https://bugs.freedesktop.org/show_bug.cgi?id=69062
--- Comment #6 from LRN lrn1986@gmail.com --- (In reply to comment #5)
How about setting env var R600_DEBUG=nodma? Try adding that to /etc/environment .
Added to /etc/environment, rebooted - no visible improvements (should the fact that this variable is set be mentioned in Xorg log or dmesg? i don't see anything specific there).
Do you want logs with more verbosity? I remember that there's a boot parameter that gives verbose logs for radeon driver.
Another bit of info (may be related): lightspark has huge problems playing youtube videos (frames jump around in time, , some parts of the picture are corrupted, and the whole thing looks choppy). Iceweasel's stderr is full of "GL error 1282"