https://bugs.freedesktop.org/show_bug.cgi?id=18953

--- Comment #4 from Michel Dänzer michel@daenzer.net 2011-09-12 01:36:29 PDT --- (In reply to comment #3)

But may be mine is a different error ?

It most definitely is. If you can still reproduce your problem with current upstream versions of xserver (1.11) and xf86-video-ati (6.14.2), please file your own bug.