nixos/kmscon: Fix session & gpu clashing

[hustlerone]

Things done

Adjusted the systemd unit for kmscon to fix #385497

This however will completely disable DRM for kmscon and now seats will be unspecified.
If someone absolutely requires DRM for kmscon, let me know.

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[hustlerone]

As per fbcon fashion, there's rendering issues. I guess the PR will get much more complicated if this doesn't turn out to be a fluke.

[hustlerone]

Ok, this might just be a greetd issue I'm having.

Either way, it would be great if someone would test this under "normal" circumstances

[liff]

Enabled with GNOME and it seems to work like it used to. Didn’t test extensively, but at least GDM is able to start now.

[hustlerone]

Yeah, I suspect the current rev we're in actually works as intended and we actually have to disable DRM specifically.

There also is the option to patch kmscon to allow it to free the gpu on demand. However, we would also have to modify every single derivation that starts a graphical session or needs /dev/dri/cardX so it also tells kmscon to free the GPU, just so we use DRM for kmscon (and I don't know any benefits of using DRM over fbcon for kmscon)

[hustlerone]

how do i undo this git has played me

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/5352

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/5372

[SuperSandro2000]

I couldn't use kmscon for a while because it broke SDDM and with this patch I found 2 issues immediately:

• performance is pretty poor
• different TTYs do not work correctly. eg when opening vim on 1 it also displays at 3 but also the login prompt at the same time

[hustlerone]

Maybe that's why specifying a seat is necessary? Try specifying a seat as an extra argument.

There is a bandaid fix so kmscon can free the GPU but that would require wrapping every single binary that will have to access the GPU (wrap Plasma, GNOME, Sway, Hyprland, etc.). Even then performance isn't guaranteed.

[SuperSandro2000]

Yeah, we cannot wrap every other binary. There must be another way.