The Documentation/DMA-API-HOWTO.txt states that dma_map_sg returns the numer of the created entries in the DMA address space. However the subsequent calls to dma_sync_sg_for_{device,cpu} and dma_unmap_sg must be called with the original number of entries passed to dma_map_sg. The sg_table->nents in turn holds the result of the dma_map_sg call as stated in include/linux/scatterlist.h.
This driver creatively uses sg_table->orig_nents to store the size of the allocate 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 fixes the sg_table->nents entries in the sg_table objects exported by the dmabuf related functions, so the other drivers, which might share buffers with i915 could rely on the nents and orig_nents values.
Signed-off-by: Marek Szyprowski m.szyprowski@samsung.com --- For more information, see '[PATCH v2 00/21] DRM: fix struct sg_table nents vs. orig_nents misuse' thread: https://lkml.org/lkml/2020/5/4/373 --- drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 9 +++++---- drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c | 5 +++-- 2 files changed, 8 insertions(+), 6 deletions(-)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c index 7db5a79..98159df 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c @@ -48,9 +48,10 @@ 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)) { + st->nents = dma_map_sg_attrs(attachment->dev, + st->sgl, st->orig_nents, dir, + DMA_ATTR_SKIP_CPU_SYNC); + if (!st->nents) { ret = -ENOMEM; goto err_free_sg; } @@ -74,7 +75,7 @@ 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, + sg->sgl, sg->orig_nents, 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 debaf7b..5723525 100644 --- a/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c +++ b/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c @@ -28,7 +28,8 @@ 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)) { + st->nents = dma_map_sg(attachment->dev, st->sgl, st->orig_nents, dir); + if (!st->nents) { err = -ENOMEM; goto err_st; } @@ -46,7 +47,7 @@ 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_sg(attachment->dev, st->sgl, st->orig_nents, dir); sg_free_table(st); kfree(st); }