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

[Stefan Schake]

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. 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. This helps in the common case, the belts&braces fix would be to
enable the VC4 HVS background color fill incurring a fixed overhead that
in most cases is wasted because the composition ends up being opaque.

Thanks,
Stefan