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Subject[PATCH v10 08/30] 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()

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 <>
Reviewed-by: Robin Murphy <>
Reviewed-by: Michael J. Ruhl <>
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,
- ret = -ENOMEM;
+ 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_unmap_sgtable(attachment->dev, sg, dir, DMA_ATTR_SKIP_CPU_SYNC);

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);
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