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

[Laurent Pinchart]

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.
> 
> Those semantics don't seem particularly good to me. I think we would want
> the per-pixel alpha and global alpha both to be effecive at the same time.
> You can always decide to ignore the per-pixel alpha by using a pixel format
> without alpha.

That makes sense to me. However, it also brings a new question: how should a 
driver that supports either global alpha or pixel alpha but not both signal 
that to userspace, and how should it reacts when userspace selects a format 
with an alpha channel and set a global alpha value other than fully opaque ? 
To make things more complex, note that some drivers support combining global 
alpha and pixel alpha only for a subset of the formats with an alpha channel 
(for instance for ARGB 1555 formats, but not for ARGB 8888 formats).

> Also, where's the userspace that wants this feature?
> 
> <snip>
> 
> > 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.

> Oh, and you should plug this into the state dumper as well.
> 
> > +
> > 
> >  	/* Plane rotation */
> >  	unsigned int rotation;
> > @@ -481,6 +485,7 @@ enum drm_plane_type {
> >   * @funcs: helper functions
> >   * @properties: property tracking for this plane
> >   * @type: type of plane (overlay, primary, cursor)
> > + * @alpha_property: alpha property for this plane
> >   * @zpos_property: zpos property for this plane
> >   * @rotation_property: rotation property for this plane
> >   * @helper_private: mid-layer private data
> > @@ -556,6 +561,7 @@ struct drm_plane {
> >  	 */
> >  	
> >  	struct drm_plane_state *state;
> > +	struct drm_property *alpha_property;
> >  	struct drm_property *zpos_property;
> >  	struct drm_property *rotation_property;
> >  };

-- 
Regards,

Laurent Pinchart