Problem scenario
Ubuntu won't boot. You get "nouveau drm failed to idle channel" upon boot up. What should you do?
Possible Solution #1
Press enter. You should then see a prompt to log in.
Possible Solution #2
Press Ctrl-Alt-F3
Possible Solution #3 (if it does not happen at boot up, but you see a similar message)
See this external posting, but remember to read the question too for context.