[PATCH] drm/gem-shmem: Optimize DMA mapping for exported buffers

[Christian König]

Am 25.03.25 um 14:37 schrieb Jacek Lawrynowicz:
> Use DMA_ATTR_SKIP_CPU_SYNC flag for exported buffers during DMA mapping.
> The same flag is already used by drm_gem_map_dma_buf() for imported
> buffers.
>
> Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz at linux.intel.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c |  8 ++++++--
>  include/drm/drm_gem.h                  | 12 ++++++++++++
>  2 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index d99dee67353a1..8938d8e3de52f 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -699,7 +699,7 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_get_sg_table);
>  static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_object *shmem)
>  {
>  	struct drm_gem_object *obj = &shmem->base;
> -	int ret;
> +	int ret, flags = 0;
>  	struct sg_table *sgt;
>  
>  	if (shmem->sgt)
> @@ -716,8 +716,12 @@ static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
>  		ret = PTR_ERR(sgt);
>  		goto err_put_pages;
>  	}
> +
> +	if (drm_gem_is_exported())
> +		flags |= DMA_ATTR_SKIP_CPU_SYNC;

We should probably just unconditionally set this flag or not at all.

Otherwise we could run into quite some surprises.

Regards,
Christian.

> +
>  	/* Map the pages for use by the h/w. */
> -	ret = dma_map_sgtable(obj->dev->dev, sgt, DMA_BIDIRECTIONAL, 0);
> +	ret = dma_map_sgtable(obj->dev->dev, sgt, DMA_BIDIRECTIONAL, flags);
>  	if (ret)
>  		goto err_free_sgt;
>  
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index 2bf893eabb4b2..7c355a44d0a49 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -589,6 +589,18 @@ static inline bool drm_gem_is_imported(const struct drm_gem_object *obj)
>  	return obj->dma_buf && (obj->dma_buf->priv != obj);
>  }
>  
> +/**
> + * drm_gem_is_exported() - Tests if GEM object's buffer has been exported
> + * @obj: the GEM object
> + *
> + * Returns:
> + * True if the GEM object's buffer has been exported, false otherwise
> + */
> +static inline bool drm_gem_is_exported(const struct drm_gem_object *obj)
> +{
> +	return obj->dma_buf && (obj->dma_buf->priv == obj);
> +}
> +
>  #ifdef CONFIG_LOCKDEP
>  /**
>   * drm_gem_gpuva_set_lock() - Set the lock protecting accesses to the gpuva list.