If you are making regular upgrades to your Linux distribution, refrain yourself from upgrading to xorg-server-1.19.x version. The latest X versions does not handle the V-sync redraw properly.
The latest Xorg server working without issue with GM is v1.18.4.
If you still need to use Xorg 1.19x version, you have to enable the ShadowPrimary option in xorg.conf (see example below). The impact on input latency is negligible. Measured latency is 2 to 3 frames.
Option "ShadowPrimary" "boolean"
This option enables a so-called "shadow primary" buffer for fast CPU access to pixel data, and separate scanout buffers for each display controller (CRTC). This may improve performance for some 2D workloads, potentially at the expense of other (e.g. 3D, video) workloads. Note in particular that enabling this option currently disables page flipping. The default is off.
Section "Device"
Identifier "Radeon"
Driver "radeon"
Option "ShadowPrimary" "on"
EndSection
<@admin, can this thread be flagged sticky?>