https://bugs.freedesktop.org/show_bug.cgi?id=79850
--- Comment #18 from Sylvain BERTRAND sylvain.bertrand@gmail.com --- Ok... I did a bit of gdb.
It's a bloody mess, many things are optimized out and breakpoints are not working consistently.
I got the DRI2CanFlip function to return FALSE for those lines: dri2.c:934 and dri2.c:945
That said, I was unlucky about can_flip debugging, since I hit only the TRUE returning code paths of DRI2CanFlip.