https://bugs.freedesktop.org/show_bug.cgi?id=28375
--- Comment #15 from Rafał Miłecki zajec5@gmail.com 2010-06-04 13:48:12 PDT --- I can confirm it's voltage setting that causes problem. Dropping it from r600.c::r600_pm_misc makes d-r-t work again.
I didn't test your patch Alex, but it does not make much sense in my case.
1) Looking through all power modes I noticed only 2 different voltages: 950 and 1200. 2) Step is 250 anyway: 0018: USHORT usVoltageStep = 0x00fa (250)