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    Subject[PATCH v6 08/36] drm: exynos: 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.

    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 <m.szyprowski@samsung.com>
    ---
    drivers/gpu/drm/exynos/exynos_drm_g2d.c | 10 +++++-----
    1 file changed, 5 insertions(+), 5 deletions(-)

    diff --git a/drivers/gpu/drm/exynos/exynos_drm_g2d.c b/drivers/gpu/drm/exynos/exynos_drm_g2d.c
    index fcee33a43aca..7014a8cd971a 100644
    --- a/drivers/gpu/drm/exynos/exynos_drm_g2d.c
    +++ b/drivers/gpu/drm/exynos/exynos_drm_g2d.c
    @@ -395,8 +395,8 @@ static void g2d_userptr_put_dma_addr(struct g2d_data *g2d,
    return;

    out:
    - dma_unmap_sg(to_dma_dev(g2d->drm_dev), g2d_userptr->sgt->sgl,
    - g2d_userptr->sgt->nents, DMA_BIDIRECTIONAL);
    + dma_unmap_sgtable(to_dma_dev(g2d->drm_dev), g2d_userptr->sgt,
    + DMA_BIDIRECTIONAL, 0);

    pages = frame_vector_pages(g2d_userptr->vec);
    if (!IS_ERR(pages)) {
    @@ -511,10 +511,10 @@ static dma_addr_t *g2d_userptr_get_dma_addr(struct g2d_data *g2d,

    g2d_userptr->sgt = sgt;

    - if (!dma_map_sg(to_dma_dev(g2d->drm_dev), sgt->sgl, sgt->nents,
    - DMA_BIDIRECTIONAL)) {
    + ret = dma_map_sgtable(to_dma_dev(g2d->drm_dev), sgt,
    + DMA_BIDIRECTIONAL, 0);
    + if (ret) {
    DRM_DEV_ERROR(g2d->dev, "failed to map sgt with dma region.\n");
    - ret = -ENOMEM;
    goto err_sg_free_table;
    }

    --
    2.17.1
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