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samples: vfio-mdev/mbochs: 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()
function.

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.

While touching this code, also add missing call to dma_unmap_sgtable.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
This commit is contained in:
Marek Szyprowski 2020-04-30 13:57:22 +02:00
parent 62296b395a
commit be0704beb2
1 changed files with 2 additions and 1 deletions
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3
samples/vfio-mdev/mbochs.c
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@ -846,7 +846,7 @@ static struct sg_table *mbochs_map_dmabuf(struct dma_buf_attachment *at,
if (sg_alloc_table_from_pages(sg, dmabuf->pages, dmabuf->pagecount,
0, dmabuf->mode.size, GFP_KERNEL) < 0)
goto err2;
if (!dma_map_sg(at->dev, sg->sgl, sg->nents, direction))
if (dma_map_sgtable(at->dev, sg, direction, 0))
goto err3;
return sg;
@ -868,6 +868,7 @@ static void mbochs_unmap_dmabuf(struct dma_buf_attachment *at,
dev_dbg(dev, "%s: %d\n", __func__, dmabuf->id);
dma_unmap_sgtable(at->dev, sg, direction, 0);
sg_free_table(sg);
kfree(sg);
}