drm: armada: 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 <m.szyprowski@samsung.com>

diff --git a/drivers/gpu/drm/armada/armada_gem.c b/drivers/gpu/drm/armada/armada_gem.c
index 8005614..a63008c 100644 (file)
--- 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));
        }