Re: [PATCH v2 7/7] dma-buf: Write down some rules for vmap usage

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Dec 03, 2020 at 03:02:59PM +0100, Thomas Zimmermann wrote:
> Dma-buf's vmap and vunmap callbacks are undocumented and various
> exporters currently have slightly different semantics for them. Add
> documentation on how to implement and use these interfaces correctly.
> 
> v2:
> 	* document vmap semantics in struct dma_buf_ops
> 	* add TODO item for reviewing and maybe fixing dma-buf exporters
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>

I still don't think this works, we're breaking dma_buf_vmap for everyone
else here.

> ---
>  Documentation/gpu/todo.rst | 15 +++++++++++++
>  include/drm/drm_gem.h      |  4 ++++
>  include/linux/dma-buf.h    | 45 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 64 insertions(+)
> 
> diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> index 009d8e6c7e3c..32bb797a84fc 100644
> --- a/Documentation/gpu/todo.rst
> +++ b/Documentation/gpu/todo.rst
> @@ -505,6 +505,21 @@ Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Daniel Vette
>  Level: Intermediate
>  
>  
> +Enforce rules for dma-buf vmap and pin ops
> +------------------------------------------
> +
> +Exporter implementations of vmap and pin in struct dma_buf_ops (and consequently
> +struct drm_gem_object_funcs) use a variety of locking semantics. Some rely on
> +the caller holding the dma-buf's reservation lock, some do their own locking,
> +some don't require any locking. VRAM helpers even used to pin as part of vmap.
> +
> +We need to review each exporter and enforce the documented rules.
> +
> +Contact: Christian König, Daniel Vetter, Thomas Zimmermann <tzimmermann@suse.de>
> +
> +Level: Advanced
> +
> +
>  Core refactorings
>  =================
>  
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index 5e6daa1c982f..1864c6a721b1 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -138,6 +138,8 @@ struct drm_gem_object_funcs {
>  	 * drm_gem_dmabuf_vmap() helper.
>  	 *
>  	 * This callback is optional.
> +	 *
> +	 * See also struct dma_buf_ops.vmap
>  	 */
>  	int (*vmap)(struct drm_gem_object *obj, struct dma_buf_map *map);
>  
> @@ -148,6 +150,8 @@ struct drm_gem_object_funcs {
>  	 * drm_gem_dmabuf_vunmap() helper.
>  	 *
>  	 * This callback is optional.
> +	 *
> +	 * See also struct dma_buf_ops.vunmap
>  	 */
>  	void (*vunmap)(struct drm_gem_object *obj, struct dma_buf_map *map);
>  
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index cf72699cb2bc..dc81fdc01dda 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -267,7 +267,52 @@ struct dma_buf_ops {
>  	 */
>  	int (*mmap)(struct dma_buf *, struct vm_area_struct *vma);
>  
> +	/**
> +	 * @vmap:

There's already a @vmap and @vunamp kerneldoc at the top comment, that
needs to be removed.
-Daniel

> +	 *
> +	 * Returns a virtual address for the buffer.
> +	 *
> +	 * Notes to callers:
> +	 *
> +	 * - Callers must hold the struct dma_buf.resv lock before calling
> +	 *   this interface.
> +	 *
> +	 * - Callers must provide means to prevent the mappings from going
> +	 *   stale, such as holding the reservation lock or providing a
> +	 *   move-notify callback to the exporter.
> +	 *
> +	 * Notes to implementors:
> +	 *
> +	 * - Implementations must expect pairs of @vmap and @vunmap to be
> +	 *   called frequently and should optimize for this case.
> +	 *
> +	 * - Implementations should avoid additional operations, such as
> +	 *   pinning.
> +	 *
> +	 * - Implementations may expect the caller to hold the dma-buf's
> +	 *   reservation lock to protect against concurrent calls and
> +	 *   relocation.
> +	 *
> +	 * - Implementations may provide additional guarantees, such as working
> +	 *   without holding the reservation lock.
> +	 *
> +	 * This callback is optional.
> +	 *
> +	 * Returns:
> +	 *
> +	 * 0 on success or a negative error code on failure.
> +	 */
>  	int (*vmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
> +
> +	/**
> +	 * @vunmap:
> +	 *
> +	 * Releases the address previously returned by @vmap.
> +	 *
> +	 * This callback is optional.
> +	 *
> +	 * See also @vmap()
> +	 */
>  	void (*vunmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
>  };
>  
> -- 
> 2.29.2
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch