(In reply to comment #18)
> (In reply to comment #17)
> > Make sure you have the updated RLC ucode in addition to the UVD ucode. 
> > Unfortunately, I can't reproduce this on the UEFI systems I have access to.
> 
> Benjamin and Parag have already indicated which firmware versions they are
> using, and I feel it's safe to assume that Christian is also using the
> proper versions as well.

It's worth double checking.  I've forgotten to update my ucode a number of
times and run into similar problems.  Christian was using out of date ucode and
when he fixed it his UEFI system started working fine.

We can take a look at register dumps.  grab radeonreg:
http://cgit.freedesktop.org/~airlied/radeontool/
and dump the regs when booted with legacy vbios vs. UEFI and attach the
outputs.

boot with legacy bios (as root):
radeonreg regs all > legacy.regs
boot with UEFI (as root):
radeonreg regs all > uefi.regs