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 --- For more information, see '[PATCH v5 00/38] DRM: fix struct sg_table nents vs. orig_nents misuse' thread: https://lore.kernel.org/linux-iommu/20200513132114.6046-1-m.szyprowski@samsu...
This patch has been resurrected on Oleksandr Andrushchenko request. The patch was a part of v2 patchset, but then I've dropped it as it only fixes the debug output, thus I didn't consider it so critical. --- 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 f0b85e0..ba4bdc5 100644 --- a/drivers/gpu/drm/xen/xen_drm_front_gem.c +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.c @@ -215,7 +215,7 @@ struct drm_gem_object * 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; }