RE: [Intel-gfx] [PATCH v9 08/32] drm: i915: fix common struct sg_table related issues

["Ruhl, Michael J" <michael.j.ruhl@intel.com>]

>-----Original Message-----
>From: Intel-gfx <intel-gfx-bounces@lists.freedesktop.org> On Behalf Of
>Marek Szyprowski
>Sent: Wednesday, August 26, 2020 2:33 AM
>To: dri-devel@lists.freedesktop.org; iommu@lists.linux-foundation.org;
>linaro-mm-sig@lists.linaro.org; linux-kernel@vger.kernel.org
>Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>; David Airlie
><airlied@linux.ie>; intel-gfx@lists.freedesktop.org; Robin Murphy
><robin.murphy@arm.com>; Christoph Hellwig <hch@lst.de>; linux-arm-
>kernel@lists.infradead.org; Marek Szyprowski
><m.szyprowski@samsung.com>
>Subject: [Intel-gfx] [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@samsung.com>
>---
> 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?

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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