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

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Date:   Fri, 4 Aug 2023 16:59:00 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Sebastian Wick <sebastian.wick@...hat.com>
Cc:     Jessica Zhang <quic_jesszhan@...cinc.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        quic_abhinavk@...cinc.com, ppaalanen@...il.com,
        contact@...rsion.fr, laurent.pinchart@...asonboard.com,
        ville.syrjala@...ux.intel.com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        freedreno@...ts.freedesktop.org,
        wayland-devel@...ts.freedesktop.org
Subject: Re: [PATCH RFC v5 02/10] drm: Introduce solid fill DRM plane property

On Fri, 4 Aug 2023 at 16:44, Sebastian Wick <sebastian.wick@...hat.com> wrote:
>
> On Fri, Aug 4, 2023 at 3:27 PM Dmitry Baryshkov
> <dmitry.baryshkov@...aro.org> wrote:
> >
> > On Fri, 28 Jul 2023 at 20:03, Jessica Zhang <quic_jesszhan@...cinc.com> 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@...cinc.com>
> > > ---
> > >  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.
>
> The only reason I see against this is that there might be some
> hardware which supports only RGB but not alpha on planes and they
> could then not use this property.

Fair enough.

> Maybe another COLOR_FILL enum value
> with alpha might be better? Maybe just doing the alpha via the alpha
> property is good enough.

One of our customers has a use case for setting the opaque solid fill,
while keeping the plane's alpha intact.

-- 
With best wishes
Dmitry