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·· -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1938999 Title: External displays not working on Thinkpad T490 with ThinkPad Thunderbolt 3 Dock To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1938999/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
