Created attachment 103116 [details]
Screenshot of Gnome shell

Please have a look at the screenshots. Rendering of most windows is totally
garbled including window decoration. This is not about Crossfire/SLI/Multi GPU
rendering, just plain single GPU.

Gnome-shell's own rendering is mostly fine. Panel, lock screen animations,
menus and dashboard don't show distortions. 
KDE's/Kwin is worse, even the panel's are broken, but it's able to draw window
decorations.
glxgears's rendering is fine.
Linux console (no X11) is also fine.


Hardware:
PowerColor AX7990 6GBD5-2DHJ, which is one of those fake 7990s that actually
use 2x7970 Tahiti XT on a single PCB and not 2x Malta. Other GPUs (6950/7950)
were working fine with that system.

Software:
Fedora 19/20 (From stock LiveCD image to current updates incl. Kernel 3.15.4)
There were no (drawing!) issues when I was using catalyst.