[PATCH v2 17/21] drm: rcar-du: fix sg_table nents vs. orig_nents misuse

Marek Szyprowski m.szyprowski at samsung.com
Mon May 4 12:53:55 UTC 2020

The Documentation/DMA-API-HOWTO.txt states that dma_map_sg returns the
numer of the created entries in the DMA address space. However the
subsequent calls to dma_sync_sg_for_{device,cpu} and dma_unmap_sg must be
called with the original number of entries passed to dma_map_sg. The
sg_table->nents in turn holds the result of the dma_map_sg call as stated
in include/linux/scatterlist.h. Adapt the code to obey those rules.

Signed-off-by: Marek Szyprowski <m.szyprowski at samsung.com>
For more information, see '[PATCH v2 00/21] DRM: fix struct sg_table nents
vs. orig_nents misuse' thread: https://lkml.org/lkml/2020/5/4/373
 drivers/media/platform/vsp1/vsp1_drm.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/media/platform/vsp1/vsp1_drm.c b/drivers/media/platform/vsp1/vsp1_drm.c
index a4a45d6..b54a30f 100644
--- a/drivers/media/platform/vsp1/vsp1_drm.c
+++ b/drivers/media/platform/vsp1/vsp1_drm.c
@@ -912,8 +912,9 @@ 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,
+	sgt->nents = dma_map_sg_attrs(vsp1->bus_master, sgt->sgl,
+			sgt->orig_nents, DMA_TO_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
+	return sgt->nents;
@@ -921,7 +922,7 @@ 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_sg_attrs(vsp1->bus_master, sgt->sgl, sgt->orig_nents,

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