As of March 18, 2021, DisplayLink support in Ubuntu 21.04 (Hirsute Hippo) is even more broken than I wrote about in my earlier blog posting describing how to make it work. They’ve just put Linux kernel version 5.11 into the release package stream, and evdi, the kernel module required for DisplayLink to work, isn’t yet compatible with 5.11. In addition, they’ve released an upgrade for a package called mesa, and the new mesa version is incompatible with evdi even with kernel version 5.10, which is otherwise compatible with DisplayLink. Therefore, if you want to keep using DisplayLink, you’re going to have to disable upgrades to both the kernel and the mesa packages until fixes are available.
Note: You may not run into the mesa issue if you’re using Wayland. I’m not sure, since I use Xorg, not Wayland, for my DisplayLink hub. I know for a fact that Xorg won’t start because of the mesa/evdi incompatibility, but I don’t know about Wayland. If Wayland won’t start either then you can probably use the solution described here for that as well.
If you’ve already upgraded and broken DisplayLink, you’re going to need to figure out how to log into your computer without DisplayLink working in order to fix this. The two options for doing that are logging in on a VT or logging in from another computer over SSH. However, you can only log in on a VT if you plug a monitor directly into your computer (i.e., not into your DisplayLink hub) and reboot, because the kernel doesn’t configure VTs unless a monitor is plugged in at boot-time.
In general, what you are going to need to do is: (1) make sure that you have the last compatible kernel and mesa releases installed; (2) put holds on those old versions so they won’t get upgraded; (3) uninstall the 5.11 kernel. The script I’ve linked to below does all everything you need. If you already ran my modified DisplayLink installer from my other blog posting successfully before today’s Hirsute updates broke things, rebooting after running the script should cause DisplayLink to start working again. Otherwise, if you’re just coming to this now, then first run the script and then run my DisplayLink installer and you should be all set.
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Here is the script, which you’ll need to run as root (as always, use at your own risk, read and understand the script before running it, I’ve probably done something stupid here so caveat emptor, no warranties, etc.).