• Marek Szyprowski's avatar
    drm: core: fix common struct sg_table related issues · 6c6fa39c
    Marek Szyprowski authored
    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.
    Signed-off-by: default avatarMarek Szyprowski <m.szyprowski@samsung.com>
    Reviewed-by: default avatarAndrzej Hajda <a.hajda@samsung.com>
    Reviewed-by: default avatarRobin Murphy <robin.murphy@arm.com>
    6c6fa39c
drm_prime.c 29.5 KB