Before a buffer swap the new QEglFSHooks::waitForVSync method is
called which looks at QT_QPA_EGLFS_FORCEVSYNC and - if that is set
and non-null - calls ioctl with the FBIO_WAITFORVSYNC request on
the framebuffer device.
This is required on some embedded platforms where the driver does not
support VSYNC yet the Kernel provides a generic implementation.
I tested this using QML_RENDER_TIMING=1 which proofs that the frame
rate for an example of mine drops from >125fps to a straight ~60fps
with a few frames that take ~33ms (i.e. 30fps) as expected for VSYNC.
To prevent excessive open/close calls on the frame buffer device
per frame, the file descriptor is now cached. To keep the QEglFSHooks
interface as clean as possible this is done via a global static in
qeglfshooks_stub.cpp and initialized and freed in platformInit and
platformDestroy.
Change-Id: I4d31b227c65ff22aa089db0fbc62c89a59cbb6c7
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
4f14b42f7d