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    Subject[PATCH v10 21/30] drm: xen: fix common struct sg_table related issues
    Date
    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.

    Fix the code to refer to proper nents or orig_nents entries. This driver
    reports the number of the pages in the imported scatterlist, so it should
    refer to sg_table->orig_nents entry.

    Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
    Acked-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
    ---
    drivers/gpu/drm/xen/xen_drm_front_gem.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.c b/drivers/gpu/drm/xen/xen_drm_front_gem.c
    index 39ff95b75357..0e57c80058b2 100644
    --- a/drivers/gpu/drm/xen/xen_drm_front_gem.c
    +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.c
    @@ -216,7 +216,7 @@ xen_drm_front_gem_import_sg_table(struct drm_device *dev,
    return ERR_PTR(ret);

    DRM_DEBUG("Imported buffer of size %zu with nents %u\n",
    - size, sgt->nents);
    + size, sgt->orig_nents);

    return &xen_obj->base;
    }
    --
    2.17.1
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