https://bugzilla.kernel.org/show_bug.cgi?id=60523
--- Comment #20 from Alex Deucher alexdeucher@gmail.com --- Christian, Can you try this patch: http://lists.freedesktop.org/archives/dri-devel/2013-August/043464.html I the vblank period on your monitor is short enough that it's causing the driver to select the multi-head case to avoid mclk switching. You should also try the patch in comment 17.