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[PATCH] arm: dma: fix sharing of coherent DMA memory without struct page

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  [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page Shuah Khan <shuahkh@osg.samsung.com> - 2017-04-05 18:10 +0200
    Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without  struct page Russell King - ARM Linux <linux@armlinux.org.uk> - 2017-04-06 01:20 +0200
      Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without  struct page Shuah Khan <shuahkh@osg.samsung.com> - 2017-04-10 17:00 +0200
    Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without  struct page Marek Szyprowski <m.szyprowski@samsung.com> - 2017-04-06 14:10 +0200

#1617090 — [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page

When coherent DMA memory without struct page is shared, importer
fails to find the page and runs into kernel page fault when it
tries to dmabuf_ops_attach/map_sg/map_page the invalid page found
in the sg_table. Please see www.spinics.net/lists/stable/msg164204.html
for more information on this problem.

This solution allows coherent DMA memory without struct page to be
shared by providing a way for the exporter to tag the DMA buffer as
a special buffer without struct page association and passing the
information in sg_table to the importer. This information is used
in attach/map_sg to avoid cleaning D-cache and mapping.

The details of the change are:

Framework:
- Add a new dma_attrs field to struct scatterlist.
- Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify
  Coherent memory without struct page.
- Add a new dma_check_dev_coherent() interface to check if memory is
  the device coherent area. There is no way to tell where the memory
  returned by dma_alloc_attrs() came from.

Exporter logic:
- Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set
  DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is
  done in the exporter context.
- Add logic to arm_dma_get_sgtable() to identify memory without struct
  page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is
  set, arm_dma_get_sgtable() will set page as the cpu_addr and update
  dma_address and dma_attrs fields in struct scatterlist for this sgl.
  This is done in exporter context when buffer is exported. With this
  Note: This change is made on top of Russell King's patch that added
  !pfn_valid(pfn) check to arm_dma_get_sgtable() to error out on invalid
  pages. Coherent memory without struct page will trigger this error.

Importer logic:
- Add logic to vb2_dc_dmabuf_ops_attach() to identify memory without
  struct page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute when it copies
  the sg_table from the exporter. It will copy dma_attrs and dma_address
  fields. With this logic, dmabuf_ops_attach will no longer trip on an
  invalid page.
- Add logic to arm_dma_map_sg() to avoid mapping the page when sg_table
  has DMA_ATTR_DEV_COHERENT_NOPAGE buffer.
- Add logic to arm_dma_unmap_sg() to do nothing for sg entries with
  DMA_ATTR_DEV_COHERENT_NOPAGE attribute.

Without this change the following use-case that runs into kernel
pagefault when importer tries to attach the exported buffer.

With this change it works: (what a relief after watching pagefaults for
weeks!!)

gst-launch-1.0 filesrc location=~/GH3_MOV_HD.mp4 ! qtdemux ! h264parse ! v4l2video4dec capture-io-mode=dmabuf ! v4l2video7convert output-io-mode=dmabuf-import ! kmssink force-modesetting=true

I am sending RFC patch to get feedback on the approach and see if I missed
anything.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
---
 arch/arm/mm/dma-mapping.c                      | 34 ++++++++++++++++++++++----
 drivers/base/dma-coherent.c                    | 25 +++++++++++++++++++
 drivers/media/v4l2-core/videobuf2-dma-contig.c |  6 +++++
 include/linux/dma-mapping.h                    |  8 ++++++
 include/linux/scatterlist.h                    |  1 +
 5 files changed, 69 insertions(+), 5 deletions(-)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index 63eabb0..75c6692 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -939,13 +939,28 @@ int arm_dma_get_sgtable(struct device *dev, struct sg_table *sgt,
 		 void *cpu_addr, dma_addr_t handle, size_t size,
 		 unsigned long attrs)
 {
-	struct page *page = pfn_to_page(dma_to_pfn(dev, handle));
+	unsigned long pfn = dma_to_pfn(dev, handle);
+	struct page *page;
 	int ret;
 
+	/* If the PFN is not valid, we do not have a struct page. */
+	if (!pfn_valid(pfn)) {
+		/* If memory is from per-device coherent area, use cpu_addr */
+		if (attrs & DMA_ATTR_DEV_COHERENT_NOPAGE)
+			page = cpu_addr;
+		else
+			return -ENXIO;
+	} else
+		page = pfn_to_page(pfn);
+
 	ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
 	if (unlikely(ret))
 		return ret;
 
+	if (attrs & DMA_ATTR_DEV_COHERENT_NOPAGE)
+		sgt->sgl->dma_address = handle;
+
+	sgt->sgl->dma_attrs = attrs;
 	sg_set_page(sgt->sgl, page, PAGE_ALIGN(size), 0);
 	return 0;
 }
@@ -1080,10 +1095,17 @@ int arm_dma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
 #ifdef CONFIG_NEED_SG_DMA_LENGTH
 		s->dma_length = s->length;
 #endif
-		s->dma_address = ops->map_page(dev, sg_page(s), s->offset,
+		/*
+		 * there is no struct page for this DMA buffer.
+		 * s->dma_address is the handle
+		 */
+		if (!(s->dma_attrs & DMA_ATTR_DEV_COHERENT_NOPAGE)) {
+			s->dma_address = ops->map_page(dev, sg_page(s),
+						s->offset,
 						s->length, dir, attrs);
-		if (dma_mapping_error(dev, s->dma_address))
-			goto bad_mapping;
+			if (dma_mapping_error(dev, s->dma_address))
+				goto bad_mapping;
+		}
 	}
 	return nents;
 
@@ -1112,7 +1134,9 @@ void arm_dma_unmap_sg(struct device *dev, struct scatterlist *sg, int nents,
 	int i;
 
 	for_each_sg(sg, s, nents, i)
-		ops->unmap_page(dev, sg_dma_address(s), sg_dma_len(s), dir, attrs);
+		if (!(s->dma_attrs & DMA_ATTR_DEV_COHERENT_NOPAGE))
+			ops->unmap_page(dev, sg_dma_address(s),
+					sg_dma_len(s), dir, attrs);
 }
 
 /**
diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
index 640a7e6..d08cf44 100644
--- a/drivers/base/dma-coherent.c
+++ b/drivers/base/dma-coherent.c
@@ -209,6 +209,31 @@ int dma_alloc_from_coherent(struct device *dev, ssize_t size,
 EXPORT_SYMBOL(dma_alloc_from_coherent);
 
 /**
+ * dma_check_dev_coherent() - checks if memory is from the device coherent area
+ *
+ * @dev:	device whose coherent area is checked to validate memory
+ * @dma_handle:	dma handle associated with the allocated memory
+ * @vaddr:	the virtual address to the allocated area.
+ *
+ * Returns true if memory does belong to the per-device cohrent area.
+ * false otherwise.
+ */
+bool dma_check_dev_coherent(struct device *dev, dma_addr_t dma_handle,
+				void *vaddr)
+{
+	struct dma_coherent_mem *mem = dev ? dev->dma_mem : NULL;
+
+	if (mem && vaddr >= mem->virt_base &&
+	    vaddr < (mem->virt_base + (mem->size << PAGE_SHIFT)) &&
+	    dma_handle >= mem->device_base &&
+	    dma_handle < (mem->device_base + (mem->size << PAGE_SHIFT)))
+		return true;
+
+	return false;
+}
+EXPORT_SYMBOL(dma_check_dev_coherent);
+
+/**
  * dma_release_from_coherent() - try to free the memory allocated from per-device coherent memory pool
  * @dev:	device from which the memory was allocated
  * @order:	the order of pages allocated
diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c b/drivers/media/v4l2-core/videobuf2-dma-contig.c
index fb6a177..f7caf2b 100644
--- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
+++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
@@ -161,6 +161,9 @@ static void *vb2_dc_alloc(struct device *dev, unsigned long attrs,
 	if ((buf->attrs & DMA_ATTR_NO_KERNEL_MAPPING) == 0)
 		buf->vaddr = buf->cookie;
 
+	if (dma_check_dev_coherent(dev, buf->dma_addr, buf->cookie))
+		buf->attrs |= DMA_ATTR_DEV_COHERENT_NOPAGE;
+
 	/* Prevent the device from being released while the buffer is used */
 	buf->dev = get_device(dev);
 	buf->size = size;
@@ -248,6 +251,9 @@ static int vb2_dc_dmabuf_ops_attach(struct dma_buf *dbuf, struct device *dev,
 	rd = buf->sgt_base->sgl;
 	wr = sgt->sgl;
 	for (i = 0; i < sgt->orig_nents; ++i) {
+		if (rd->dma_attrs & DMA_ATTR_DEV_COHERENT_NOPAGE)
+			wr->dma_address = rd->dma_address;
+		wr->dma_attrs = rd->dma_attrs;
 		sg_set_page(wr, sg_page(rd), rd->length, rd->offset);
 		rd = sg_next(rd);
 		wr = sg_next(wr);
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 0977317..9f3ec53 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -70,6 +70,12 @@
 #define DMA_ATTR_PRIVILEGED		(1UL << 9)
 
 /*
+ * DMA_ATTR_DEV_COHERENT_NOPAGE: This is a hint to the DMA-mapping sub-system
+ * that this memory isn't backed by struct page.
+ */
+#define DMA_ATTR_DEV_COHERENT_NOPAGE	(1UL << 10)
+
+/*
  * A dma_addr_t can hold any valid DMA or bus address for the platform.
  * It can be given to a device to use as a DMA source or target.  A CPU cannot
  * reference a dma_addr_t directly because there may be translation between
@@ -160,6 +166,8 @@ static inline int is_device_dma_capable(struct device *dev)
  */
 int dma_alloc_from_coherent(struct device *dev, ssize_t size,
 				       dma_addr_t *dma_handle, void **ret);
+bool dma_check_dev_coherent(struct device *dev, dma_addr_t dma_handle,
+				void *vaddr);
 int dma_release_from_coherent(struct device *dev, int order, void *vaddr);
 
 int dma_mmap_from_coherent(struct device *dev, struct vm_area_struct *vma,
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index cb3c8fe..7da610b 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -18,6 +18,7 @@ struct scatterlist {
 #ifdef CONFIG_NEED_SG_DMA_LENGTH
 	unsigned int	dma_length;
 #endif
+	unsigned long	dma_attrs;
 };
 
 /*
-- 
2.7.4

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#1617419 — Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page

On Wed, Apr 05, 2017 at 10:02:42AM -0600, Shuah Khan wrote:
> When coherent DMA memory without struct page is shared, importer
> fails to find the page and runs into kernel page fault when it
> tries to dmabuf_ops_attach/map_sg/map_page the invalid page found
> in the sg_table. Please see www.spinics.net/lists/stable/msg164204.html
> for more information on this problem.
> 
> This solution allows coherent DMA memory without struct page to be
> shared by providing a way for the exporter to tag the DMA buffer as
> a special buffer without struct page association and passing the
> information in sg_table to the importer. This information is used
> in attach/map_sg to avoid cleaning D-cache and mapping.
> 
> The details of the change are:
> 
> Framework:
> - Add a new dma_attrs field to struct scatterlist.
> - Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify
>   Coherent memory without struct page.
> - Add a new dma_check_dev_coherent() interface to check if memory is
>   the device coherent area. There is no way to tell where the memory
>   returned by dma_alloc_attrs() came from.
> 
> Exporter logic:
> - Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set
>   DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is
>   done in the exporter context.
> - Add logic to arm_dma_get_sgtable() to identify memory without struct
>   page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is
>   set, arm_dma_get_sgtable() will set page as the cpu_addr and update
>   dma_address and dma_attrs fields in struct scatterlist for this sgl.
>   This is done in exporter context when buffer is exported. With this

This sentence appears to just end...

I'm not convinced that coherent allocations should be setting the "page"
of a scatterlist to anything that isn't a real struct page or NULL.  It
is, after all, an error to look up the virtual address etc of the
scatterlist entry or kmap it when it isn't backed by a struct page.

I'm actually already passing non-page backed memory through the DMA API
in armada-drm, although not entirely correctly, and etnaviv handles it
fine:

        } else if (dobj->linear) {
                /* Single contiguous physical region - no struct page */
                if (sg_alloc_table(sgt, 1, GFP_KERNEL))
                        goto free_sgt;
                sg_dma_address(sgt->sgl) = dobj->dev_addr;
                sg_dma_len(sgt->sgl) = dobj->obj.size;

This is not quite correct, as it assumes (which is safe for it currently)
that the DMA address is the same on all devices.  On Dove, which is where
this is used, that is the case, but it's not true elsewhere.  Also note
that I'm avoid calling dma_map_sg() and dma_unmap_sg() - there's no iommus
to be considered.

I'd suggest that this follows the same pattern - setting the DMA address
(more appropriately for generic code) and the DMA length, while leaving
the virtual address members NULL/0.  However, there's also the
complication of setting up any IOMMUs that would be necessary.  I haven't
looked at that, or how it could work.

I also think this should be documented in the dmabuf API that it can
pass such scatterlists that are DMA-parameter only.

Lastly, I'd recommend that anything using this does _not_ provide
functional kmap/kmap_atomic support for these - kmap and kmap_atomic
are both out because there's no struct page anyway (and their use would
probably oops the kernel in this scenario.)  I avoided mmap support in
armada drm, but if there's a pressing reason and real use case for the
importer to mmap() the buffers in userspace, it's something I could be
convinced of.

What I'm quite certain of is that we do _not_ want to be passing
coherent memory allocations into the streaming DMA API, not even with
a special attribute.  The DMA API is about gaining coherent memory
(shared ownership of the buffer), or mapping system memory to a
specified device (which can imply unique ownership.)  Trying to mix
the two together muddies the separation that we have there, and makes
it harder to explain.  As can be seen from this patch, we'd end up
needing to add this special DMA_ATTR_DEV_COHERENT_NOPAGE everywhere,
which is added complexity on top of stuff that is not required for
this circumstance.

I can see why you're doing it, to avoid having to duplicate more of
the generic code in drm_prime, but I don't think plasting over this
problem in arch code by adding this special flag is a particularly
good way forward.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

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#1619997 — Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page

On 04/05/2017 05:14 PM, Russell King - ARM Linux wrote:
> On Wed, Apr 05, 2017 at 10:02:42AM -0600, Shuah Khan wrote:
>> When coherent DMA memory without struct page is shared, importer
>> fails to find the page and runs into kernel page fault when it
>> tries to dmabuf_ops_attach/map_sg/map_page the invalid page found
>> in the sg_table. Please see www.spinics.net/lists/stable/msg164204.html
>> for more information on this problem.
>>
>> This solution allows coherent DMA memory without struct page to be
>> shared by providing a way for the exporter to tag the DMA buffer as
>> a special buffer without struct page association and passing the
>> information in sg_table to the importer. This information is used
>> in attach/map_sg to avoid cleaning D-cache and mapping.
>>
>> The details of the change are:
>>
>> Framework:
>> - Add a new dma_attrs field to struct scatterlist.
>> - Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify
>>   Coherent memory without struct page.
>> - Add a new dma_check_dev_coherent() interface to check if memory is
>>   the device coherent area. There is no way to tell where the memory
>>   returned by dma_alloc_attrs() came from.
>>
>> Exporter logic:
>> - Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set
>>   DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is
>>   done in the exporter context.
>> - Add logic to arm_dma_get_sgtable() to identify memory without struct
>>   page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is
>>   set, arm_dma_get_sgtable() will set page as the cpu_addr and update
>>   dma_address and dma_attrs fields in struct scatterlist for this sgl.
>>   This is done in exporter context when buffer is exported. With this
> 
> This sentence appears to just end...
> 
> I'm not convinced that coherent allocations should be setting the "page"
> of a scatterlist to anything that isn't a real struct page or NULL.  It
> is, after all, an error to look up the virtual address etc of the
> scatterlist entry or kmap it when it isn't backed by a struct page.
> 
> I'm actually already passing non-page backed memory through the DMA API
> in armada-drm, although not entirely correctly, and etnaviv handles it
> fine:
> 
>         } else if (dobj->linear) {
>                 /* Single contiguous physical region - no struct page */
>                 if (sg_alloc_table(sgt, 1, GFP_KERNEL))
>                         goto free_sgt;
>                 sg_dma_address(sgt->sgl) = dobj->dev_addr;
>                 sg_dma_len(sgt->sgl) = dobj->obj.size;
> 
> This is not quite correct, as it assumes (which is safe for it currently)
> that the DMA address is the same on all devices.  On Dove, which is where
> this is used, that is the case, but it's not true elsewhere.  Also note
> that I'm avoid calling dma_map_sg() and dma_unmap_sg() - there's no iommus
> to be considered.

I see. That is not the case for the drivers involved in my use-case. exynos
has iommu and this s5p-mfc exporting buffers to exynos-gsc use-case does
work when iommu is enabled.

> 
> I'd suggest that this follows the same pattern - setting the DMA address
> (more appropriately for generic code) and the DMA length, while leaving
> the virtual address members NULL/0.  However, there's also the
> complication of setting up any IOMMUs that would be necessary.  I haven't
> looked at that, or how it could work.
> 
> I also think this should be documented in the dmabuf API that it can
> pass such scatterlists that are DMA-parameter only.
> 
> Lastly, I'd recommend that anything using this does _not_ provide
> functional kmap/kmap_atomic support for these - kmap and kmap_atomic
> are both out because there's no struct page anyway (and their use would
> probably oops the kernel in this scenario.)  I avoided mmap support in
> armada drm, but if there's a pressing reason and real use case for the
> importer to mmap() the buffers in userspace, it's something I could be
> convinced of.
> 
> What I'm quite certain of is that we do _not_ want to be passing
> coherent memory allocations into the streaming DMA API, not even with
> a special attribute.  The DMA API is about gaining coherent memory
> (shared ownership of the buffer), or mapping system memory to a
> specified device (which can imply unique ownership.)  Trying to mix
> the two together muddies the separation that we have there, and makes
> it harder to explain.  As can be seen from this patch, we'd end up
> needing to add this special DMA_ATTR_DEV_COHERENT_NOPAGE everywhere,
> which is added complexity on top of stuff that is not required for
> this circumstance.

The ownership can be tricky as you mentioned. In this particular use-case,
there is a clear ownership definition because of the way v4l2 export/import
works and also the qbuf/dqbuf rules. However, there might be other use-cases
ownership isn't clearly established.

> 
> I can see why you're doing it, to avoid having to duplicate more of
> the generic code in drm_prime, but I don't think plasting over this
> problem in arch code by adding this special flag is a particularly
> good way forward.
> 

Right. I went with this approach to avoid duplicating the code. It does
come with the complexity of needing to check the attribute in a few
places.

With the current code, we still have the issue of pagefault. Your patch
that adds a check for invalid doesn't cover all cases.

My goal behind this patch is two fold. 1. Fix the pagefault with a
definitive test and 2. see if per-device coherent memory can be passed
through.

The first goal is still worth while. Would it be reasonable to use
dma_check_dev_coherent() to test for this case in arm_dma_get_sgtable()
or even from dma_get_sgtable_attrs() and fail early? This will avoid
false negatives with the invalid page test. If this sounds reasonable,
I can spin this work to do that instead.

thanks,
-- Shuah

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#1617909 — Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page

Hi Shuah,

On 2017-04-05 18:02, Shuah Khan wrote:
> When coherent DMA memory without struct page is shared, importer
> fails to find the page and runs into kernel page fault when it
> tries to dmabuf_ops_attach/map_sg/map_page the invalid page found
> in the sg_table. Please see www.spinics.net/lists/stable/msg164204.html
> for more information on this problem.
>
> This solution allows coherent DMA memory without struct page to be
> shared by providing a way for the exporter to tag the DMA buffer as
> a special buffer without struct page association and passing the
> information in sg_table to the importer. This information is used
> in attach/map_sg to avoid cleaning D-cache and mapping.
>
> The details of the change are:
>
> Framework:
> - Add a new dma_attrs field to struct scatterlist.
> - Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify
>    Coherent memory without struct page.
> - Add a new dma_check_dev_coherent() interface to check if memory is
>    the device coherent area. There is no way to tell where the memory
>    returned by dma_alloc_attrs() came from.
>
> Exporter logic:
> - Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set
>    DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is
>    done in the exporter context.
> - Add logic to arm_dma_get_sgtable() to identify memory without struct
>    page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is
>    set, arm_dma_get_sgtable() will set page as the cpu_addr and update
>    dma_address and dma_attrs fields in struct scatterlist for this sgl.
>    This is done in exporter context when buffer is exported. With this
>    Note: This change is made on top of Russell King's patch that added
>    !pfn_valid(pfn) check to arm_dma_get_sgtable() to error out on invalid
>    pages. Coherent memory without struct page will trigger this error.
>
> Importer logic:
> - Add logic to vb2_dc_dmabuf_ops_attach() to identify memory without
>    struct page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute when it copies
>    the sg_table from the exporter. It will copy dma_attrs and dma_address
>    fields. With this logic, dmabuf_ops_attach will no longer trip on an
>    invalid page.
> - Add logic to arm_dma_map_sg() to avoid mapping the page when sg_table
>    has DMA_ATTR_DEV_COHERENT_NOPAGE buffer.
> - Add logic to arm_dma_unmap_sg() to do nothing for sg entries with
>    DMA_ATTR_DEV_COHERENT_NOPAGE attribute.
>
> Without this change the following use-case that runs into kernel
> pagefault when importer tries to attach the exported buffer.
>
> With this change it works: (what a relief after watching pagefaults for
> weeks!!)
>
> gst-launch-1.0 filesrc location=~/GH3_MOV_HD.mp4 ! qtdemux ! h264parse ! v4l2video4dec capture-io-mode=dmabuf ! v4l2video7convert output-io-mode=dmabuf-import ! kmssink force-modesetting=true
>
> I am sending RFC patch to get feedback on the approach and see if I missed
> anything.

Frankly, once You decided to hack around dma-buf and issues with coherent,
carved out memory, it might be a bit better to find the ultimate solution
instead of the another hack. Please note that it will still not allow to
share a buffer allocated from carved-out memory and a device, which is
behind IOMMU.

I thought a bit about this and the current shape of dma-buf code.

IMHO the proper way of solving all those issues would be to replace
dma-buf internal representation of the memory from struct scatter_list
to pfn array. This would really solve the problem of buffers which
cannot be properly represented by scatter lists/struct pages and would
even allow sharing buffers between all kinds of devices. Scatter-lists
are also quite over-engineered structures to represent a single buffer
(pfn array is a bit more compact representation). Also there is a lots
of buggy code which use scatter-list in a bit creative way (like
assuming that each page maps to a single scatter list entry for
example). The only missing piece, required for such change would be
extending DMA-mapping with dma_map_pfn() interface.

This would be however quite large task, especially taking into account
all current users of DMA-buf framework...

> Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
> ---
>   arch/arm/mm/dma-mapping.c                      | 34 ++++++++++++++++++++++----
>   drivers/base/dma-coherent.c                    | 25 +++++++++++++++++++
>   drivers/media/v4l2-core/videobuf2-dma-contig.c |  6 +++++
>   include/linux/dma-mapping.h                    |  8 ++++++
>   include/linux/scatterlist.h                    |  1 +
>   5 files changed, 69 insertions(+), 5 deletions(-)
 > [...]

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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