** Changed in: linux (Ubuntu Hirsute)
Status: In Progress => Fix Committed
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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
Status in linux source package in Hirsute:
Fix Committed
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'.
SRU Justifications:
===============
[Impact]
External displays not working on Thinkpad T490 with ThinkPad Thunderbolt
3 Dock
[Fix]
Some SKL systems DP output fails when AUX timeout of LTTPR detection.
Disable the LTTPR detection until GLK to fix the link training.
[Test]
Verified on hardware, external DP work fine.
Also tested on I+N GPU laptop and a iGPU only laptop, all displays with dock
are good.
[Where problems could occur]
It may break output of external DP.
This patch from 5.12-rc5, only Hirsute kernel is SRUed.
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