diff mbox series

[09/15] device core: Add ability to handle multiple dma offsets

Message ID 20200519203419.12369-10-james.quinlan@broadcom.com
State Superseded
Headers show
Series PCI: brcmstb: enable PCIe for STB chips | expand

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Context Check Description
robh/checkpatch warning total: 0 errors, 2 warnings, 161 lines checked

Commit Message

Jim Quinlan May 19, 2020, 8:34 p.m. UTC
The device variable 'dma_pfn_offset' is used to do a single
linear map between cpu addrs and dma addrs.  The variable
'dma_map' is added to struct device to point to an array
of multiple offsets which is required for some devices.

Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
---
 drivers/of/address.c        | 50 ++++++++++++++++++++++++++++++++++---
 include/linux/device.h      |  9 ++++++-
 include/linux/dma-mapping.h | 44 ++++++++++++++++++++++++++++++++
 kernel/dma/Kconfig          | 12 +++++++++
 4 files changed, 111 insertions(+), 4 deletions(-)

Comments

Greg KH May 20, 2020, 5:43 a.m. UTC | #1
On Tue, May 19, 2020 at 04:34:07PM -0400, Jim Quinlan wrote:
> diff --git a/include/linux/device.h b/include/linux/device.h
> index ac8e37cd716a..6cd916860b5f 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -493,6 +493,8 @@ struct dev_links_info {
>   * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
>   *		DMA limit than the device itself supports.
>   * @dma_pfn_offset: offset of DMA memory range relatively of RAM
> + * @dma_map:	Like dma_pfn_offset but used when there are multiple
> + *		pfn offsets for multiple dma-ranges.
>   * @dma_parms:	A low level driver may set these to teach IOMMU code about
>   * 		segment limitations.
>   * @dma_pools:	Dma pools (if dma'ble device).
> @@ -578,7 +580,12 @@ struct device {
>  					     allocations such descriptors. */
>  	u64		bus_dma_limit;	/* upstream dma constraint */
>  	unsigned long	dma_pfn_offset;
> -
> +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> +	const void *dma_offset_map;	/* Like dma_pfn_offset, but for
> +					 * the unlikely case of multiple
> +					 * offsets. If non-null, dma_pfn_offset
> +					 * will be 0. */
> +#endif
>  	struct device_dma_parameters *dma_parms;
>  
>  	struct list_head	dma_pools;	/* dma pools (if dma'ble) */

I'll defer to Christoph here, but I thought we were trying to get rid of
stuff like this from struct device, not add new things to it for dma
apis.  And why is it a void *?

thanks,

greg k-h
Nicolas Saenz Julienne May 20, 2020, 11:28 a.m. UTC | #2
Hi Jim,
thanks for having a go at this! My two cents.

On Tue, 2020-05-19 at 16:34 -0400, Jim Quinlan wrote:
> The device variable 'dma_pfn_offset' is used to do a single
> linear map between cpu addrs and dma addrs.  The variable
> 'dma_map' is added to struct device to point to an array
> of multiple offsets which is required for some devices.
> 
> Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
> ---

[...]

> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -493,6 +493,8 @@ struct dev_links_info {
>   * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
>   *		DMA limit than the device itself supports.
>   * @dma_pfn_offset: offset of DMA memory range relatively of RAM
> + * @dma_map:	Like dma_pfn_offset but used when there are multiple
> + *		pfn offsets for multiple dma-ranges.
>   * @dma_parms:	A low level driver may set these to teach IOMMU code
> about
>   * 		segment limitations.
>   * @dma_pools:	Dma pools (if dma'ble device).
> @@ -578,7 +580,12 @@ struct device {
>  					     allocations such descriptors. */
>  	u64		bus_dma_limit;	/* upstream dma constraint */
>  	unsigned long	dma_pfn_offset;
> -
> +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> +	const void *dma_offset_map;	/* Like dma_pfn_offset, but for
> +					 * the unlikely case of multiple
> +					 * offsets. If non-null, dma_pfn_offset
> +					 * will be 0. */

I get a bad feeling about separating the DMA offset handling into two distinct
variables. Albeit generally frowned upon, there is a fair amount of trickery
around dev->dma_pfn_offset all over the kernel. usb_alloc_dev() comes to mind
for example. And this obviously doesn't play well with it. I feel a potential
solution to multiple DMA ranges should completely integrate with the current
device DMA handling code, without special cases, on top of that, be transparent
to the user.

In more concrete terms, I'd repackage dev->bus_dma_limit and
dev->dma_pfn_offset into a list/array of DMA range structures and adapt/create
the relevant getter/setter functions so as for DMA users not to have to worry
about the specifics of a device's DMA constraints. For example, instead of
editing dev->dma_pfn_offset, you'd be passing a DMA range structure to the
device core, and let it take the relevant decisions on how to handle it
internally (overwrite, add a new entry, merge them, etc...).

Easier said than done. :)

Regards,
Nicolas
Jim Quinlan May 20, 2020, 1:50 p.m. UTC | #3
On Wed, May 20, 2020 at 1:43 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Tue, May 19, 2020 at 04:34:07PM -0400, Jim Quinlan wrote:
> > diff --git a/include/linux/device.h b/include/linux/device.h
> > index ac8e37cd716a..6cd916860b5f 100644
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -493,6 +493,8 @@ struct dev_links_info {
> >   * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
> >   *           DMA limit than the device itself supports.
> >   * @dma_pfn_offset: offset of DMA memory range relatively of RAM
> > + * @dma_map: Like dma_pfn_offset but used when there are multiple
> > + *           pfn offsets for multiple dma-ranges.
> >   * @dma_parms:       A low level driver may set these to teach IOMMU code about
> >   *           segment limitations.
> >   * @dma_pools:       Dma pools (if dma'ble device).
> > @@ -578,7 +580,12 @@ struct device {
> >                                            allocations such descriptors. */
> >       u64             bus_dma_limit;  /* upstream dma constraint */
> >       unsigned long   dma_pfn_offset;
> > -
> > +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> > +     const void *dma_offset_map;     /* Like dma_pfn_offset, but for
> > +                                      * the unlikely case of multiple
> > +                                      * offsets. If non-null, dma_pfn_offset
> > +                                      * will be 0. */
> > +#endif
> >       struct device_dma_parameters *dma_parms;
> >
> >       struct list_head        dma_pools;      /* dma pools (if dma'ble) */
>
> I'll defer to Christoph here, but I thought we were trying to get rid of
> stuff like this from struct device, not add new things to it for dma
Hi Greg,

I wasn't aware of this policy.  I put it in 'struct device' because
just like the existing dma_pfn_offset; it seemed to be the only way to
pull this off.  I'll certainly follow any ideas on alternative
strategies from Christoph et al.

> apis.  And why is it a void *?
Just wanted to minimize the number of lines I've added to device.h, no
other reason.

Thanks,
Jim
>
> thanks,
>
> greg k-h
Greg KH May 20, 2020, 2:03 p.m. UTC | #4
On Wed, May 20, 2020 at 09:50:36AM -0400, Jim Quinlan wrote:
> On Wed, May 20, 2020 at 1:43 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Tue, May 19, 2020 at 04:34:07PM -0400, Jim Quinlan wrote:
> > > diff --git a/include/linux/device.h b/include/linux/device.h
> > > index ac8e37cd716a..6cd916860b5f 100644
> > > --- a/include/linux/device.h
> > > +++ b/include/linux/device.h
> > > @@ -493,6 +493,8 @@ struct dev_links_info {
> > >   * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
> > >   *           DMA limit than the device itself supports.
> > >   * @dma_pfn_offset: offset of DMA memory range relatively of RAM
> > > + * @dma_map: Like dma_pfn_offset but used when there are multiple
> > > + *           pfn offsets for multiple dma-ranges.
> > >   * @dma_parms:       A low level driver may set these to teach IOMMU code about
> > >   *           segment limitations.
> > >   * @dma_pools:       Dma pools (if dma'ble device).
> > > @@ -578,7 +580,12 @@ struct device {
> > >                                            allocations such descriptors. */
> > >       u64             bus_dma_limit;  /* upstream dma constraint */
> > >       unsigned long   dma_pfn_offset;
> > > -
> > > +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> > > +     const void *dma_offset_map;     /* Like dma_pfn_offset, but for
> > > +                                      * the unlikely case of multiple
> > > +                                      * offsets. If non-null, dma_pfn_offset
> > > +                                      * will be 0. */
> > > +#endif
> > >       struct device_dma_parameters *dma_parms;
> > >
> > >       struct list_head        dma_pools;      /* dma pools (if dma'ble) */
> >
> > I'll defer to Christoph here, but I thought we were trying to get rid of
> > stuff like this from struct device, not add new things to it for dma
> Hi Greg,
> 
> I wasn't aware of this policy.  I put it in 'struct device' because
> just like the existing dma_pfn_offset; it seemed to be the only way to
> pull this off.  I'll certainly follow any ideas on alternative
> strategies from Christoph et al.
> 
> > apis.  And why is it a void *?
> Just wanted to minimize the number of lines I've added to device.h, no
> other reason.

How would using a real type make this more lines?  Never use a void *
unless you have to, we want the compiler to check our errors for us :)

thanks,

greg k-h
Christoph Hellwig May 20, 2020, 5:42 p.m. UTC | #5
If you don't Cc me on the whole series I have absolutely no way to
review it.  Don't ever do these stupid partial Ccs as they cause nothing
but pain.
Jim Quinlan May 20, 2020, 6:26 p.m. UTC | #6
Sorry, I meant to put you on the to-list for all patches.   The last
time I sent out this many patches using a collective cc-list for all
patches I was told to reduce my cc-list.
Jim

On Wed, May 20, 2020 at 1:42 PM Christoph Hellwig <hch@lst.de> wrote:
>
> If you don't Cc me on the whole series I have absolutely no way to
> review it.  Don't ever do these stupid partial Ccs as they cause nothing
> but pain.
Dan Williams May 20, 2020, 10:36 p.m. UTC | #7
On Wed, May 20, 2020 at 11:27 AM Jim Quinlan <james.quinlan@broadcom.com> wrote:
>
> Sorry, I meant to put you on the to-list for all patches.   The last
> time I sent out this many patches using a collective cc-list for all
> patches I was told to reduce my cc-list.

You'd be forgiven. There are some developers that are ok to go read
the thread on something like lore if they are cc'd only a subset and
some that require the whole thread copied to them. Perhaps we need an
entry in MAINTAINERS that makes this preference discoverable? To date
I have been manually keeping track of those who want full threads and
those that would prefer to just be cc'd on the cover letter and the
one patch that directly affects their maintenance area.

Certainly blindly cc'ing everyone recommended by
scripts/get_maintainers.pl is overkill, but finding that subset is a
bit of an art.
Christoph Hellwig May 21, 2020, 8:19 a.m. UTC | #8
On Wed, May 20, 2020 at 03:36:16PM -0700, Dan Williams wrote:
> Certainly blindly cc'ing everyone recommended by
> scripts/get_maintainers.pl is overkill, but finding that subset is a
> bit of an art.

Yes.  But I'd rather be not Cced and just find the complete thread on
a list.  But all the lists I'm on and have managed to read through
yesterday didn't have the full series either.
Jim Quinlan May 22, 2020, 2:31 p.m. UTC | #9
Hi Nicolas,

On Wed, May 20, 2020 at 7:28 AM Nicolas Saenz Julienne
<nsaenzjulienne@suse.de> wrote:
>
> Hi Jim,
> thanks for having a go at this! My two cents.
>
> On Tue, 2020-05-19 at 16:34 -0400, Jim Quinlan wrote:
> > The device variable 'dma_pfn_offset' is used to do a single
> > linear map between cpu addrs and dma addrs.  The variable
> > 'dma_map' is added to struct device to point to an array
> > of multiple offsets which is required for some devices.
> >
> > Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
> > ---
>
> [...]
>
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -493,6 +493,8 @@ struct dev_links_info {
> >   * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
> >   *           DMA limit than the device itself supports.
> >   * @dma_pfn_offset: offset of DMA memory range relatively of RAM
> > + * @dma_map: Like dma_pfn_offset but used when there are multiple
> > + *           pfn offsets for multiple dma-ranges.
> >   * @dma_parms:       A low level driver may set these to teach IOMMU code
> > about
> >   *           segment limitations.
> >   * @dma_pools:       Dma pools (if dma'ble device).
> > @@ -578,7 +580,12 @@ struct device {
> >                                            allocations such descriptors. */
> >       u64             bus_dma_limit;  /* upstream dma constraint */
> >       unsigned long   dma_pfn_offset;
> > -
> > +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> > +     const void *dma_offset_map;     /* Like dma_pfn_offset, but for
> > +                                      * the unlikely case of multiple
> > +                                      * offsets. If non-null, dma_pfn_offset
> > +                                      * will be 0. */
>
> I get a bad feeling about separating the DMA offset handling into two distinct
> variables. Albeit generally frowned upon, there is a fair amount of trickery
> around dev->dma_pfn_offset all over the kernel. usb_alloc_dev() comes to mind
> for example. And this obviously doesn't play well with it.

The trickery should only be present when
CONFIG_DMA_PFN_OFFSET_MAP=y**.  Otherwise it does no harm.  Also, I
feel that if dev-dma_pfn_offset is valid then so is
dev->dma_pfn_offset_map -- they both use the same mechanism in the
same places.  I am merely
extending something that has been in Linux for a long time..

Further,  I could have had dma_pfn_offset_map  subsume dma_pfn_offset
but I wanted to leave it alone since folks would complain that it
would go from an addition to an if-clause and an inline function.  But
if I did go that way there would  only be one mechanism that would
cover both cases.

> I feel a potential
> solution to multiple DMA ranges should completely integrate with the current
> device DMA handling code, without special cases, on top of that, be transparent
> to the user.

Having dma_pfn_offset_map subsume  dma_pfn_offset would integrate the
current  code too.  And I am not sure what you mean by being
"transparent to the user" -- the writer of the PCIe endpoint driver is
going to do some DMA calls and they have no idea if this mechanism is
in play or not.

>
> In more concrete terms, I'd repackage dev->bus_dma_limit and
> dev->dma_pfn_offset into a list/array of DMA range structures

This is sort of what I am doing except I defined my own structure.
Using the of_range structure would require one to do the same extra
calculations over  and over for a DMA call; this is why I  defined my
structure that has all of the needed precomputed variables.

> and adapt/create
> the relevant getter/setter functions so as for DMA users not to have to worry
> about the specifics of a device's DMA constraints.

I'm not sure I understand where these getter/setter functions would
exist or what they would do.

> editing dev->dma_pfn_offset, you'd be passing a DMA range structure to the
> device core, and let it take the relevant decisions on how to handle it

How and where would the device core operate for these getter/setters?
In how many places in the code?  The way I see it, any solution has to
adjust the value when doing dma2phys and phys2dma conversions, and the
most efficient place to do that is in the two DMA header files (the
second one is for ARM).

> internally (overwrite, add a new entry, merge them, etc...).
I'm concerned that  this would be overkill; I am just trying to get a
driver upstream for some baroque PCIe RC HW I'm not sure if we should
set up something elaborate when the demand is not there.

I'll be posting a v2.  ChrisophH has sent me some personal feedback
which I am incorporating; so feel free to discuss your ideas with him
as well because I really want consensus on any large changes in
direction.

Thanks,
Jim

** CONFIG_DMA_OF_PFN_OFFSET_MAP=y only occurs when building for
ARCH_BRCMSTB.  However, ARCH_BRCMSTB is set by the ARM64 defconfig and
the ARM multi_v7_defconfig, so it would be activated for those
defconfigs.  This may(a)  get us kicked out of those defconfigs  or
(b) we may have to keep DMA_OF_PFN_OFFSET_MAP off in those defconfigs
and only turn it on solely for Broadcom STB Linux.

>
> Easier said than done. :)
>
> Regards,
> Nicolas
>
diff mbox series

Patch

diff --git a/drivers/of/address.c b/drivers/of/address.c
index 96d8cfb14a60..7dfff618af6a 100644
--- a/drivers/of/address.c
+++ b/drivers/of/address.c
@@ -947,6 +947,8 @@  int of_dma_get_range(struct device *dev, struct device_node *np, u64 *dma_addr,
 	struct of_range_parser parser;
 	struct of_range range;
 	u64 dma_start = U64_MAX, dma_end = 0, dma_offset = 0;
+	bool dma_multi_pfn_offset = false;
+	int num_ranges = 0;
 
 	while (node) {
 		ranges = of_get_property(node, "dma-ranges", &len);
@@ -977,10 +979,18 @@  int of_dma_get_range(struct device *dev, struct device_node *np, u64 *dma_addr,
 		pr_debug("dma_addr(%llx) cpu_addr(%llx) size(%llx)\n",
 			 range.bus_addr, range.cpu_addr, range.size);
 
+		num_ranges++;
 		if (dma_offset && range.cpu_addr - range.bus_addr != dma_offset) {
-			pr_warn("Can't handle multiple dma-ranges with different offsets on node(%pOF)\n", node);
-			/* Don't error out as we'd break some existing DTs */
-			continue;
+			dma_multi_pfn_offset = true;
+			if (!IS_ENABLED(CONFIG_DMA_PFN_OFFSET_MAP)) {
+				pr_warn("Can't handle multiple dma-ranges with different offsets on node(%pOF)\n", node);
+				/*
+				 * Don't error out as we'd break some
+				 * existing DTs that are using configs
+				 * w/o CONFIG_DMA_PFN_OFFSET_MAP set.
+				 */
+				continue;
+			}
 		}
 		dma_offset = range.cpu_addr - range.bus_addr;
 
@@ -991,6 +1001,40 @@  int of_dma_get_range(struct device *dev, struct device_node *np, u64 *dma_addr,
 			dma_end = range.bus_addr + range.size;
 	}
 
+#ifdef CONFIG_DMA_PFN_OFFSET_MAP
+	if (dma_multi_pfn_offset) {
+		size_t r_size = (num_ranges + 1)
+			* sizeof(struct dma_pfn_offset_region);
+		struct dma_pfn_offset_region *r;
+
+		if (!dev)
+			return -EINVAL;
+
+		dma_offset = 0;
+		r = devm_kcalloc(dev, 1, r_size, GFP_KERNEL);
+		if (!r)
+			return -ENOMEM;
+		dev->dma_offset_map = (const void *) r;
+		of_dma_range_parser_init(&parser, node);
+
+		/*
+		 * Record all info for DMA ranges array.  We could
+		 * just use the of_range struct, but if we did that it
+		 * would require more calculations for phys_to_dma and
+		 * dma_to_phys conversions.
+		 */
+		for_each_of_range(&parser, &range) {
+			r->cpu_beg = range.cpu_addr;
+			r->cpu_end = r->cpu_beg + range.size;
+			r->dma_beg = range.bus_addr;
+			r->dma_end = r->dma_beg + range.size;
+			r->pfn_offset = PFN_DOWN(range.cpu_addr)
+				- PFN_DOWN(range.bus_addr);
+			r++;
+		}
+	}
+#endif
+
 	if (dma_start >= dma_end) {
 		ret = -EINVAL;
 		pr_debug("Invalid DMA ranges configuration on node(%pOF)\n",
diff --git a/include/linux/device.h b/include/linux/device.h
index ac8e37cd716a..6cd916860b5f 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -493,6 +493,8 @@  struct dev_links_info {
  * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
  *		DMA limit than the device itself supports.
  * @dma_pfn_offset: offset of DMA memory range relatively of RAM
+ * @dma_map:	Like dma_pfn_offset but used when there are multiple
+ *		pfn offsets for multiple dma-ranges.
  * @dma_parms:	A low level driver may set these to teach IOMMU code about
  * 		segment limitations.
  * @dma_pools:	Dma pools (if dma'ble device).
@@ -578,7 +580,12 @@  struct device {
 					     allocations such descriptors. */
 	u64		bus_dma_limit;	/* upstream dma constraint */
 	unsigned long	dma_pfn_offset;
-
+#ifdef CONFIG_DMA_PFN_OFFSET_MAP
+	const void *dma_offset_map;	/* Like dma_pfn_offset, but for
+					 * the unlikely case of multiple
+					 * offsets. If non-null, dma_pfn_offset
+					 * will be 0. */
+#endif
 	struct device_dma_parameters *dma_parms;
 
 	struct list_head	dma_pools;	/* dma pools (if dma'ble) */
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 330ad58fbf4d..288d8089faf7 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -256,6 +256,38 @@  static inline void dma_direct_sync_sg_for_cpu(struct device *dev,
 size_t dma_direct_max_mapping_size(struct device *dev);
 
 #ifdef CONFIG_HAS_DMA
+#ifdef CONFIG_DMA_PFN_OFFSET_MAP
+struct dma_pfn_offset_region {
+	phys_addr_t	cpu_beg;
+	phys_addr_t	cpu_end;
+	dma_addr_t	dma_beg;
+	dma_addr_t	dma_end;
+	unsigned long	pfn_offset;
+};
+
+static inline unsigned long dma_pfn_offset_frm_dma_addr(const void *arr,
+							dma_addr_t dma_addr)
+{
+	const struct dma_pfn_offset_region *m;
+
+	for (m = arr; m->cpu_end; m++)
+		if (dma_addr >= m->dma_beg && dma_addr < m->dma_end)
+			return m->pfn_offset;
+	return 0;
+}
+
+static inline unsigned long dma_pfn_offset_frm_phys_addr(const void *arr,
+							 phys_addr_t paddr)
+{
+	const struct dma_pfn_offset_region *m;
+
+	for (m = arr; m->cpu_end; m++)
+		if (paddr >= m->cpu_beg && paddr < m->cpu_end)
+			return m->pfn_offset;
+	return 0;
+}
+#endif /* CONFIG_DMA_PFN_OFFSET_MAP */
+
 #include <asm/dma-mapping.h>
 
 static inline const struct dma_map_ops *get_dma_ops(struct device *dev)
@@ -575,6 +607,18 @@  static inline unsigned long dma_get_merge_boundary(struct device *dev)
 {
 	return 0;
 }
+#ifdef CONFIG_DMA_PFN_OFFSET_MAP
+static inline unsigned long dma_pfn_offset_frm_dma_addr(const void *arr,
+							dma_addr_t dma_addr)
+{
+	return 0;
+}
+static inline unsigned long dma_pfn_offset_frm_phys_addr(const void *arr,
+							 phys_addr_t paddr)
+{
+	return 0;
+}
+#endif /* CONFIG_DMA_PFN_OFFSET_MAP */
 #endif /* CONFIG_HAS_DMA */
 
 static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
index 4c103a24e380..b4c3de521bf9 100644
--- a/kernel/dma/Kconfig
+++ b/kernel/dma/Kconfig
@@ -195,3 +195,15 @@  config DMA_API_DEBUG_SG
 	  is technically out-of-spec.
 
 	  If unsure, say N.
+
+config DMA_PFN_OFFSET_MAP
+	bool "Uses a DMA range map to calculate PFN offset"
+	default n
+	help
+	  Some devices have a dma-range that gets converted to
+	  a dev->dma_pfn_offset value.  This option is for the
+	  atypical case of there being multiple dma-ranges requiring
+	  multiple pfn offsets, which are selected from when
+	  converting to phys to dma and vice versa.
+
+	  If unsure, say N.