[PATCH v3 1/8] drm/blend: Add a generic alpha property

[Daniel Vetter]

On Wed, Feb 21, 2018 at 02:07:57PM +0100, Maxime Ripard wrote:
> On Mon, Feb 19, 2018 at 10:58:40PM +0100, Daniel Vetter wrote:
> > On Mon, Feb 19, 2018 at 9:19 PM, Laurent Pinchart
> > <laurent.pinchart at ideasonboard.com> wrote:
> > > Hi Ville,
> > >
> > > On Friday, 16 February 2018 20:20:41 EET Ville Syrjälä wrote:
> > >> On Fri, Feb 16, 2018 at 06:39:29PM +0100, Maxime Ripard wrote:
> > >> > Some drivers duplicate the logic to create a property to store a per-plane
> > >> > alpha.
> > >> >
> > >> > This is especially useful if we ever want to support extra protocols for
> > >> > Wayland like:
> > >> > https://lists.freedesktop.org/archives/wayland-devel/2017-August/034741.ht
> > >> > ml
> > >> >
> > >> > Let's create a helper in order to move that to the core.
> > >> >
> > >> > Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > >> > Reviewed-by: Boris Brezillon <boris.brezillon at bootlin.com>
> > >> > Signed-off-by: Maxime Ripard <maxime.ripard at bootlin.com>
> > >> > ---
> > >> >
> > >> >  Documentation/gpu/kms-properties.csv |  2 +-
> > >> >  drivers/gpu/drm/drm_atomic.c         |  4 ++++-
> > >> >  drivers/gpu/drm/drm_atomic_helper.c  |  4 ++++-
> > >> >  drivers/gpu/drm/drm_blend.c          | 32 +++++++++++++++++++++++++++++-
> > >> >  include/drm/drm_blend.h              |  1 +-
> > >> >  include/drm/drm_plane.h              |  6 +++++-
> > >> >  6 files changed, 48 insertions(+), 1 deletion(-)
> > >> >
> > >> > diff --git a/Documentation/gpu/kms-properties.csv
> > >> > b/Documentation/gpu/kms-properties.csv index 927b65e14219..25ad3503d663
> > >> > 100644
> > >> > --- a/Documentation/gpu/kms-properties.csv
> > >> > +++ b/Documentation/gpu/kms-properties.csv
> > >> > @@ -99,5 +99,5 @@ radeon,DVI-I,“coherent”,RANGE,"Min=0,
> > >> > Max=1",Connector,TBD>
> > >> >  ,,"""underscan vborder""",RANGE,"Min=0, Max=128",Connector,TBD
> > >> >  ,Audio,“audio”,ENUM,"{ ""off"", ""on"", ""auto"" }",Connector,TBD
> > >> >  ,FMT Dithering,“dither”,ENUM,"{ ""off"", ""on"" }",Connector,TBD
> > >> >
> > >> > -rcar-du,Generic,"""alpha""",RANGE,"Min=0, Max=255",Plane,TBD
> > >> > +,,"""alpha""",RANGE,"Min=0, Max=Driver dependant",Plane,Opacity of the
> > >> > plane from transparent (0) to fully opaque (MAX). If this property is set
> > >> > to a value different than max, and that the pixel will define an alpha
> > >> > component, the property will have precendance and the pixel value will be
> > >> > ignored.
> > 
> > Please don't document new properties in that csv file, it's an
> > unreadable mess. Instead follow how we document standardized
> > properties nowadays in full-blown sections. For plane blending we
> > have:
> > 
> > https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html#plane-composition-properties
> 
> Ack
> 
> > >> > diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h
> > >> > index 8185e3468a23..5a6f29524f12 100644
> > >> > --- a/include/drm/drm_plane.h
> > >> > +++ b/include/drm/drm_plane.h
> > >> > @@ -42,6 +42,7 @@ struct drm_modeset_acquire_ctx;
> > >> >   * plane (in 16.16)
> > >> >   * @src_w: width of visible portion of plane (in 16.16)
> > >> >   * @src_h: height of visible portion of plane (in 16.16)
> > >> > + * @alpha: opacity of the plane
> > >> >   * @rotation: rotation of the plane
> > >> >   * @zpos: priority of the given plane on crtc (optional)
> > >> >   * Note that multiple active planes on the same crtc can have an
> > >> >   identical
> > >> > @@ -105,6 +106,9 @@ struct drm_plane_state {
> > >> >     uint32_t src_x, src_y;
> > >> >     uint32_t src_h, src_w;
> > >> >
> > >> > +   /* Plane opacity */
> > >> > +   u8 alpha;
> > >>
> > >> We may want to make that u16. The general we expect 16bpc for most color
> > >> related things, but since this is a range prop I suppose we should just
> > >> expose the actual hardware range. But making it u16 might avoid some head
> > >> scratching for the first person to have hardware with higher precision.
> > >> Either that or we should make the prop creation fail if the driver asks
> > >> for more bits than we have in the state.
> > >
> > > I'm tempted to go one step further and always make the alpha property 16-bits
> > > wide for new users (we can't do so for existing users as it could break
> > > userspace), and let drivers convert that internally to the range they need.
> > > There could however be drawbacks I don't foresee.
> > 
> > I think scaling the range to match the hw is the most sensible (yes
> > I'm flip-flopping around here). And once someone needs more than u8,
> > we can extend the internal representation easily. The external
> > representation in the property is an u64, that /should/ be enough for
> > the next few years :-)
> 
> Just to make sure we're on the same page, you want to keep the u8, and
> if the hardware uses say an u16, the driver for that hardware will do
> the upscaling?

The idea is that we'd set the u16 limit in the property and so inform
userspace that a different range applies. But that's probably going to be
ignored.

Could we do the property itself as u16 range, and (for now, only
internally in drm in drm_plane_state) throw the lower u8 bits away? Or
just let drivers do this.

Sorry that I'm flip-flopping around on this, but we just have an ongoing
discussion about a range/size mixup in the CTM uapi, I think assuming that
all userspace will correctly scale is not realistic. So larger scale in
the uapi (but maybe not internally) from the start seems like a good idea.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch