[PATCH v2 03/21] drm: armada: fix sg_table nents vs. orig_nents misuse

[Marek Szyprowski]

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. Adapt the code to obey those rules.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
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/armada/armada_gem.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/armada/armada_gem.c b/drivers/gpu/drm/armada/armada_gem.c
index 976685f..749647f 100644
--- a/drivers/gpu/drm/armada/armada_gem.c
+++ b/drivers/gpu/drm/armada/armada_gem.c
@@ -407,8 +407,10 @@ 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;
+		sgt->nents = dma_map_sg(attach->dev, sgt->sgl, sgt->orig_nents,
+					dir);
+		if (sgt->nents == 0) {
+			num = sgt->orig_nents;
 			goto release;
 		}
 	} else if (dobj->page) {
@@ -418,7 +420,9 @@ 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)
+		sgt->nents = dma_map_sg(attach->dev, sgt->sgl, sgt->orig_nents,
+					dir);
+		if (sgt->nents == 0)
 			goto free_table;
 	} else if (dobj->linear) {
 		/* Single contiguous physical region - no struct page */
@@ -449,11 +453,11 @@ 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_sg(attach->dev, sgt->sgl, sgt->orig_nents, dir);
 
 	if (dobj->obj.filp) {
 		struct scatterlist *sg;
-		for_each_sg(sgt->sgl, sg, sgt->nents, i)
+		for_each_sg(sgt->sgl, sg, sgt->orig_nents, i)
 			put_page(sg_page(sg));
 	}
 
-- 
1.9.1