I'm running Ubuntu 20.04 on a ThinkPad P14s Gen 1 with internal 4K display. I'm currently running the Intel Mesa drivers on X11, but the behavior I'm about to describe is the same with the proprietary Nvidia drivers.
I have two LG UltraFine 4K external displays, that were originally designed for Macs. These are great displays, as they have an unmatched pixel density, connect via USB-C and can also charge the computer through the same cable. They don't have buttons, but there is third-party support for changing the brightness via a custom USB protocol.
- 22MD4KA-B 21.5" Model, 65W power delivery
- 24MD4KL-B 24" Model, 85W power delivery, support for daisy chaining.
So with my Macbook Pro I have the 22" plugged into the 24" and the 24" plugged into the computer, it charges and drives both displays.
With my Thinkpad, the 21.5" on its own works perfectly. The computer charges, I get full 4k resolution, I can use the custom drivers to change the brightness. Perfect support basically.
When I use the daisy-chained setup as with my Macbook, only one of the Displays works. It seems a bit random, but usually its the last one in the chain, the 21.5". When I unplug it, the 24" turns on and displays correctly. When I re-plug it, the 24" continues to display, the 21.5" stays black. (I know I can drive two external Displays, because I can connect another one using the HDMI port and then both work). It would be nice if I could run both through the daisy-chain (then I'd not need the internal display), but I guess it has to do with how the display lines are soldered on the motherboard. That's not a big deal, I'm fine to use just one external one, but I would like to be able to choose which one without having to change the cables.
The main issue though is, that the 24" model does not charge my laptop. Neither with both displays attached nor with just the 24" one on its own. Maybe it's power delivery is too high? The Thinkpad came with a 65W brick, maybe it doesn't support 85W? But shouldn't there be a way to negotiate the power draw? If I use my 96W power brick that came with my Macbook and use the same cable as for connecting to the display, it charges the Thinkpad.
(I actually want to buy a second 24" one for symmetry reasons, but I cannot until I can charge my Thinkpad with it, so for now I'm just using the 21.5" one and have to re-plug it to the 24" afterwards)
Maybe I need to change some Thunderbolt options? I don't even know where to start debugging this? If I google for "ubuntu power draw support" I only find answers about monitoring CPU power draw, not how much a certain USB port is using or would support or something like that?
