Re: [PATCH v9 08/32] drm: i915: fix common struct sg_table related issues

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On 2020-09-01 20:38, Ruhl, Michael J wrote:
-----Original Message-----
From: Intel-gfx <intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
Marek Szyprowski
Sent: Wednesday, August 26, 2020 2:33 AM
To: dri-devel@xxxxxxxxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx;
linaro-mm-sig@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>; David Airlie
<airlied@xxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx; Robin Murphy
<robin.murphy@xxxxxxx>; Christoph Hellwig <hch@xxxxxx>; linux-arm-
kernel@xxxxxxxxxxxxxxxxxxx; Marek Szyprowski
<m.szyprowski@xxxxxxxxxxx>
Subject:  [PATCH v9 08/32] drm: i915: fix common struct sg_table
related issues

The Documentation/DMA-API-HOWTO.txt states that the dma_map_sg()
function
returns the number of the created entries in the DMA address space.
However the subsequent calls to the dma_sync_sg_for_{device,cpu}() and
dma_unmap_sg must be called with the original number of the entries
passed to the dma_map_sg().

struct sg_table is a common structure used for describing a non-contiguous
memory buffer, used commonly in the DRM and graphics subsystems. It
consists of a scatterlist with memory pages and DMA addresses (sgl entry),
as well as the number of scatterlist entries: CPU pages (orig_nents entry)
and DMA mapped pages (nents entry).

It turned out that it was a common mistake to misuse nents and orig_nents
entries, calling DMA-mapping functions with a wrong number of entries or
ignoring the number of mapped entries returned by the dma_map_sg()
function.

This driver creatively uses sg_table->orig_nents to store the size of the
allocated scatterlist and ignores the number of the entries returned by
dma_map_sg function. The sg_table->orig_nents is (mis)used to properly
free the (over)allocated scatterlist.

This patch only introduces the common DMA-mapping wrappers operating
directly on the struct sg_table objects to the dmabuf related functions,
so the other drivers, which might share buffers with i915 could rely on
the properly set nents and orig_nents values.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c       | 11 +++--------
drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c |  7 +++----
2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
index 2679380159fc..8a988592715b 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
@@ -48,12 +48,9 @@ static struct sg_table *i915_gem_map_dma_buf(struct
dma_buf_attachment *attachme
		src = sg_next(src);
	}

-	if (!dma_map_sg_attrs(attachment->dev,
-			      st->sgl, st->nents, dir,
-			      DMA_ATTR_SKIP_CPU_SYNC)) {
-		ret = -ENOMEM;

You have dropped this error value.

Do you now if this is a benign loss?

True, dma_map_sgtable() will return -EINVAL rather than -ENOMEM for failure. A quick look through other .map_dma_buf callbacks suggests they're returning a motley mix of error values and NULL for failure cases, so I'd imagine that importers shouldn't be too sensitive to the exact value.

Robin.


M

+	ret = dma_map_sgtable(attachment->dev, st, dir,
DMA_ATTR_SKIP_CPU_SYNC);
+	if (ret)
		goto err_free_sg;
-	}

	return st;

@@ -73,9 +70,7 @@ static void i915_gem_unmap_dma_buf(struct
dma_buf_attachment *attachment,
{
	struct drm_i915_gem_object *obj = dma_buf_to_obj(attachment-
dmabuf);

-	dma_unmap_sg_attrs(attachment->dev,
-			   sg->sgl, sg->nents, dir,
-			   DMA_ATTR_SKIP_CPU_SYNC);
+	dma_unmap_sgtable(attachment->dev, sg, dir,
DMA_ATTR_SKIP_CPU_SYNC);
	sg_free_table(sg);
	kfree(sg);

diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
b/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
index debaf7b18ab5..be30b27e2926 100644
--- a/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
+++ b/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
@@ -28,10 +28,9 @@ static struct sg_table *mock_map_dma_buf(struct
dma_buf_attachment *attachment,
		sg = sg_next(sg);
	}

-	if (!dma_map_sg(attachment->dev, st->sgl, st->nents, dir)) {
-		err = -ENOMEM;
+	err = dma_map_sgtable(attachment->dev, st, dir, 0);
+	if (err)
		goto err_st;
-	}

	return st;

@@ -46,7 +45,7 @@ static void mock_unmap_dma_buf(struct
dma_buf_attachment *attachment,
			       struct sg_table *st,
			       enum dma_data_direction dir)
{
-	dma_unmap_sg(attachment->dev, st->sgl, st->nents, dir);
+	dma_unmap_sgtable(attachment->dev, st, dir, 0);
	sg_free_table(st);
	kfree(st);
}
--
2.17.1

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