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Subject[PATCH v10 20/30] drm: vmwgfx: 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()

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 <>
Acked-by: Roland Scheidegger <>
drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c | 17 ++++-------------
1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
index c7f10b2c93d2..13c31e2d7254 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
@@ -362,8 +362,7 @@ static void vmw_ttm_unmap_from_dma(struct vmw_ttm_tt *vmw_tt)
struct device *dev = vmw_tt->dev_priv->dev->dev;

- dma_unmap_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.nents,
+ dma_unmap_sgtable(dev, &vmw_tt->sgt, DMA_BIDIRECTIONAL, 0);
vmw_tt->sgt.nents = vmw_tt->sgt.orig_nents;

@@ -383,16 +382,8 @@ static void vmw_ttm_unmap_from_dma(struct vmw_ttm_tt *vmw_tt)
static int vmw_ttm_map_for_dma(struct vmw_ttm_tt *vmw_tt)
struct device *dev = vmw_tt->dev_priv->dev->dev;
- int ret;
- ret = dma_map_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.orig_nents,
- if (unlikely(ret == 0))
- return -ENOMEM;

- vmw_tt->sgt.nents = ret;
- return 0;
+ return dma_map_sgtable(dev, &vmw_tt->sgt, DMA_BIDIRECTIONAL, 0);

@@ -449,10 +440,10 @@ static int vmw_ttm_map_dma(struct vmw_ttm_tt *vmw_tt)
if (unlikely(ret != 0))
goto out_sg_alloc_fail;

- if (vsgt->num_pages > vmw_tt->sgt.nents) {
+ if (vsgt->num_pages > vmw_tt->sgt.orig_nents) {
uint64_t over_alloc =
sgl_size * (vsgt->num_pages -
- vmw_tt->sgt.nents);
+ vmw_tt->sgt.orig_nents);

ttm_mem_global_free(glob, over_alloc);
vmw_tt->sg_alloc_size -= over_alloc;
 \ /
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