[PATCH v10 04/30] drm: armada: fix common struct sg_table related issues

[Marek Szyprowski]

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 at samsung.com>
---
 drivers/gpu/drm/armada/armada_gem.c | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/armada/armada_gem.c b/drivers/gpu/drm/armada/armada_gem.c
index 8005614d2e6b..a63008ce284d 100644
--- a/drivers/gpu/drm/armada/armada_gem.c
+++ b/drivers/gpu/drm/armada/armada_gem.c
@@ -379,7 +379,7 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach,
 	struct armada_gem_object *dobj = drm_to_armada_gem(obj);
 	struct scatterlist *sg;
 	struct sg_table *sgt;
-	int i, num;
+	int i;
 
 	sgt = kmalloc(sizeof(*sgt), GFP_KERNEL);
 	if (!sgt)
@@ -395,22 +395,18 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach,
 
 		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);
-			if (IS_ERR(page)) {
-				num = i;
+			if (IS_ERR(page))
 				goto release;
-			}
 
 			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))
 			goto release;
-		}
 	} else if (dobj->page) {
 		/* Single contiguous page */
 		if (sg_alloc_table(sgt, 1, GFP_KERNEL))
@@ -418,7 +414,7 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach,
 
 		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 */
@@ -432,8 +428,9 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach,
 	return sgt;
 
  release:
-	for_each_sg(sgt->sgl, sg, num, i)
-		put_page(sg_page(sg));
+	for_each_sgtable_sg(sgt, sg, i)
+		if (sg_page(sg))
+			put_page(sg_page(sg));
  free_table:
 	sg_free_table(sgt);
  free_sgt:
@@ -449,11 +446,12 @@ 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));
 	}
 
-- 
2.17.1