Re: [PATCH 1/2] drm: document DRM_IOCTL_PRIME_HANDLE_TO_FD and PRIME_FD_TO_HANDLE

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Feb 16, 2023 at 01:09:41PM +0000, Simon Ser wrote:
> Signed-off-by: Simon Ser <contact@emersion.fr>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen@gmail.com>
> Cc: Daniel Stone <daniel@fooishbar.org>
> ---
>  include/uapi/drm/drm.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
> index 4cb956a52aee..54b2313c8332 100644
> --- a/include/uapi/drm/drm.h
> +++ b/include/uapi/drm/drm.h
> @@ -1012,7 +1012,24 @@ extern "C" {
>  #define DRM_IOCTL_UNLOCK		DRM_IOW( 0x2b, struct drm_lock)
>  #define DRM_IOCTL_FINISH		DRM_IOW( 0x2c, struct drm_lock)
>  
> +/**
> + * DRM_IOCTL_PRIME_HANDLE_TO_FD - Convert a GEM handle to a DMA-BUF FD.
> + *
> + * User-space sets &drm_prime_handle.handle with the GEM handle to export and
> + * &drm_prime_handle.flags, and gets back a DMA-BUF file descriptor in
> + * &drm_prime_handle.fd.

I think it'd be good to mention here that export could fail for any number
of driver specific reason, e.g. because the driver doesn't support dma-buf
export at all, or it's not supported for this specific gem handle because
it's a special one. And therefore generic clients need to be able to cope
with some kind of fallback path ideally.

Also I think it'd be good to mention that export support is announced with
the DRM_PRIME_CAP_EXPORT flag in the DRM_CAP_PRIME GETCAP ioctl
capability.

> + */
>  #define DRM_IOCTL_PRIME_HANDLE_TO_FD    DRM_IOWR(0x2d, struct drm_prime_handle)
> +/**
> + * DRM_IOCTL_PRIME_FD_TO_HANDLE - Convert a DMA-BUF FD to a GEM handle.
> + *
> + * User-space sets &drm_prime_handle.fd with a DMA-BUF file descriptor to
> + * import, and gets back a GEM handle in &drm_prime_handle.handle.
> + * &drm_prime_handle.flags is unused.

Same here, some mention that import can fail for any reason (many drivers
do not support importing to anything else than the exporting device). Also
DRM_PRIME_CAP_IMPORT for querying.

> + *
> + * If an existing GEM handle refers to the memory object backing the DMA-BUF,
> + * that GEM handle is returned.

I think we need to state clearly that this means importers who get unknown
buffers must have a userspace lookup structure to catch these handle
deduplications and refcount objects properly.

With these additions: Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> + */
>  #define DRM_IOCTL_PRIME_FD_TO_HANDLE    DRM_IOWR(0x2e, struct drm_prime_handle)
>  
>  #define DRM_IOCTL_AGP_ACQUIRE		DRM_IO(  0x30)
> -- 
> 2.39.1
> 
> 

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