Re: [PATCH RFC v5 02/10] drm: Introduce solid fill DRM plane property

[Dmitry Baryshkov]

On Fri, 28 Jul 2023 at 20:03, Jessica Zhang <quic_jesszhan@xxxxxxxxxxx> wrote:
>
> Document and add support for solid_fill property to drm_plane. In
> addition, add support for setting and getting the values for solid_fill.
>
> To enable solid fill planes, userspace must assign a property blob to
> the "solid_fill" plane property containing the following information:
>
> struct drm_mode_solid_fill {
>         u32 version;
>         u32 r, g, b;
> };
>
> Signed-off-by: Jessica Zhang <quic_jesszhan@xxxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_atomic_state_helper.c |  9 +++++
>  drivers/gpu/drm/drm_atomic_uapi.c         | 55 +++++++++++++++++++++++++++++++
>  drivers/gpu/drm/drm_blend.c               | 30 +++++++++++++++++
>  include/drm/drm_blend.h                   |  1 +
>  include/drm/drm_plane.h                   | 35 ++++++++++++++++++++
>  include/uapi/drm/drm_mode.h               | 24 ++++++++++++++
>  6 files changed, 154 insertions(+)
>

[skipped most of the patch]

> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 43691058d28f..53c8efa5ad7f 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -259,6 +259,30 @@ struct drm_mode_modeinfo {
>         char name[DRM_DISPLAY_MODE_LEN];
>  };
>
> +/**
> + * struct drm_mode_solid_fill - User info for solid fill planes
> + *
> + * This is the userspace API solid fill information structure.
> + *
> + * Userspace can enable solid fill planes by assigning the plane "solid_fill"
> + * property to a blob containing a single drm_mode_solid_fill struct populated with an RGB323232
> + * color and setting the pixel source to "SOLID_FILL".
> + *
> + * For information on the plane property, see drm_plane_create_solid_fill_property()
> + *
> + * @version: Version of the blob. Currently, there is only support for version == 1
> + * @r: Red color value of single pixel
> + * @g: Green color value of single pixel
> + * @b: Blue color value of single pixel
> + */
> +struct drm_mode_solid_fill {
> +       __u32 version;
> +       __u32 r;
> +       __u32 g;
> +       __u32 b;

Another thought about the drm_mode_solid_fill uABI. I still think we
should add alpha here. The reason is the following:

It is true that we have  drm_plane_state::alpha and the plane's
"alpha" property. However it is documented as "the plane-wide opacity
[...] It can be combined with pixel alpha. The pixel values in the
framebuffers are expected to not be pre-multiplied by the global alpha
associated to the plane.".

I can imagine a use case, when a user might want to enable plane-wide
opacity, set "pixel blend mode" to "Coverage" and then switch between
partially opaque framebuffer and partially opaque solid-fill without
touching the plane's alpha value.

-- 
With best wishes
Dmitry