Hey,
On 02/23/2018 03:34 PM, Stefan Schake wrote:
Hey Eric,
On Thu, Feb 22, 2018 at 9:47 PM, Eric Anholt <eric@xxxxxxxxxx> wrote:
Stefan Schake <stschake@xxxxxxxxx> writes:
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@xxxxxxxxx>
---
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).
Couldn't we just ignore the alpha channel on the primary plane, on the
assumption that it's supposed to be all zeroes below it? Or are we not
premultiplied, so we do still need to multiply the primary plane's
colors by the alpha value? I couldn't find any obvious DRM
documentation about whether alpha is premultiplied.
That would work, too (for a black background anyway). Though the easiest place
to spoof this is presumably in the kernel. From what I have seen, everything in
Android is already premultiplied, but technically it can specify
either. Not sure if this
is correctly handled in drm_hwcomposer yet.
To me Erics solution sounds pragmatic, and would be easy to reason about going
forward. Making sure that all of the kernel drivers behave the same way might be
the hardest part.
Stefan: Are you looking at an entirely kernel side fix now, or are you pushing
this series forward?
Rob.
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