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.
To avoid such issues, lets use a common dma-mapping wrappers operating
directly on the struct sg_table objects and use scatterlist page
iterators where possible. This, almost always, hides references to the
nents and orig_nents entries, making the code robust, easier to follow
and copy/paste safe.
Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
For more information, see '[PATCH v5 00/38] DRM: fix struct sg_table nents
vs. orig_nents misuse' thread:
https://lore.kernel.org/linux-iommu/20200513132114.6046-1-m.szyprowski@xxxxxxxxxxx/T/
---
drivers/gpu/drm/armada/armada_gem.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/drivers/gpu/drm/armada/armada_gem.c b/drivers/gpu/drm/armada/armada_gem.c
index 976685f..0d61da5 100644
--- a/drivers/gpu/drm/armada/armada_gem.c
+++ b/drivers/gpu/drm/armada/armada_gem.c
@@ -395,7 +395,7 @@ int armada_gem_pwrite_ioctl(struct drm_device *dev, void *data,
mapping = dobj->obj.filp->f_mapping;
- for_each_sg(sgt->sgl, sg, count, i) {
+ for_each_sgtable_sg(sgt, sg, i) {
struct page *page;
page = shmem_read_mapping_page(mapping, i);
@@ -407,8 +407,8 @@ int armada_gem_pwrite_ioctl(struct drm_device *dev, void *data,
sg_set_page(sg, page, PAGE_SIZE, 0);
}
- if (dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir) == 0) {
- num = sgt->nents;
+ if (dma_map_sgtable(attach->dev, sgt, dir, 0)) {
+ num = count;
goto release;
}
} else if (dobj->page) {
@@ -418,7 +418,7 @@ int armada_gem_pwrite_ioctl(struct drm_device *dev, void *data,
sg_set_page(sgt->sgl, dobj->page, dobj->obj.size, 0);
- if (dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir) == 0)
+ if (dma_map_sgtable(attach->dev, sgt, dir, 0))
goto free_table;
} else if (dobj->linear) {
/* Single contiguous physical region - no struct page */
@@ -449,11 +449,11 @@ static void armada_gem_prime_unmap_dma_buf(struct dma_buf_attachment *attach,
int i;
if (!dobj->linear)
- dma_unmap_sg(attach->dev, sgt->sgl, sgt->nents, dir);
+ dma_unmap_sgtable(attach->dev, sgt, dir, 0);
if (dobj->obj.filp) {
struct scatterlist *sg;
- for_each_sg(sgt->sgl, sg, sgt->nents, i)
+ for_each_sgtable_sg(sgt, sg, i)
put_page(sg_page(sg));
}
--
1.9.1
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