Add support for forced VSYNC using the EGLFS platform plugin.

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>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/plugins/platforms/eglfs/qeglfshooks.h b/src/plugins/platforms/eglfs/qeglfshooks.h
index f3b6a28e70..fc1ee16073 100644
--- a/src/plugins/platforms/eglfs/qeglfshooks.h
+++ b/src/plugins/platforms/eglfs/qeglfshooks.h
@@ -69,6 +69,7 @@ public:
     virtual bool hasCapability(QPlatformIntegration::Capability cap) const;
     virtual QEglFSCursor *createCursor(QEglFSScreen *screen) const;
     virtual bool filterConfig(EGLDisplay display, EGLConfig config) const;
+    virtual void waitForVSync() const;
 
     virtual const char *fbDeviceName() const;
 };