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()

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 <>
 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 
index 8005614d2e6b..bedd8937d8a1 100644
--- a/drivers/gpu/drm/armada/armada_gem.c
+++ b/drivers/gpu/drm/armada/armada_gem.c
@@ -395,7 +395,7 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment 
                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 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment 
                        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 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment 
                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)

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