[RFC PATCH hwc] drm_hwcomposer: set CRTC background color when available

[Maarten Lankhorst]

Op 29-06-18 om 12:27 schreef Stefan Schake:
> Hey Maarten,
>
> On Fri, Jun 29, 2018 at 12:05 PM, Maarten Lankhorst
> <maarten.lankhorst at linux.intel.com> wrote:
>> Op 22-02-18 om 04:54 schreef Stefan Schake:
>>> 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.
>>>
>>> On some platforms (e.g. VC4) a background color fill has a cycle cost for
>>> the hardware composer and is not enabled by default. Instead, userland can
>>> request a background color through a CRTC property. Use this property to
>>> specify the implicit black background Android expects.
>>>
>>> Signed-off-by: Stefan Schake <stschake at gmail.com>
>>> ---
>>> Kernel changes for this (background_color) are available here:
>>>
>>> https://github.com/stschake/linux/commits/background-upstream
>>>
>>> Sending as RFC because I'm not entirely clear on whose responsibility
>>> this should be, on most DRM drivers it seems to be implicit. I think
>>> a case could also be made that VC4 should not accept alpha formats on
>>> the lowest layer or enable background color fill when given one anyway.
>>> On the other hand, userland control over background color seems desirable
>>> regardless and is a feature of multiple hardware composers (i915, vc4, omap).
>> Ping? Would be nice if we were moving forward. :)
> I was unclear if DRM specified a black background when writing this.
> Someone pointed me towards the excerpt in the docs that explicitly
> mandates black background fill and so I ended up writing a VC4 patch
> that automatically sets it when required instead of the optional property
> used here.
>
> Adding a background_color property would still be desirable, but I'm
> unclear on what the userspace would be at the moment. drm_hwc doesn't
> need any background color other than black and since that is the DRM
> default, it wouldn't need to use a property.
>
> Thanks,
> Stefan

It would still be useful for e-paper displays where the background color would be better suited
as white, or when we want to blend a different color than black. But I fear for now there's no
real userspace. :(

Which is unfortunate, because the series are easy to implement, but it's just a case where there's
no need from userspace yet for the hw capability.

~Maarten