Re: [PATCH v9 03/32] drm: core: fix common struct sg_table related issues

Robin Murphy
2020-09-01T17:46:47Z

On 2020-08-26 07:32, Marek Szyprowski wrote:
> 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>
> ---
> drivers/gpu/drm/drm_cache.c | 2 +-
> drivers/gpu/drm/drm_gem_shmem_helper.c | 14 +++++++++-----
> drivers/gpu/drm/drm_prime.c | 11 ++++++-----
> 3 files changed, 16 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
> index 03e01b000f7a..0fe3c496002a 100644
> --- a/drivers/gpu/drm/drm_cache.c
> +++ b/drivers/gpu/drm/drm_cache.c
> @@ -127,7 +127,7 @@ drm_clflush_sg(struct sg_table *st)
> struct sg_page_iter sg_iter;
>
> mb(); /*CLFLUSH is ordered only by using memory barriers*/
> - for_each_sg_page(st->sgl, &sg_iter, st->nents, 0)
> + for_each_sgtable_page(st, &sg_iter, 0)
> drm_clflush_page(sg_page_iter_page(&sg_iter));
> mb(); /*Make sure that all cache line entry is flushed*/
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 4b7cfbac4daa..47d8211221f2 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -126,8 +126,8 @@ void drm_gem_shmem_free_object(struct drm_gem_object *obj)
> drm_prime_gem_destroy(obj, shmem->sgt);
> } else {
> if (shmem->sgt) {
> - dma_unmap_sg(obj->dev->dev, shmem->sgt->sgl,
> - shmem->sgt->nents, DMA_BIDIRECTIONAL);
> + dma_unmap_sgtable(obj->dev->dev, shmem->sgt,
> + DMA_BIDIRECTIONAL, 0);
> sg_free_table(shmem->sgt);
> kfree(shmem->sgt);
> }
> @@ -424,8 +424,7 @@ void drm_gem_shmem_purge_locked(struct drm_gem_object *obj)
>
> WARN_ON(!drm_gem_shmem_is_purgeable(shmem));
>
> - dma_unmap_sg(obj->dev->dev, shmem->sgt->sgl,
> - shmem->sgt->nents, DMA_BIDIRECTIONAL);
> + dma_unmap_sgtable(obj->dev->dev, shmem->sgt, DMA_BIDIRECTIONAL, 0);
> sg_free_table(shmem->sgt);
> kfree(shmem->sgt);
> shmem->sgt = NULL;
> @@ -697,12 +696,17 @@ struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_object *obj)
> goto err_put_pages;
> }
> /* Map the pages for use by the h/w. */
> - dma_map_sg(obj->dev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL);
> + ret = dma_map_sgtable(obj->dev->dev, sgt, DMA_BIDIRECTIONAL, 0);
> + if (ret)
> + goto err_free_sgt;
>
> shmem->sgt = sgt;
>
> return sgt;
>
> +err_free_sgt:
> + sg_free_table(sgt);
> + kfree(sgt);

Should this be a separate patch to add the missing error handling to the
existing code first?

Otherwise the rest of the mechanical conversion looks straightforward
enough, and I'm not the separation-of-concerns police (for this
subsystem, at least), so either way,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> err_put_pages:
> drm_gem_shmem_put_pages(shmem);
> return ERR_PTR(ret);
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 5d181bf60a44..c45b0cc6e31d 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -617,6 +617,7 @@ struct sg_table *drm_gem_map_dma_buf(struct dma_buf_attachment *attach,
> {
> struct drm_gem_object *obj = attach->dmabuf->priv;
> struct sg_table *sgt;
> + int ret;
>
> if (WARN_ON(dir == DMA_NONE))
> return ERR_PTR(-EINVAL);
> @@ -626,11 +627,12 @@ struct sg_table *drm_gem_map_dma_buf(struct dma_buf_attachment *attach,
> else
> sgt = obj->dev->driver->gem_prime_get_sg_table(obj);
>
> - if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
> - DMA_ATTR_SKIP_CPU_SYNC)) {
> + ret = dma_map_sgtable(attach->dev, sgt, dir,
> + DMA_ATTR_SKIP_CPU_SYNC);
> + if (ret) {
> sg_free_table(sgt);
> kfree(sgt);
> - sgt = ERR_PTR(-ENOMEM);
> + sgt = ERR_PTR(ret);
> }
>
> return sgt;
> @@ -652,8 +654,7 @@ void drm_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
> if (!sgt)
> return;
>
> - dma_unmap_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
> - DMA_ATTR_SKIP_CPU_SYNC);
> + dma_unmap_sgtable(attach->dev, sgt, dir, DMA_ATTR_SKIP_CPU_SYNC);
> sg_free_table(sgt);
> kfree(sgt);
> }
>