[PATCH v9 14/32] drm: omapdrm: fix common struct sg_table related issues

Marek Szyprowski
2020-08-26T06:38:25Z

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
checks for a buffer contiguity in DMA address space, so it should test
sg_table->nents entry.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
drivers/gpu/drm/omapdrm/omap_gem.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/omapdrm/omap_gem.c b/drivers/gpu/drm/omapdrm/omap_gem.c
index ff0c4b0c3fd0..a7a9a0afe2b6 100644
--- a/drivers/gpu/drm/omapdrm/omap_gem.c
+++ b/drivers/gpu/drm/omapdrm/omap_gem.c
@@ -48,7 +48,7 @@ struct omap_gem_object {
* OMAP_BO_MEM_DMA_API flag set)
*
* - buffers imported from dmabuf (with the OMAP_BO_MEM_DMABUF flag set)
- * if they are physically contiguous (when sgt->orig_nents == 1)
+ * if they are physically contiguous (when sgt->nents == 1)
*
* - buffers mapped through the TILER when dma_addr_cnt is not zero, in
* which case the DMA address points to the TILER aperture
@@ -1279,7 +1279,7 @@ struct drm_gem_object *omap_gem_new_dmabuf(struct drm_device *dev, size_t size,
union omap_gem_size gsize;

/* Without a DMM only physically contiguous buffers can be supported. */
- if (sgt->orig_nents != 1 && !priv->has_dmm)
+ if (sgt->nents != 1 && !priv->has_dmm)
return ERR_PTR(-EINVAL);

gsize.bytes = PAGE_ALIGN(size);
@@ -1293,7 +1293,7 @@ struct drm_gem_object *omap_gem_new_dmabuf(struct drm_device *dev, size_t size,

omap_obj->sgt = sgt;

- if (sgt->orig_nents == 1) {
+ if (sgt->nents == 1) {
omap_obj->dma_addr = sg_dma_address(sgt->sgl);
} else {
/* Create pages list from sgt */
--
2.17.1