The Current State Of Radeon Power Management
First up is the AMD Radeon HD 6950 2GB graphics card from Sapphire.
When the system was idling at the Unity desktop, the stock (default) configuration left the graphics card running with a core clock of 800MHz, which is the card's highest power state. In the middle state, the Cayman GPU was idling at 500MHz. In the lowest power state, the GPU was clocked at 250MHz, which is the same as what the Catalyst driver put the graphics card at when running in its stock configuration. When using the open-source Radeon driver's dynamic power management mode, the core idled at 500MHz (the middle state) and not the lowest clock speed of 250MHz.
When the Radeon HD 6950 was idling, the lowest core temperature was reported when using the Catalyst driver. While the open-source driver had also dropped the Cayman core down to 250MHz, the average temperature under the Catalyst driver was 34.5C where as the low power state was at 38C, likely due to other power-saving features only implemented in the Catalyst driver. In the stock configuration without any power management tweaks, the graphics card idled at just under 60C, or a 25C increase over the Catalyst driver in its lowest power state. That is quite a warm difference.
In terms of the voltage fed to the core when controlled by the open-source Radeon DRM (since voltage monitoring isn't exposed to user-space on the Catalyst driver), in the stock configuration the HD 6950 was at 1140mV, but only 1100mV when forced to the high power state. The low state was at 900mV and the middle voltage was reduced to 1020mV, as was the dynamic PM mode.