Renoir kernel warnings and crashes with external monitors (DP mst)

[Qu Wenruo]

Hi guys,

Firstly thanks for the awesome renoir CPUs.

However it looks like there are still some bugs in the Linux support,
especially with external monitors.

My setup:
- Laptop: Lenovo IdeaPad S540 13ARE (Or, Xiaoxin Pro 13 in Chinese market)
  4800U CPU, with 5.7.4 Arch kernel. (Also tried 5.8-rc kernels, even
more crashes)
  The eDP display is 2560x1600 natively, but to cooperate with external
monitors, it's
  set to 1920x1200.
  And under my daily usage, the lid is closed, thus the eDP display
shouldn't be involved.

- Two external monitors connected using USB-c dock
  The dock is HP usb-c dock G4, which uses DP alt mode MST for external
displays.
  So not DisplayLink garbage, and all monitors are handled by Vega
internal GPU.

  Both external displays are 1080P 60hz, so definitely inside the spec
for DP 1.2.

- X11 environment
  xf86-video-amdgpu, along with mesa stack.

With such setup, the kernel boots with warning already related to DP MST:
https://paste.opensuse.org/54293967

Furthermore, under certain GPU load (?), the graphic stack just crashes
completely.

The reproducer is a little complex to setup (involves steam Linux and
CNC remastered collection, which needs to download 15G contents):

- Steam, runtime from Arch repo

- Purchase "Command & Conquer Remastered Collection"

https://store.steampowered.com/app/1213210/Command__Conquer_Remastered_Collection/

- Use Proton 4.11-13
  As Proton 5.x failed to start the game
- Run the game

Then the graphic stack would crash, the external monitors lost signal,
and internal display won't react to any input, have to force shutdown
the system.

Furthermore, if not using the external monitors (disconnected from
dock), the game starts without any problem.
Thus it should be a bug in the MST routine.

The full dmesg (without crash) can be fetched here:
https://paste.opensuse.org/92544722

No call trace for the crash though, if really needed, I could setup
netconsole to try to catch the dying message.

Thanks again for the awesome hardware and the awesome opensource graphic
stack!
Qu

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