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Thunderbolt dock works through front port, but not back port

rated 0 times [ 0] [ 0] / answers: 1 / hits: 1205 / 3 Years ago, sat, july 31, 2021, 5:38:14

I have a Lenovo Yoga c930 and a Lenovo Thinkpad Thunderbolt 3 Gen 2 dock. This dock features two USB C/Thunderbolt 3 ports, let's call them the front one and the back one.

The dock's manual explicitly states the laptop should be connected to the back port, however I find that the dock only works (in a sense to be specified below) when connected to the front port. However, I experience issues which I suspect are due to being connected through the front port, so I want to try and get the back port working.

When I connect the laptop to the front (not recommended by manual) port:

(1a) USB devices connected to the dock are seen and usable by the laptop.

(1b) A monitor connected to the dock's DisplayPort port is seen by the laptop and receives the image correctly.

(1c) The laptop charges at a slow rate (so slow that it decharges while playing a game).

(1d) I experience occasional USB issues (USB functionality ceases suddenly until I reboot).

When I connect the laptop to the back (recommended by manual) port:

(2a) USB devices connected to the dock work.

(2b) The same monitor connected to the same DisplayPort port does not work. The output of xrandr claims to see it and says it's connected, and I can see it in the Ubuntu display settings, but the monitor itself says "No signal". However, if I boot into Windows 10, I get an image on the monitor no problem.

(2c) The thunderbolt dock itself does not seem to be seen by the system. For example, if I run boltctl, the output says the dock is disconnected, as does the Thunderbolt settings manager in the Ubuntu settings.

(2d) The laptop charges, possibly faster than when connected via the front port, but I'm not sure.

I am willing to chalk up problems (1c) and (1d) to using the wrong port on the dock. Therefore I am interested in fixing problems (2b) and (2c).

Additional information obtained after this question was posted:

05/06/2021 I booted into Windows and used the pre-installed "Lenovo Vantage" program to download and install a BIOS update. I then downloaded and installed the driver and the firmware update at this link. No change in behavior.

06/06/2021 I made a live USB stick of Ubuntu 20.04 (i.e. the same version I'm using currently) and booted into it in "Try Ubuntu" mode. The back port works! That is, USB devices continue to function (tested a mouse and a hard drive), and the laptop charges, but now the monitor connected through DisplayPort also gets an image. However, boltctl does not see the dock. This makes me suspect that the back port of the dock is in fact USB C and not Thunderbolt 3 (although I don't know enough about either protocol to really know if that statement even makes sense). Of course, when I boot back into my usual Ubuntu 20.04 install, the problems have not been fixed. As far as I know, I am using the latest version of 20.04 (the About page in the settings says 20.04.2).