There are some folks on the Windows side of the fence that have had great success under-volting their Vega M chips to improve thermals (and because TDP is shared between the CPU and GPU on Kaby Lake G, also their overall performance).

Following information from the amdgpu kernel documentation I applied the following kernel boot parameters:

amdgpu.runpm=0 amdgpu.ppfeaturemask=0xffffffff

Note: I needed the runpm=0 so that the Vega M is active at all times, otherwise it powers down and I'm unable to make any changes to the pstates using the sysfs API.

Then doing the following I was able to see the current pstates:

cat /sys/bus/pci/drivers/amdgpu/0000:01:00.0/pp_od_clk_voltage

OD_SCLK:
0:        225MHz        750mV
1:        400MHz        750mV
2:        535MHz        750mV
3:        715MHz        750mV
4:        850MHz        750mV
5:        960MHz        750mV
6:        985MHz        750mV
7:       1011MHz        750mV
OD_MCLK:
0:        300MHz        750mV
1:        500MHz        750mV
2:        700MHz        750mV
OD_RANGE:
SCLK:     225MHz       1011MHz
MCLK:     300MHz        700MHz
VDDC:     750mV         750mV

Then this to support updating the pstates:

echo "manual" > /sys/bus/pci/drivers/amdgpu/0000:01:00.0/power_dpm_force_performance_level

Then I could push in new pstates values, provided the voltage was *exactly* 750mV.  I could set a higher clock speed for a pstate, but I was unable to make any changes to the voltage.

echo "s 0 226 750" > /sys/bus/pci/drivers/amdgpu/0000:01:00.0/pp_od_clk_voltage

works and increases the MHz from 225 -> 226 for pstate 0, but

echo "s 0 226 749" > /sys/bus/pci/drivers/amdgpu/0000:01:00.0/pp_od_clk_voltage
-bash: echo: write error: Invalid argument

Checking dmesg I see that the voltage range is constrained between 750 and 750.

[ 4362.487021] amdgpu: [powerplay] OD voltage is out of range [750 - 750] mV

Would it be possible to add support for under-volting Vega M using the sysfs API?

I also noticed that it's not possible to set any pstate higher than 1011 MHz, perhaps that's a harder sell, but it might also be nice to be able to overclock the Vega M a little!

P.S. I tried finding these values in the kernel source code, but I was extremely confused about the way they are defined.  I figured worse case scenario I could manually modify these limits and recompile the kernel.

Thanks!
Rob