(In reply to James Le Cuirot from comment #0)
> This has similarities to bug #69120 but I believe that to be a different
> issue because it involves much older kernel versions and a lot has changed
> since then, plus it used to work for me until 3.18.

Yeah, that is indeed a completely different issue, so opening up a new bug
report was the right thing to do.

> [drm:rv770_dpm_set_power_state [radeon]] *ERROR*
> rv770_restrict_performance_levels_before_switch failed
> 
> This led me to try booting with radeon.dpm=0. Under the high profile, low
> quality is smooth and high quality improves to just jumpy. Under the dynpm
> method, both are smooth.
> 
> I have two displays connected using Zaphod mode, both normally at 1080p. If
> I disconnect the second, playback is smooth. If I set the second to some low
> resolution like 720x480 but play 1080p video on the first, playback is
> smooth. I'm not sure whether this behaviour is a symptom or a cause.

Thanks for the detailed report, but unfortunately I can't help much and Alex
need to take a look at this.

The issue is that the driver send a message to the SMU to raise the clocks for
playback and with two connected monitors that fails for some reason.