[Bug 202043] New: amdgpu: Vega 56 SCLK drops to 700 Mhz when undervolting

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=202043

            Bug ID: 202043
           Summary: amdgpu: Vega 56 SCLK drops to 700 Mhz when
                    undervolting
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.19.8, 4.20.0-rc6
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri at kernel-bugs.osdl.org
          Reporter: antifermion at protonmail.com
        Regression: No

When undervolting my Sapphire Pulse Vega 56 by just 1mV, SCLK immediately drops
down to 700 Mhz and pstate 1-2 under load (`gputest /test=fur /width=1920
/height=1080`).

Script to undervolt:
```
echo "s 7 1630 1199" > /sys/class/drm/card0/device/pp_od_clk_voltage
echo "c" > /sys/class/drm/card0/device/pp_od_clk_voltage
```

Stock voltage would be 1200 on the Vega 64 Bios.
The same behavior can be observed with the stock Vega 56 Bios.
Undervolting the memory by 1mV results in similar behavior.
Overvolting by 1mV has no discernable effect.

`echo r > pp_od_clk_voltage` does not work to go back to the normal behavior.
Instead, I need to use `echo "s 7 1630 1200" > pp_od_clk_voltage` as above.

Without undervolting, SCLK is around 1330 Mhz, which matches the behavior on
Windows, where undervolting by around 150 mV is no problem and increases clock.

With an increased power limit of 300W, the clocks increase to around 1100 Mhz
while the card uses the full 300W.
It even maxes that limit with a significant underclock/undervolt which would
pull around 200W on Windows.

I tested with current Manjaro (4.19.8-2-MANJARO), as well as Kubuntu 18.10 with
stock (4.18) and 4.20 from
https://github.com/M-Bab/linux-kernel-amdgpu-binaries.

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