Comment # 170 on bug 66963 from Thierry Vignaud

I don't know if it can helps you (Alex), but as of kernel-3.12.6/mesa-10.0.1,
my RV670 ([Radeon HD 3690/3850]) is pretty reliable when booting wit dpm
(usually for resuming hibernation):
- it consistently fails on the first electric power on
  (the screen blacked out, "no signal")
- it consistently works smoothly after powering off then
  powering on after a couple seconds: it resumes smoothly

Does it helps you figure what can be wrong?