[PATCH] drm/vc4: Ignore alpha on primary plane

[Ville Syrjälä]

On Fri, Mar 02, 2018 at 09:13:14AM -0800, Eric Anholt wrote:
> Ville Syrjälä <ville.syrjala at linux.intel.com> writes:
> 
> > On Fri, Mar 02, 2018 at 04:06:58PM +0100, Stefan Schake wrote:
> >> Hey Ville,
> >> 
> >> On Fri, Mar 2, 2018 at 3:43 PM, Ville Syrjälä
> >> <ville.syrjala at linux.intel.com> wrote:
> >> > On Fri, Mar 02, 2018 at 04:39:22PM +0200, Ville Syrjälä wrote:
> >> >> If you want the plane to always be opaque you shouldn't expose any
> >> >> formats with alpha.
> >> >>
> >> >> Also what happens if one disables the primary plane? Or does the driver
> >> >> not allow that?
> >> >
> >> > Or just makes the plane not cover the entire screen?
> >> 
> >> We've exposed alpha formats in the past so disabling that now would break
> >> userspace, certainly Android that chooses alpha-everything.
> >
> > So it refuses to even run on hardware that can't do per-pixel alpha on
> > the primary plane?
> >
> >> The VC4 HVS
> >> has no fixed planes so I'll acknowledge that the concept of a primary plane
> >> is somewhat dubious and userspace could disable it or make it not cover the
> >> entire screen, making this ineffective.
> >> 
> >> But then ultimately there doesn't seem to be a standard for what the display
> >> is supposed to be if you have transparent pixels with no plane to blend to
> >> below.
> >
> > The standard is black. IMO it's a driver bug if it fails to do that.
> 
> If the plane is premultiplied (isn't that what DRM planes are supposed
> to be?  I can't find any docs),

Yeah I think pre-multiplied is what was agreed on because that was
what everyone's cursor planes were doing already. But I guess we didn't
actually document that anywhere :(

Should we add something like 
+/*
+ * Note that using any format with alpha (A) implies pre-multiplied
+ * alpha blending: Dc = Sc + (1.0 - Sa) * Dc, where Sa is source alpha,
+ * Sc is source color, and Dc is destination color.
+ */
to drm_fourcc.h perhaps?

Not sure if that's the best place, nor am I sure the wording is good
enough. Opinions welcome.

igt could really use some alpha blending tests as well...

> then blending against black is the same
> as not doing any blending at all.

Ah, yes indeeed. Hmm. And with my i915 hat on that actually means that we
should in theory be able to expose alpha formats on i915 primary planes
because there can never be anything but black underneath them on account
of the fixed zorder. But we can't make that promise for the sprite planes,
so probably not worth the hassle for primary planes either. Sufficiently
smart userspace should be able figure this fact out on its own I
suppose.

-- 
Ville Syrjälä
Intel OTC