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 <>
 drivers/misc/fastrpc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 7939c55daceb..9d6867749316 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -518,7 +518,7 @@ fastrpc_map_dma_buf(struct dma_buf_attachment *attachment,
        table = &a->sgt;
-       if (!dma_map_sg(attachment->dev, table->sgl, table->nents, dir))
+       if (!dma_map_sgtable(attachment->dev, table, dir, 0))
                return ERR_PTR(-ENOMEM);
        return table;
@@ -528,7 +528,7 @@ static void fastrpc_unmap_dma_buf(struct dma_buf_attachment 
                                  struct sg_table *table,
                                  enum dma_data_direction dir)
-       dma_unmap_sg(attach->dev, table->sgl, table->nents, dir);
+       dma_unmap_sgtable(attach->dev, table, dir, 0);
 static void fastrpc_release(struct dma_buf *dmabuf)

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