The specific problem I have is as follows. I also use my PC as a mediacenter. I like to use XBMC as mediacenter software which can only use GL as video out (vo). I use Metacity as my window manager then, without compositing. I use an Ati Radeon 9600 videocard now, I used to have an Nvidia card before, which worked just fine w/ XBMC. The old Ati card is not supported by the proprietry 'fglrx' driver from Ati/AMD. So I use the (exellent) open Xorg driver that's simply called "radeon". I activate the TV-out on the Radon to watch my movies and DVD's of course. No clone mode: I disable the CRT monitor (xrandr --output VGA-0 --off).
Using XBMC to watch moviesis somewhat the same thing as using mplayer with the command line option "-vo gl". On my TV the video tears in XBMC and w/ 'mplayer -vo gl' and I get the error from mplayer:
If I use 'mplayer -vo gl' on the CRT monitor I get the (classic) error that my PC is 'too slow too play this'. If I use 'xv' as vo then there's no error. That's why I think vsync is broken in the 'radeon' driver for GL. I don't know exactly which version of the radeon driver I use, but I think it's the latest because I use Ubuntu Lucid (10.04)and Synaptic says about radeon: version 6.13.0.
do_wait: drmWaitVBlank returned -1, IRQs don't seem to be working correctly. Try adjusting the vblank_mode configuration parameter
P.S. Every thing works fine when I use xv for video output. But not all applications can use xv (or x11): some only use gl (i.e. XBMC).