https://bugs.freedesktop.org/show_bug.cgi?id=79980
--- Comment #17 from Alex Deucher agd5f@yahoo.com --- (In reply to comment #15)
This bug is caused by TAHITI_mc2.bin firmware. The old firmware works good.
Did you test a new kernel with the old firmware or an old kernel without the new firmware patch? It could be some other change if you did the latter.