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Subject[PATCH v10 22/30] xen: gntdev: fix common struct sg_table related issues
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 <>
Acked-by: Juergen Gross <>
drivers/xen/gntdev-dmabuf.c | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
index b1b6eebafd5d..4c13cbc99896 100644
--- a/drivers/xen/gntdev-dmabuf.c
+++ b/drivers/xen/gntdev-dmabuf.c
@@ -247,10 +247,9 @@ static void dmabuf_exp_ops_detach(struct dma_buf *dma_buf,

if (sgt) {
if (gntdev_dmabuf_attach->dir != DMA_NONE)
- dma_unmap_sg_attrs(attach->dev, sgt->sgl,
- sgt->nents,
- gntdev_dmabuf_attach->dir,
+ dma_unmap_sgtable(attach->dev, sgt,
+ gntdev_dmabuf_attach->dir,

@@ -288,8 +287,8 @@ dmabuf_exp_ops_map_dma_buf(struct dma_buf_attachment *attach,
sgt = dmabuf_pages_to_sgt(gntdev_dmabuf->pages,
if (!IS_ERR(sgt)) {
- if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
+ if (dma_map_sgtable(attach->dev, sgt, dir,
@@ -633,7 +632,7 @@ dmabuf_imp_to_refs(struct gntdev_dmabuf_priv *priv, struct device *dev,

/* Now convert sgt to array of pages and check for page validity. */
i = 0;
- for_each_sg_page(sgt->sgl, &sg_iter, sgt->nents, 0) {
+ for_each_sgtable_page(sgt, &sg_iter, 0) {
struct page *page = sg_page_iter_page(&sg_iter);
* Check if page is valid: this can happen if we are given
 \ /
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