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Two 4k external monitors through a docking station - Why is this seemingly effortless for Windows but basically impossible for Linux? (feddit.dk)

submitted 3 months ago by [email protected] to c/linux

52 comments fedilink hide all child comments

I've ran into this situation multiple times at my current and previous jobs. I really want to avoid Windows and use something better, but I can't live without two external monitors.

On Windows, it "just works". I don't have to do anything.

On Linux (I tried Linux Mint today) it doesn't work. First, it only connected one of the monitors, the other one did not register. Then I switched to a different cable from the computer to the docking station and it connected both screens - however, they were locked to 30fps. I could not make them work at 60fps (and this is a major dealbreaker, I cannot live with 30fps).

This isn't really a tech support question, I'm more trying to understand what fundamentally causes this situation. Why is Linux still struggling with pretty basic functionality that Windows does with zero setup? Is it the vendor of the laptop and docking station that aren't properly supporting Linux? Or is it some other problem?

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[–] [email protected] 7 points 3 months ago (1 children)

Your best bet when it comes to USB-C docks is using a computer with a Thunderbolt 4 port. While I'm not 100 % sure, I think one of the few spec changes comparing Thunderbolt 4 to 3 is that TB4 certification requires two independent DisplayPort streams. If you can then find a Thunderbolt 4 certified Dock with either two independent DisplayPort outputs or even two Thunderbolt outputs (physically USB-C), you can connect your monitors via DP to DP or USB-C to DP. This configuration should work on pretty much any OS as long as Thunderbolt is working properly.

USB 4 or Thunderbolt 3 can support multiple DisplayPort streams, but it's often hard to find exact specifications on what's supported with which computer and dock. Many docks have multiple DisplayPort ports, but they are linked via MST (essentially daisy-chaining) so they share a single DisplayPort stream. This tends to work fine under Windows as long as you don't run into bandwidth limitations, and I think Linux supports it as well, but this might vary by distro. I know macOS does not support MST at all.

Even worse than that, a lot of the cheaper (but also more expensive) multi-display USB-C docks use DisplayLink, a proprietary technology that essentially uses software rendering to output to multiple displays. This requires a proprietary driver to get working, so I'm not sure how well it works under Linux if at all.

Your best bet for Linux is to run Wayland and an up-to-date kernel (we're currently at 6.9.x) with a well-supported DE like KDE or GNOME.

[–] [email protected] 4 points 3 months ago

I'm forced to use Displaylink for one of my 4k monitor's. The second one is directly connected with a Displayport.

You can see a noticeable lag on the Displaylink Version.