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

[Eric Anholt <eric@anholt.net>]

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Ville Syrjälä <ville.syrjala@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@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), then blending against black is the same
as not doing any blending at all.

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