[PATCH] drm: document pre-multiplied assumptions

[Ville Syrjälä]

On Wed, Sep 29, 2021 at 09:54:14AM +0000, Simon Ser wrote:
> When a plane is missing the "alpha blend mode" property, KMS drivers
> will use the pre-multiplied mode.
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>

Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Another option would be to sprinkle immutable props to all drivers
that claim to do alpha blending and don't currently expose the
property.

> ---
>  drivers/gpu/drm/drm_blend.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
> index ec37cbfabb50..eebb32ba84d7 100644
> --- a/drivers/gpu/drm/drm_blend.c
> +++ b/drivers/gpu/drm/drm_blend.c
> @@ -185,6 +185,9 @@
>   *		 plane does not expose the "alpha" property, then this is
>   *		 assumed to be 1.0
>   *
> + *	When a plane is missing this property, the plane uses the
> + *	"Pre-multiplied" equation.
> + *
>   * Note that all the property extensions described here apply either to the
>   * plane or the CRTC (e.g. for the background color, which currently is not
>   * exposed and assumed to be black).
> -- 
> 2.33.0
> 

-- 
Ville Syrjälä
Intel