[PATCH v5 29/38] drm: rcar-du: fix common struct sg_table related issues

Marek Szyprowski m.szyprowski at samsung.com
Wed May 13 13:32:36 UTC 2020

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 <m.szyprowski at samsung.com>
For more information, see '[PATCH v5 00/38] DRM: fix struct sg_table nents
vs. orig_nents misuse' thread:
 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 b/drivers/gpu/drm/rcar-du/rcar_du_vsp.c
index 5e4faf2..2fc1816 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 b/drivers/media/platform/vsp1/vsp1_drm.c
index a4a45d6..86d5e3f 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,
+	return dma_map_sgtable(vsp1->bus_master, sgt, DMA_TO_DEVICE,
@@ -921,8 +921,8 @@ void vsp1_du_unmap_sg(struct device *dev, struct sg_table *sgt)
 	struct vsp1_device *vsp1 = dev_get_drvdata(dev);
-	dma_unmap_sg_attrs(vsp1->bus_master, sgt->sgl, sgt->nents,
+	dma_unmap_sgtable(vsp1->bus_master, sgt, DMA_TO_DEVICE,

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