Hey Robert,

On Thu, Feb 22, 2018 at 11:04 AM, Robert Foss <robert.f...@collabora.com> wrote:
> Hey Stefan,
> On 02/22/2018 04:54 AM, Stefan Schake wrote:
>> Android assumes an implicit black background layer is always present
>> behind all layers it specifies for composition. drm_hwcomposer currently
>> punts responsibility for this to the kernel/DRM platform and puts layers
>> with per-pixel alpha content on the primary plane when requested.
> I wasn't aware of this assumption, but given that it is the case,
> the patch looks like a good fix for the problem.
> Unfortunately I don't have a hardware platform up and running to test the
> patch with at the moment.

HWC1 has this ominous comment:

* IMPORTANT NOTE: There is an implicit layer containing opaque black
* pixels behind all the layers in the list. It is the responsibility of
* the hwcomposer module to make sure black pixels are output (or blended
* from).

It's not repeated in the HWC2 docs, but I think the assumption carried over
given that Android prefers all things RGBA. Admittedly it's rare that you
don't have a full-size opaque layer in the composition like a background
picture, so the cases where this causes corrupted rendering on VC4 are limited
to some time during boot after the animation finishes but the launcher isn't
up yet.

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