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()

Fix the code to refer to proper nents or orig_nents entries. This driver
checks for a buffer contiguity in DMA address space, so it should test
sg_table->nents entry.

Signed-off-by: Marek Szyprowski <>
 drivers/gpu/drm/omapdrm/omap_gem.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/omapdrm/omap_gem.c 
index ff0c4b0c3fd0..a7a9a0afe2b6 100644
--- a/drivers/gpu/drm/omapdrm/omap_gem.c
+++ b/drivers/gpu/drm/omapdrm/omap_gem.c
@@ -48,7 +48,7 @@ struct omap_gem_object {
         *   OMAP_BO_MEM_DMA_API flag set)
         * - buffers imported from dmabuf (with the OMAP_BO_MEM_DMABUF flag set)
-        *   if they are physically contiguous (when sgt->orig_nents == 1)
+        *   if they are physically contiguous (when sgt->nents == 1)
         * - buffers mapped through the TILER when dma_addr_cnt is not zero, in
         *   which case the DMA address points to the TILER aperture
@@ -1279,7 +1279,7 @@ struct drm_gem_object *omap_gem_new_dmabuf(struct 
drm_device *dev, size_t size,
        union omap_gem_size gsize;
        /* Without a DMM only physically contiguous buffers can be supported. */
-       if (sgt->orig_nents != 1 && !priv->has_dmm)
+       if (sgt->nents != 1 && !priv->has_dmm)
                return ERR_PTR(-EINVAL);
        gsize.bytes = PAGE_ALIGN(size);
@@ -1293,7 +1293,7 @@ struct drm_gem_object *omap_gem_new_dmabuf(struct 
drm_device *dev, size_t size,
        omap_obj->sgt = sgt;
-       if (sgt->orig_nents == 1) {
+       if (sgt->nents == 1) {
                omap_obj->dma_addr = sg_dma_address(sgt->sgl);
        } else {
                /* Create pages list from sgt */

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