Add module to use kmscon instead of linux-console for VTs

This required some changes to systemd unit handling:

* Add an option to specify that a unit is just a symlink
* Allow specified units to overwrite systemd-provided ones
* Have gettys.target require autovt@1.service instead of getty@1.service

Signed-off-by: Shea Levy <shea@shealevy.com>

1 files changed, 64 insertions, 0 deletions

diff --git a/nixos/modules/services/ttys/kmscon.nix b/nixos/modules/services/ttys/kmscon.nix
new file mode 100644
index 000000000000..97fe7a1ca1b8
--- /dev/null
+++ b/nixos/modules/services/ttys/kmscon.nix
@@ -0,0 +1,64 @@
+{ config, pkgs, ... }:
+let
+  inherit (pkgs.lib) mkOption types mkIf optionalString;
+
+  cfg = config.services.kmscon;
+
+  configDir = pkgs.writeTextFile { name = "kmscon-config"; destination = "/kmscon.conf"; text = cfg.extraConfig; };
+in {
+  options = {
+    services.kmscon = {
+      enable = mkOption {
+        description = "Use kmscon as the virtual console instead of gettys";
+        type = types.bool;
+        default = false;
+      };
+
+      hwRender = mkOption {
+        description = "Whether to use 3D hardware acceleration to render the console";
+        type = types.bool;
+        default = false;
+      };
+
+      extraConfig = mkOption {
+        description = "Extra contents of the kmscon.conf file";
+        type = types.lines;
+        default = "";
+        example = "font-size=14";
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    # Largely copied from unit provided with kmscon source
+    systemd.units."kmsconvt@.service".text = ''
+      [Unit]
+      Description=KMS System Console on %I
+      Documentation=man:kmscon(1)
+      After=systemd-user-sessions.service
+      After=plymouth-quit-wait.service
+      After=systemd-logind.service
+      Requires=systemd-logind.service
+      Before=getty.target
+      Conflicts=getty@%i.service
+      OnFailure=getty@%i.service
+      IgnoreOnIsolate=yes
+      ConditionPathExists=/dev/tty0
+
+      [Service]
+      ExecStart=${pkgs.kmscon}/bin/kmscon "--vt=%I" --seats=seat0 --no-switchvt --configdir ${configDir} --login -- ${pkgs.shadow}/bin/login -p
+      UtmpIdentifier=%I
+      TTYPath=/dev/%I
+      TTYReset=yes
+      TTYVHangup=yes
+      TTYVTDisallocate=yes
+    '';
+
+    systemd.units."autovt@.service".linkTarget = "${config.systemd.units."kmsconvt@.service".unit}/kmsconvt@.service";
+
+    services.kmscon.extraConfig = mkIf cfg.hwRender ''
+      drm
+      hwaccel
+    '';
+  };
+}