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.

dma_map_sgtable() function returns zero or an error code, so adjust the
return value check for the vsp1_du_map_sg() function.

Signed-off-by: Marek Szyprowski <>
Reviewed-by: Laurent Pinchart <>
 drivers/gpu/drm/rcar-du/rcar_du_vsp.c  | 3 +--
 drivers/media/platform/vsp1/vsp1_drm.c | 8 ++++----
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/rcar-du/rcar_du_vsp.c 
index f1a81c9b184d..a27bff999649 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_vsp.c
+++ b/drivers/gpu/drm/rcar-du/rcar_du_vsp.c
@@ -197,9 +197,8 @@ int rcar_du_vsp_map_fb(struct rcar_du_vsp *vsp, struct 
drm_framebuffer *fb,
                        goto fail;
                ret = vsp1_du_map_sg(vsp->vsp, sgt);
-               if (!ret) {
+               if (ret) {
-                       ret = -ENOMEM;
                        goto fail;
diff --git a/drivers/media/platform/vsp1/vsp1_drm.c 
index a4a45d68a6ef..86d5e3f4b1ff 100644
--- a/drivers/media/platform/vsp1/vsp1_drm.c
+++ b/drivers/media/platform/vsp1/vsp1_drm.c
@@ -912,8 +912,8 @@ int vsp1_du_map_sg(struct device *dev, struct sg_table *sgt)
         * skip cache sync. This will need to be revisited when support for
         * non-coherent buffers will be added to the DU driver.
-       return dma_map_sg_attrs(vsp1->bus_master, sgt->sgl, sgt->nents,
-                               DMA_TO_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
+       return dma_map_sgtable(vsp1->bus_master, sgt, DMA_TO_DEVICE,
+                              DMA_ATTR_SKIP_CPU_SYNC);
@@ -921,8 +921,8 @@ void vsp1_du_unmap_sg(struct device *dev, struct sg_table 
        struct vsp1_device *vsp1 = dev_get_drvdata(dev);
-       dma_unmap_sg_attrs(vsp1->bus_master, sgt->sgl, sgt->nents,
-                          DMA_TO_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
+       dma_unmap_sgtable(vsp1->bus_master, sgt, DMA_TO_DEVICE,
+                         DMA_ATTR_SKIP_CPU_SYNC);

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