[PATCH v9 19/32] drm: v3d: fix common struct sg_table related issues

Marek Szyprowski
2020-08-26T06:37:15Z

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@samsung.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
---
drivers/gpu/drm/v3d/v3d_mmu.c | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/v3d/v3d_mmu.c b/drivers/gpu/drm/v3d/v3d_mmu.c
index 3b81ea28c0bb..5a453532901f 100644
--- a/drivers/gpu/drm/v3d/v3d_mmu.c
+++ b/drivers/gpu/drm/v3d/v3d_mmu.c
@@ -90,18 +90,17 @@ 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) >=
+ BUG_ON(page_address + (PAGE_SIZE >> V3D_MMU_PAGE_SHIFT) >=
BIT(24));
-
- for (i = 0; i < sg_dma_len(sgl) >> V3D_MMU_PAGE_SHIFT; i++)
+ for (i = 0; i < PAGE_SIZE >> V3D_MMU_PAGE_SHIFT; i++)
v3d->pt[page++] = pte + i;
}

--
2.17.1