https://bugs.freedesktop.org/show_bug.cgi?id=39202
--- Comment #27 from Michel Dänzer michel@daenzer.net 2011-08-18 03:00:16 PDT --- (In reply to comment #25)
There is a sequence of commits in drm, some of them related to radeon and vblank.
I guess it's most likely commit 19b01b5fbf0b78930b3b06ee6080539c17b5d1fd ('drm/kernel: vblank wait on crtc > 1') that makes the difference. Without that commit, the X driver completely disables vsync functionality with your card, as it can't work on some of the CRTCs.
What I don't understand is why the suggested workarounds don't restore the previous behaviour... did you leave all of them (disabling page flipping and SwapBuffersWait — verify in Xorg.0.log that they're really disabled — and vblank_mode=0) enabled at the same time? Missing any one of them could result in vsync being effectively enabled.