Built drm next yesterday was running drm fixes from a couple of weeks ago previously, card is rv790.

Have powered up twice since and both times seen logged -

[drm:rv770_dpm_set_power_state] *ERROR* rv770_restrict_performance_levels_before_switch failed

Today was 79 sec after boot, though I don't startx automatically so it may have been soon after that.

Clock switching does seem to be working OK.