Re: [PATCH v5 02/16] drm/shmem: Provide a generic {begin, end}_cpu_access() implementation

[Akash Goel <akash.goel@arm.com>]

On 10/30/25 14:05, Boris Brezillon wrote:
> The default implementation simply takes care of invalidating/flushing
> caches around CPU accesses. It takes care of both the exporter and
> the importers, which forces us to overload the default
> ::[un]map_dma_buf() implementation provided by drm_gem.c to store the
> sgt.
>
> v5:
> - New patch
>
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> ---
>   drivers/gpu/drm/drm_gem_shmem_helper.c | 114 +++++++++++++++++++++++++
>   include/drm/drm_gem_shmem_helper.h     |  10 +++
>   2 files changed, 124 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index dc94a27710e5..e49c75739c20 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -893,6 +893,120 @@ struct drm_gem_object *drm_gem_shmem_prime_import_no_map(struct drm_device *dev,
>   }
>   EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_no_map);
>
> +/**
> + * drm_gem_shmem_prime_unmap_dma_buf - Default unmap_dma_buf() for exported buffers
> + * @attach: attachment
> + * @sgt: SG table to unmap
> + * @dir: type of access done by this attachment
> + *
> + * Default implementation for dma_buf_ops::map_dma_buf(). This is just a wrapper
> + * around drm_gem_map_dma_buf() that lets us set the dma_buf_attachment::priv
> + * to the sgt so that drm_gem_shmem_prime_{begin,end}_cpu_access() can sync
> + * around CPU accesses.
> + */
> +struct sg_table *
> +drm_gem_shmem_prime_map_dma_buf(struct dma_buf_attachment *attach,
> +                             enum dma_data_direction dir)
> +{
> +     struct sg_table *sgt = drm_gem_map_dma_buf(attach, dir);
> +
> +     if (!IS_ERR(sgt))
> +             attach->priv = sgt;
> +
> +     return sgt;
> +}
> +EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_map_dma_buf);
> +
> +/**
> + * drm_gem_shmem_prime_unmap_dma_buf - Default unmap_dma_buf() for exported buffers
> + * @attach: attachment
> + * @sgt: SG table to unmap
> + * @dir: type of access done by this attachment
> + *
> + * Default implementation for dma_buf_ops::unmap_dma_buf(). This is just a wrapper
> + * around drm_gem_unmap_dma_buf() that lets us reset the dma_buf_attachment::priv
> + * field so that drm_gem_shmem_prime_{begin,end}_cpu_access() don't consider it
> + * as a mapped attachment to sync against.
> + */
> +void drm_gem_shmem_prime_unmap_dma_buf(struct dma_buf_attachment *attach,
> +                                    struct sg_table *sgt,
> +                                    enum dma_data_direction dir)
> +{
> +     attach->priv = NULL;
> +     drm_gem_unmap_dma_buf(attach, sgt, dir);
> +}
> +EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_unmap_dma_buf);
> +
> +/**
> + * drm_gem_shmem_prime_begin_cpu_access - Default end_cpu_access() for exported buffers


Sorry nitpick. There is a typo here. Should be 'Default begin_cpu_access()`.


> + * @dma_buf: The exported DMA buffer this acts on
> + * @dir: direction of the access
> + *
> + * Default implementation for dma_buf_ops::begin_cpu_access(). This only takes care of
> + * cache maintenance.
> + */
> +int drm_gem_shmem_prime_begin_cpu_access(struct dma_buf *dma_buf,
> +                                      enum dma_data_direction dir)
> +{
> +     struct drm_gem_object *obj = dma_buf->priv;
> +     struct drm_device *dev = obj->dev;
> +     struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
> +     struct dma_buf_attachment *attach;
> +
> +     dma_resv_lock(obj->resv, NULL);
> +     if (shmem->sgt)
> +             dma_sync_sgtable_for_cpu(dev->dev, shmem->sgt, dir);
> +
> +     if (shmem->vaddr)
> +             invalidate_kernel_vmap_range(shmem->vaddr, shmem->base.size);
> +
> +     list_for_each_entry(attach, &dma_buf->attachments, node) {
> +             struct sg_table *sgt = attach->priv;
> +
> +             if (sgt)
> +                     dma_sync_sgtable_for_cpu(attach->dev, sgt, dir);
> +     }
> +     dma_resv_unlock(obj->resv);
> +
> +     return 0;
> +}
> +EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_begin_cpu_access);
> +
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