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