Hello,
with Polaris and Vega, setting amdgpu.ppfeaturemask=0xffffffff worked without
issues here: It unlocked pp_od_clk_voltage and didn't cause any issues for me.

But with Navi, it doesn't work. I'm still not allowed to open
/sys/class/drm/card0/device/pp_od_clk_voltage
as root with specifying that flag.

Also, I can't increase the GPU's power consumption, as
/sys/class/drm/card0/device/hwmon/hwmon0/power1_cap_max
only allows the default 100% Powertune limit, meaning I can't set any higher
value in
/sys/class/drm/card0/device/hwmon/hwmon0/power1_cap

Apart from not being able to change the aforementioned parameters, setting
amdgpu.ppfeaturemask=0xffffffff causes stuttering, even on the desktop and also
affects the mouse cursor.

This is with kernel drm-next-5.5-wip 73cdff347343504287feae8b36fa7317f04dcc61
and an MSI 5700 XT Gaming X.