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

I tested again on kernels 5.8 and 5.11 with nvidia-driver-470. Logs are
attached.

With 5.8 kernel, the external displays eventually come to live, after
connecting the dock and some waiting (~15 s). After first connection, it
seemed to be doing several trials, in the end it worked too.

With 5.11 kernel, the external displays remain black, and computer was
lagging or almost frozen.

I'll see what I can do with 5.13 kernel.

** Attachment added: "Logs from 5.8 and 5.11 kernels with nvidia v470 driver"
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1938999/+attachment/5517301/+files/logs_2021-08-11.zip

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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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