Uday Kiran Pichika authored Jun 18, 2020 and Martin Roukala committed Sep 08, 2020

These changes have been ported from AMD GPU DDX driver.

This patch adds support for setting the CRTC variable refresh property
for suitable windows flipping via the Present extension.

In order for a window to be suitable for variable refresh it must have
the _VARIABLE_REFRESH property set by the MESA and inform Modesetting
DDX driver with window property updates.

Then the window must pass the checks required to be suitable for
Present extension flips - it must cover the entire X screen and no
other window may already be flipping. And also DRM connector should
be VRR capable.

With these conditions met every CRTC for the X screen will have their
variable refresh property set to true.

Kernel Changes to support this feature in I915 driver is under development.

Tested with DOTA2, Xonotic and custom GLX apps.

Signed-off-by: Uday Kiran Pichika <pichika.uday.kiran@intel.com>