power ones concern me the same as the reset issue. In the 'on' case, we immediately call link enable, so the slot better have acknowledged the power on command. In the 'off' case, it seems pretty bad to complete the 'off' function, but we don't know if the slot is actually off yet. For notifications, I can imagine that the caller of those functions is going to want on or off at the completion of those functions, not some in-between state. Even in the case of the LED functions, where do we want the error to occur, at the time the command is issued, or maybe some time in the future when the next unrelated command comes through. So, rather than posting a new patch adding a flush everywhere that it seems a little scary, should we consider scrapping this patch altogether as unsafe? We need to wait, not only to issue the next command, but to have any sort of acknowledgment that the current command has completed. > > Because of hardware errata, some controllers generate command completion > events for some commands but not others. In the case of Intel CF118 (see > spec update reference), the controller indicates command completion only > for Slot Control writes that change the value of the following bits: > > Power Controller Control > Power Indicator Control > Attention Indicator Control > Electromechanical Interlock Control > > Changes to other bits, e.g., the interrupt enable bits, do not cause the > Command Completed bit to be set. Controllers from AMD and Nvidia are > reported to have similar errata. > > These errata cause timeouts when pcie_enable_notification() enables > interrupts. Previously that timeout occurred at boot-time. With this > change, the timeout occurs later, when we change the state of the slot > power, indicators, or interlock. This speeds up boot but causes a timeout > at the first hotplug event on the slot. Subsequent events don't timeout > because only the first (boot-time) hotplug command updates Slot Control > without touching the power/indicator/interlock controls. It sounds like we need some sort of completion mask to handle devices like this, instead of effectively removing the write-barrier for the general case. Thanks, Alex > > Link: http://www.intel.com/content/www/us/en/processors/xeon/xeon-e7-v2-spec-update.html > Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> > --- > drivers/pci/hotplug/pciehp_hpc.c | 7 +++---- > 1 file changed, 3 insertions(+), 4 deletions(-) > > diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c > index 0e76e9d9d134..f44fdb5b0b08 100644 > --- a/drivers/pci/hotplug/pciehp_hpc.c > +++ b/drivers/pci/hotplug/pciehp_hpc.c > @@ -165,6 +165,9 @@ static void pcie_write_cmd(struct controller *ctrl, u16 cmd, u16 mask) > > mutex_lock(&ctrl->ctrl_lock); > > + /* Wait for any previous command that might still be in progress */ > + pcie_wait_cmd(ctrl); > + > pcie_capability_read_word(pdev, PCI_EXP_SLTSTA, &slot_status); > if (slot_status & PCI_EXP_SLTSTA_CC) { > pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, > @@ -197,10 +200,6 @@ static void pcie_write_cmd(struct controller *ctrl, u16 cmd, u16 mask) > pcie_capability_write_word(pdev, PCI_EXP_SLTCTL, slot_ctrl); > ctrl->slot_ctrl = slot_ctrl; > > - /* > - * Wait for command completion. > - */ > - pcie_wait_cmd(ctrl); > mutex_unlock(&ctrl->ctrl_lock); > } > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-pci" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, May 29, 2015 at 04:45:34PM -0600, Alex Williamson wrote: > > Sorry, I'm digging up and old patch here and this RFC was the only copy > I could find in my mailbox. > > On Sat, 2014-06-14 at 15:21 -0600, Bjorn Helgaas wrote: > > Previously we issued a hotplug command and waited for it to complete. But > > there's no need to wait until we're ready to issue the *next* command. The > > next command will probably be much later, so the first one may have already > > completed and we may not have to actually wait at all. > > I'm seeing a regression as a result of this patch. Consider the > following function: > > int pciehp_reset_slot(struct slot *slot, int probe) > { > ... > pcie_write_cmd(ctrl, 0, ctrl_mask); > ... > pci_reset_bridge_secondary_bus(ctrl->pcie->port); > > Our command write is clearing bits that control whether the slot has > presence detection and link layer state change enabled. These are the > things that we're trying to clear to prevent a secondary bus reset from > turning into a surprise hotplug. If we don't wait for the command to > complete, what state are when in as we initiate the secondary bus reset? > On my system, it's like we never issued the write command at all, the > previous behavior where the hotplug controller detects the link down as > a surprise hotplug returns. So I'm afraid we do need to wait until > we're ready to issue the *next* command, because it's our only > indication that the current command has completed. I think that's a bug. If we do something that depends on a command completion, we'll have to wait for it. > pciehp_set_attention_status > pciehp_green_led_on > pciehp_green_led_off > pciehp_green_led_blink > pciehp_power_on_slot > pciehp_power_off_slot > pcie_enable_notification > pcie_disable_notification > > The slot power ones concern me the same as the reset issue. In the 'on' > case, we immediately call link enable, so the slot better have > acknowledged the power on command. In the 'off' case, it seems pretty > bad to complete the 'off' function, but we don't know if the slot is > actually off yet. I agree. > For notifications, I can imagine that the caller of > those functions is going to want on or off at the completion of those > functions, not some in-between state. These control interrupt generation by the hotplug controller in the switch. When disabling interrupt generation, I think we should wait for completion before we call free_irq(); waiting should prevent spurious interrupts. When enabling interrupt generation, we've already hooked up the ISR before we enable interrupt generation, so I don't see a need to wait. > Even in the case of the LED > functions, where do we want the error to occur, at the time the command > is issued, or maybe some time in the future when the next unrelated > command comes through. The only error here is the "Timeout on hotplug command" message, and we don't do anything other than print the message, so I'm not too worried about this one. > So, rather than posting a new patch adding a flush everywhere that it > seems a little scary, should we consider scrapping this patch altogether > as unsafe? We need to wait, not only to issue the next command, but to > have any sort of acknowledgment that the current command has completed. I definitely screwed up by assuming that completion was only useful for pacing writes to the command register. I think lazy checking is fine for command pacing, but we certainly need to know about completions for other things, too. If we added calls to pcie_wait_cmd() in these places: pciehp_power_on_slot pciehp_power_off_slot pcie_disable_notification pciehp_reset_slot do you think that would be enough? > > Because of hardware errata, some controllers generate command completion > > events for some commands but not others. In the case of Intel CF118 (see > > spec update reference), the controller indicates command completion only > > for Slot Control writes that change the value of the following bits: > > > > Power Controller Control > > Power Indicator Control > > Attention Indicator Control > > Electromechanical Interlock Control > > > > Changes to other bits, e.g., the interrupt enable bits, do not cause the > > Command Completed bit to be set. Controllers from AMD and Nvidia are > > reported to have similar errata. > > > > These errata cause timeouts when pcie_enable_notification() enables > > interrupts. Previously that timeout occurred at boot-time. With this > > change, the timeout occurs later, when we change the state of the slot > > power, indicators, or interlock. This speeds up boot but causes a timeout > > at the first hotplug event on the slot. Subsequent events don't timeout > > because only the first (boot-time) hotplug command updates Slot Control > > without touching the power/indicator/interlock controls. > > It sounds like we need some sort of completion mask to handle devices > like this, instead of effectively removing the write-barrier for the > general case. Thanks, You mean some quirk-based solution, where we know certain devices don't indicate command completion for certain events? That seems hard because I think there are many devices that have this erratum. It was difficult to extract this out of Intel, and I'm pretty sure other vendors have the same issue. Bjorn -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, 2015-06-01 at 16:43 -0500, Bjorn Helgaas wrote: > On Fri, May 29, 2015 at 04:45:34PM -0600, Alex Williamson wrote: > > > > Sorry, I'm digging up and old patch here and this RFC was the only copy > > I could find in my mailbox. > > > > On Sat, 2014-06-14 at 15:21 -0600, Bjorn Helgaas wrote: > > > Previously we issued a hotplug command and waited for it to complete. But > > > there's no need to wait until we're ready to issue the *next* command. The > > > next command will probably be much later, so the first one may have already > > > completed and we may not have to actually wait at all. > > > > I'm seeing a regression as a result of this patch. Consider the > > following function: > > > > int pciehp_reset_slot(struct slot *slot, int probe) > > { > > ... > > pcie_write_cmd(ctrl, 0, ctrl_mask); > > ... > > pci_reset_bridge_secondary_bus(ctrl->pcie->port); > > > > Our command write is clearing bits that control whether the slot has > > presence detection and link layer state change enabled. These are the > > things that we're trying to clear to prevent a secondary bus reset from > > turning into a surprise hotplug. If we don't wait for the command to > > complete, what state are when in as we initiate the secondary bus reset? > > On my system, it's like we never issued the write command at all, the > > previous behavior where the hotplug controller detects the link down as > > a surprise hotplug returns. So I'm afraid we do need to wait until > > we're ready to issue the *next* command, because it's our only > > indication that the current command has completed. > > I think that's a bug. If we do something that depends on a command > completion, we'll have to wait for it. > > > pciehp_set_attention_status > > pciehp_green_led_on > > pciehp_green_led_off > > pciehp_green_led_blink > > pciehp_power_on_slot > > pciehp_power_off_slot > > pcie_enable_notification > > pcie_disable_notification > > > > The slot power ones concern me the same as the reset issue. In the 'on' > > case, we immediately call link enable, so the slot better have > > acknowledged the power on command. In the 'off' case, it seems pretty > > bad to complete the 'off' function, but we don't know if the slot is > > actually off yet. > > I agree. > > > For notifications, I can imagine that the caller of > > those functions is going to want on or off at the completion of those > > functions, not some in-between state. > > These control interrupt generation by the hotplug controller in the switch. > When disabling interrupt generation, I think we should wait for completion > before we call free_irq(); waiting should prevent spurious interrupts. > When enabling interrupt generation, we've already hooked up the ISR before > we enable interrupt generation, so I don't see a need to wait. > > > Even in the case of the LED > > functions, where do we want the error to occur, at the time the command > > is issued, or maybe some time in the future when the next unrelated > > command comes through. > > The only error here is the "Timeout on hotplug command" message, and we > don't do anything other than print the message, so I'm not too worried > about this one. > > > So, rather than posting a new patch adding a flush everywhere that it > > seems a little scary, should we consider scrapping this patch altogether > > as unsafe? We need to wait, not only to issue the next command, but to > > have any sort of acknowledgment that the current command has completed. > > I definitely screwed up by assuming that completion was only useful for > pacing writes to the command register. I think lazy checking is fine for > command pacing, but we certainly need to know about completions for other > things, too. > > If we added calls to pcie_wait_cmd() in these places: > > pciehp_power_on_slot > pciehp_power_off_slot > pcie_disable_notification > pciehp_reset_slot > > do you think that would be enough? I think that would solve the problem, but the API becomes very difficult to use correctly if the programmer just needs to know that a pcie_wait_cmd() is necessary following a pcie_write_cmd() if you really want to be sure it's synchronous. pcie_write_cmd() should probably incorporate the "safe" behavior and some new _nowait version should handle the special cases where we don't need to wait > > > Because of hardware errata, some controllers generate command completion > > > events for some commands but not others. In the case of Intel CF118 (see > > > spec update reference), the controller indicates command completion only > > > for Slot Control writes that change the value of the following bits: > > > > > > Power Controller Control > > > Power Indicator Control > > > Attention Indicator Control > > > Electromechanical Interlock Control > > > > > > Changes to other bits, e.g., the interrupt enable bits, do not cause the > > > Command Completed bit to be set. Controllers from AMD and Nvidia are > > > reported to have similar errata. > > > > > > These errata cause timeouts when pcie_enable_notification() enables > > > interrupts. Previously that timeout occurred at boot-time. With this > > > change, the timeout occurs later, when we change the state of the slot > > > power, indicators, or interlock. This speeds up boot but causes a timeout > > > at the first hotplug event on the slot. Subsequent events don't timeout > > > because only the first (boot-time) hotplug command updates Slot Control > > > without touching the power/indicator/interlock controls. > > > > It sounds like we need some sort of completion mask to handle devices > > like this, instead of effectively removing the write-barrier for the > > general case. Thanks, > > You mean some quirk-based solution, where we know certain devices don't > indicate command completion for certain events? That seems hard because I > think there are many devices that have this erratum. It was difficult to > extract this out of Intel, and I'm pretty sure other vendors have the same > issue. Yeah, I was thinking of a bitmap where we could quirk non-completion of devices, but maybe it's not worth it. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, Jun 01, 2015 at 04:02:59PM -0600, Alex Williamson wrote: > On Mon, 2015-06-01 at 16:43 -0500, Bjorn Helgaas wrote: > > If we added calls to pcie_wait_cmd() in these places: > > > > pciehp_power_on_slot > > pciehp_power_off_slot > > pcie_disable_notification > > pciehp_reset_slot > > > > do you think that would be enough? > > I think that would solve the problem, but the API becomes very difficult > to use correctly if the programmer just needs to know that a > pcie_wait_cmd() is necessary following a pcie_write_cmd() if you really > want to be sure it's synchronous. pcie_write_cmd() should probably > incorporate the "safe" behavior and some new _nowait version should > handle the special cases where we don't need to wait I don't think of pcie_write_cmd() as an API, since it's static to pciehp_hpc.c, but I do like your idea. Bjorn -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c index 0e76e9d9d134..f44fdb5b0b08 100644 --- a/drivers/pci/hotplug/pciehp_hpc.c +++ b/drivers/pci/hotplug/pciehp_hpc.c @@ -165,6 +165,9 @@ static void pcie_write_cmd(struct controller *ctrl, u16 cmd, u16 mask) mutex_lock(&ctrl->ctrl_lock); + /* Wait for any previous command that might still be in progress */ + pcie_wait_cmd(ctrl); + pcie_capability_read_word(pdev, PCI_EXP_SLTSTA, &slot_status); if (slot_status & PCI_EXP_SLTSTA_CC) { pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, @@ -197,10 +200,6 @@ static void pcie_write_cmd(struct controller *ctrl, u16 cmd, u16 mask) pcie_capability_write_word(pdev, PCI_EXP_SLTCTL, slot_ctrl); ctrl->slot_ctrl = slot_ctrl; - /* - * Wait for command completion. - */ - pcie_wait_cmd(ctrl); mutex_unlock(&ctrl->ctrl_lock); }
Previously we issued a hotplug command and waited for it to complete. But there's no need to wait until we're ready to issue the *next* command. The next command will probably be much later, so the first one may have already completed and we may not have to actually wait at all. Because of hardware errata, some controllers generate command completion events for some commands but not others. In the case of Intel CF118 (see spec update reference), the controller indicates command completion only for Slot Control writes that change the value of the following bits: Power Controller Control Power Indicator Control Attention Indicator Control Electromechanical Interlock Control Changes to other bits, e.g., the interrupt enable bits, do not cause the Command Completed bit to be set. Controllers from AMD and Nvidia are reported to have similar errata. These errata cause timeouts when pcie_enable_notification() enables interrupts. Previously that timeout occurred at boot-time. With this change, the timeout occurs later, when we change the state of the slot power, indicators, or interlock. This speeds up boot but causes a timeout at the first hotplug event on the slot. Subsequent events don't timeout because only the first (boot-time) hotplug command updates Slot Control without touching the power/indicator/interlock controls. Link: http://www.intel.com/content/www/us/en/processors/xeon/xeon-e7-v2-spec-update.html Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> --- drivers/pci/hotplug/pciehp_hpc.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html