[Bug 1938999] Re: External displays not working on Thinkpad T490 with ThinkPad Thunderbolt 3 Dock

I could reproduce this issue on L15, when I insert DP cable, it show a
lot of '*ERROR* Link Training Unsuccessful'. And from the xrandr I found
the DP monitor was recognized, and I could move mouse to this external
monitor, but the screen is black.

Aug 12 23:45:05 u-ThinkPad-L15-Gen-1 kernel: i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful
Aug 12 23:45:05 u-ThinkPad-L15-Gen-1 kernel: i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful

u@u-ThinkPad-L15-Gen-1:~$ uname -an
Linux u-ThinkPad-L15-Gen-1 5.11.0-25-generic #27~20.04.1-Ubuntu SMP Tue Jul 13 17:41:23 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

$ xrandr
DP-1-3 connected 1600x900+1920+0 (normal left inverted right x axis y axis) 434mm x 236mm
1600x900 59.98*+
1280x1024 75.02 60.02··
1152x864 75.00··
1024x768 75.03 60.00··
800x600 75.00 60.32··
640x480 75.00 59.94··
720x400 70.08··

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https://bugs.launchpad.net/bugs/1938999

Title:
External displays not working on Thinkpad T490 with ThinkPad
Thunderbolt 3 Dock

Status in linux package in Ubuntu:
Confirmed

Bug description:
I have two external displays chained via Display port connected to the
ThinkPad Thunderbolt 3 Dock.

They are working with kernel 5.8.0-63 but not with the new 5.11.0-25
kernel.

I could reproduce this behavior several times for both kernels, as
follows: I booted up with disconnected dock, logged in, then I
connected it, disconnected it, and connected it again.

With the 5.11.0-25 kernel the external displays go black; the main
screen turns off and on several times. Sometimes the main screen
remains working, but UI is lagging.

I'm attaching the logs. Two separate boots for each variant are marked
'a'/'b'.

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