This changes how we synchronize through vsync. We use a mutex and a
wait condition to synchronize the threads. When presenting the frame
our main gui thread blocks and will be woken up by the vsync event
(or a timeout of max 1 frame time slot). In order to minimize the
blocked time we use the blocksForRetrace functionality from the GLX
compositor.
Given this change we no longer need to tell the compositor that we
are swapping the frame, it's blocked anyway. Also we don't need the
failsafe QTimer anymore.
With this change applied on a Nexus 5 it's succeeding the "Martin
tortures phone test". It doesn't tear anymore and has a smooth
experience.
I'm rather disappointed by the fact that we need to block in order
to get vsync. This means Android/hwcomposer is as bad as GLX. So
much for the "Android stack is so awesome", in fact it's not. Anybody
thinking it's awesome should compare to DRM/KMS and especially atomic
modesetting. Yes it's possible to present frames without tearing and
without having to block the rendering thread.
Reviewed-By: Marco Martin and Bhushan Shah
Note: qt5-qpa-hwcomposer-plugin does the vsync in a different way:
it uses a wait condition to truly block in present till the vsync.
Maybe we need to do that as well.
Based on a previous patch done by David Edmundson and heavily
inspired by qt5-qpa-hwcomposer-plugin [1].
The change requires a newer libhybris than the one used by Ubuntu. In
fact it allows to build against current master (at the day of writing [2]).
REVIEW: 125606
[1] https://github.com/mer-hybris/qt5-qpa-hwcomposer-plugin
[2] bd6df6a306
This backend interacts with libhybris to create a hwcomposer which is
used for creating the egl context and surface. The initial version of
this backend is based on test_hwcomposer.cpp provided by libhybris.
Please note that using the hwcomposer backend requires a newer libepoxy,
the latest stable release is not able to bring up OpenGLES, so one needs
a master build of libepoxy.
Notes on licensing:
libhybris is Apache 2.0 licensed, which is not compatile with GPLv2.
But it is compatible with GPLv3. Thus the source files in the hwcomposer
backend are licensed GPLv3+ and not GPLv2+ as the rest of KWin. If one
uses KWin without the hwcomposer backend (which is obviously the default)
the licence doesn't change. But if the hwcomposer backend is used the
overall license of KWin changes to GPLv3+.
