drm: radeon: fix common struct sg_table related issues

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: Christian König <[email protected]>
Link: https://patchwork.freedesktop.org/patch/371172/
Signed-off-by: Christian König <[email protected]>

Author: mszyprow

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -479,7 +479,7 @@ static int radeon_ttm_tt_pin_userptr(struct ttm_tt *ttm)
 {
 	struct radeon_device *rdev = radeon_get_rdev(ttm->bdev);
 	struct radeon_ttm_tt *gtt = (void *)ttm;
-	unsigned pinned = 0, nents;
+	unsigned pinned = 0;
 	int r;
 
 	int write = !(gtt->userflags & RADEON_GEM_USERPTR_READONLY);
@@ -519,9 +519,8 @@ static int radeon_ttm_tt_pin_userptr(struct ttm_tt *ttm)
 	if (r)
 		goto release_sg;
 
-	r = -ENOMEM;
-	nents = dma_map_sg(rdev->dev, ttm->sg->sgl, ttm->sg->nents, direction);
-	if (nents == 0)
+	r = dma_map_sgtable(rdev->dev, ttm->sg, direction, 0);
+	if (r)
 		goto release_sg;
 
 	drm_prime_sg_to_page_addr_arrays(ttm->sg, ttm->pages,
@@ -552,9 +551,9 @@ static void radeon_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
 		return;
 
 	/* free the sg table and pages again */
-	dma_unmap_sg(rdev->dev, ttm->sg->sgl, ttm->sg->nents, direction);
+	dma_unmap_sgtable(rdev->dev, ttm->sg, direction, 0);
 
-	for_each_sg_page(ttm->sg->sgl, &sg_iter, ttm->sg->nents, 0) {
+	for_each_sgtable_page(ttm->sg, &sg_iter, 0) {
 		struct page *page = sg_page_iter_page(&sg_iter);
 		if (!(gtt->userflags & RADEON_GEM_USERPTR_READONLY))
 			set_page_dirty(page);