[PATCH 1/3] drm/plane: Mention format_mod_supported in IN_FORMATS docs

[Dmitry Baryshkov]

On Tue, 21 Dec 2021 at 13:13, José Expósito <jose.exposito89 at gmail.com> wrote:
>
> Adding format modifiers without implementing the function
> "drm_plane_funcs.format_mod_supported" exposes an invalid IN_FORMATS
> blob with modifiers but no formats to user-space.

I think the fix should be applied to the generic code. The docs at
drm_plane.h clearly state that this callback is optional:

         * This optional hook is used for the DRM to determine if the given
         * format/modifier combination is valid for the plane. This allows the
         * DRM to generate the correct format bitmask (which formats apply to
         * which modifier), and to valdiate modifiers at atomic_check time.
         *
         * If not present, then any modifier in the plane's modifier
         * list is allowed with any of the plane's formats.

So, one should fix the core code in create_in_format_blob() to include
all combinations.

>
> This breaks the latest Weston [1]. For testing purposes, I extracted the
> affected code to a standalone program [2].
>
> Make clear in the IN_FORMATS documentation that implementing
> "format_mod_supported" is mandatory.

format_mod_supported() being optional or mandatory should be
documented in the format_mod_supported() docs, not in the IN_FORMAT
docs.

>
> [1] https://gitlab.freedesktop.org/wayland/weston/-/blob/9.0/libweston/backend-drm/kms.c#L431
> [2] https://github.com/JoseExposito/drm-sandbox/blob/main/in_formats.c
>
> Signed-off-by: José Expósito <jose.exposito89 at gmail.com>
> ---
>  drivers/gpu/drm/drm_plane.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 82afb854141b..347571f8909a 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -126,8 +126,11 @@
>   * IN_FORMATS:
>   *     Blob property which contains the set of buffer format and modifier
>   *     pairs supported by this plane. The blob is a struct
> - *     drm_format_modifier_blob. Without this property the plane doesn't
> - *     support buffers with modifiers. Userspace cannot change this property.
> + *     drm_format_modifier_blob and the drm_plane_funcs.format_mod_supported
> + *     function must be implemented by the driver to determine if the given
> + *     format/modifier combination is valid for the plane. Without this property
> + *     the plane doesn't support buffers with modifiers. Userspace cannot change
> + *     this property.
>   *
>   *     Note that userspace can check the &DRM_CAP_ADDFB2_MODIFIERS driver
>   *     capability for general modifier support. If this flag is set then every
> --
> 2.25.1
>


-- 
With best wishes
Dmitry