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    SubjectRe: [PATCH v9 30/32] samples: vfio-mdev/mbochs: fix common struct sg_table related issues
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    On 2020-08-26 07:33, Marek Szyprowski wrote:
    > 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.

    Reviewed-by: Robin Murphy <robin.murphy@arm.com>

    > Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
    > ---
    > samples/vfio-mdev/mbochs.c | 3 ++-
    > 1 file changed, 2 insertions(+), 1 deletion(-)
    >
    > diff --git a/samples/vfio-mdev/mbochs.c b/samples/vfio-mdev/mbochs.c
    > index 3cc5e5921682..e03068917273 100644
    > --- a/samples/vfio-mdev/mbochs.c
    > +++ b/samples/vfio-mdev/mbochs.c
    > @@ -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);
    > }
    >

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