If you mean if it's possible to change the `VDDCOffset` in SCLKDependencyTable then yeah it is, the question is if the AMDGPU driver would honor it, I think it won't.
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I would like to add a more automatic solution to do undervolting, but that would take some time to do properly. I looked at the amdgpu driver and I was not able to figure out if it's using the voltage offsets or not. It seems it has no effect, but I'm not completely sure.
I think that the driver itself can't do this. It is more related to edit the registers in the VRM controller (i.e IR3567B, NCP81022), that is what Wattool or VRMtool do. Can deal with voltage, but also frequency and more.
Can you access to i2c bus via driver?