Voltage control on Southern Island GPU using radeon driver

[Evans Jahja]

Hi, I have a HAINAN GPU below:

lspci -nn
0a:00.0 Display controller [0380]: Advanced Micro Devices, Inc. [AMD/ATI]
Sun LE [Radeon HD 8550M / R5 M230] [1002:666f]

I run linux 5.13.12 on Arch on a Lenovo B40-70 laptop.

I'm trying to understand more on how voltage control works and how I can
modify the voltage for doing overvoltage / undervoltage on my GPU. The
reason is I am observing how running programs under high GPU load (glmark2)
would lead to crashes when I use dpm=1 in either radeon or amdgpu driver,
which seems to happen when I am reaching power level 4 (sclk 900MHz), while
a lighter program like glxgears could run and switch power levels  between
0,1,2 without issue under both drivers. I believe my laptop might be
faulty, but I would like to take this opportunity to try fixing it from the
driver's side so that it can run anyway, however limited.

Right now, I have managed to increase the performance of my GPU by manually
overwriting the sclk to 630MHz in all performance_levels in radeon_pm.c,
which surprises me as overriding the clock was not possible for me to do
previously via sysfs.

I've managed to tweak both sclk and mclk (or so I believe), but I still
cannot tweak the voltage (vddc). The reason is, if I increase the sclk to
650MHz, the lockup will happen again. Changing the pl->vddc  variable does
not seem to do anything. After various tracing with printk, I understand
that on my system:

pi->voltage_control = radeon_atom_is_voltage_gpio(rdev,
SET_VOLTAGE_TYPE_ASIC_VDDC,
   VOLTAGE_OBJ_GPIO_LUT)

this returns false, while:

si_pi->voltage_control_svi2 =
radeon_atom_is_voltage_gpio(rdev, SET_VOLTAGE_TYPE_ASIC_VDDC,
   VOLTAGE_OBJ_SVID2);

This returns true, so I believe my system is using SVI2 somehow to set the
voltage. Having no experience with SVI2, I read online and found out that
SVI2 is a voltage regulator that uses Data / Clock pins to clock-in 8 bits
of information and convert it to some voltage value between OFF, 0.5V ->
1.5V, offering fine control based on some look up table.

My questions are as follows:
Is it possible for me to modify my system so that I can manually adjust the
voltage to my GPU?

Thank you very much in advance. This is the first time I deal with kernel
drivers, so any guidance on the matter helps a lot.

- Evans
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