I have been attempting to undervolt my Vega64 to reduce power consumption.

I have tested the GPU under Fedora 29, running stock kernel 4.20 and mesa 18.3, and Mint 19.1, with Kernel 5.0 and mesa 19 and the issue is identical

I have set the AMDgpu mask in grub to amdgpu.ppfeaturemask=0xffffffff.

I can confirm the GPU accepts custom values that I write into pp_od_clk_voltage, and the GPU run on these, but the voltage create a strange profile.

The voltages abide to the new settings, but only in the hysteresis/dead band around the clock in that P state.

Soon as the clocks move out of this band, the voltages go max, i.e. 1.2V.
Under 2D workload, the card seems fine, but soon as it is in 3D work loads, running in P5 and up, the voltage control is a problem, esspecialy if I'm at only 1450Mhz and running at 1.2V

If I set P state, P6, to 1550MHz @ 1000mV and P7 to 1620MHz @ 1050mV and the clock is around 1550MHz, I will see that it has set the voltage to 1000mV, but if that freq increase to, say, 1580MHz, the GPU will be set to 1200mV.

I suspected that maybe the reading I saw was incorrect, but it cannot be incorrect, as the GPU performs worse after the tweak, generally battling to leave P5 state, where stock clocks, it will stick closer to the P6 state clocks.

My GPU also unfortunately has coil whine, but this does give audio cues with load and frame rate.

The GPU changes pitch when it moves in and out of the set voltage. The GPU whine will be quieter when it landed in the reduced voltage state, and immediately increases once it jumps to the 1.2V state. 
The issue is also that it jumps between the reduced voltage and full voltage when it is boarder line, as the GPU starts to throttle when the voltages jump to 1.2V

Essentially this is how I see the GPU react to my new voltage profile (leaving the clocks at stock).

         P5                  P6                   P7
-------|     |-------------|     |--------------|     |-----1200mV
       |_____|             |_____|              |_____|
        980mV               1000mV              1050mV

So far all I can really change is the power state without causing issue, and this helps with performace when I increase it, at the cost of massive heat production