https://bugzilla.kernel.org/show_bug.cgi?id=42920
--- Comment #3 from Alex Deucher alexdeucher@gmail.com 2012-03-13 14:13:34 --- radeontool is for older chips. The register is at a different location on them. Note I said radeonreg rather than radeontool, but either will work as long as you specify the proper offset (0x5428).