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Subject[PATCH v10 07/30] 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>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by : Inki Dae <inki.dae@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 03be31427181..967a5cdc120e 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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