I am having a similar problem with my 4870. If i disconnect my second monitor I can boot with radeon.dmp = 1. If I reconnect the monitor again it will lock up shortly after boot. A bisect brings up this commit https://github.com/torvalds/linux/commit/ab70b1dde73ff4525c3cd51090c233482c50f21 Which makes sense since this commit enabled dpm by default on radeon x7xx series cards.