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 <[email protected]>
---
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/[email protected]/T/
---
drivers/gpu/drm/v3d/v3d_mmu.c | 17 +++++++----------
1 file changed, 7 insertions(+), 10 deletions(-)
diff --git a/drivers/gpu/drm/v3d/v3d_mmu.c b/drivers/gpu/drm/v3d/v3d_mmu.c
index 3b81ea2..175c257 100644
--- a/drivers/gpu/drm/v3d/v3d_mmu.c
+++ b/drivers/gpu/drm/v3d/v3d_mmu.c
@@ -90,19 +90,16 @@ void v3d_mmu_insert_ptes(struct v3d_bo *bo)
struct v3d_dev *v3d = to_v3d_dev(shmem_obj->base.dev);
u32 page = bo->node.start;
u32 page_prot = V3D_PTE_WRITEABLE | V3D_PTE_VALID;
- unsigned int count;
- struct scatterlist *sgl;
+ struct sg_dma_page_iter dma_iter;
- for_each_sg(shmem_obj->sgt->sgl, sgl, shmem_obj->sgt->nents, count) {
- u32 page_address = sg_dma_address(sgl) >> V3D_MMU_PAGE_SHIFT;
+ for_each_sgtable_dma_page(shmem_obj->sgt, &dma_iter, 0) {
+ dma_addr_t dma_addr = sg_page_iter_dma_address(&dma_iter);
+ u32 page_address = dma_addr >> V3D_MMU_PAGE_SHIFT;
u32 pte = page_prot | page_address;
- u32 i;
- BUG_ON(page_address + (sg_dma_len(sgl) >> V3D_MMU_PAGE_SHIFT) >=
- BIT(24));
-
- for (i = 0; i < sg_dma_len(sgl) >> V3D_MMU_PAGE_SHIFT; i++)
- v3d->pt[page++] = pte + i;
+ BUILD_BUG_ON(V3D_MMU_PAGE_SHIFT != PAGE_SIZE);
+ BUG_ON(page_address + 1 >= BIT(24));
+ v3d->pt[page++] = pte;
}
WARN_ON_ONCE(page - bo->node.start !=
--
1.9.1
On Wed, May 13, 2020 at 6:33 AM Marek Szyprowski
<[email protected]> wrote:
>
> 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 <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>