These are specified by ATOM_VOLTAGE_OBJECT structure in VOLTAGE_OBJECT_INFO table, so I guess the driver does initialization from VBIOS on startup and then keeps the values as initialized. It's not possible to change these via PowerPlay as it's a different table, but it would be possible to add access to them via sysfs I think.

I already started working on adding interface to read GPU load, consumption, etc, so these would be just additions to it. What I don't like on this is that the amdgpu driver crashes when I try to unload it, so when I compile a new version I have to reboot the machine, which is kinda annoying.

I would check accessing these registers tomorrow