> What does
> 
>  env | grep DRI_
> 
> say?

That printed nothing.

My session with runtime pm enabled (no radeon.runpm=0 in cmdline) had been
running for a couple of hours without problem (apart from a bit of freezing at
the start). However, just after running that command, some new radeon errors
appeared in dmesg that I haven't seen before. I think they were ring test
failures. The PC has locked up now anyway so I can only hard shut it down. I
was switching ttys with CTRL+ALT at the same time which might have caused it.