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>

nixos/modules/module-list.nix

@@ -214,6 +214,7 @@
   ./services/torrent/transmission.nix
   ./services/ttys/gpm.nix
   ./services/ttys/agetty.nix
+  ./services/ttys/kmscon.nix
   ./services/web-servers/apache-httpd/default.nix
   ./services/web-servers/jboss/default.nix
   ./services/web-servers/lighttpd/default.nix

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
+    '';
+  };
+}

nixos/modules/system/boot/systemd.nix

@@ -11,13 +11,16 @@ let
   systemd = cfg.package;
 
   makeUnit = name: unit:
-    pkgs.runCommand "unit" { inherit (unit) text; preferLocalBuild = true; }
-      (if unit.enable then  ''
+    pkgs.runCommand "unit" ({ preferLocalBuild = true; } // optionalAttrs (unit.linkTarget == null) { inherit (unit) text; })
+      (if !unit.enable then  ''
         mkdir -p $out
-        echo -n "$text" > $out/${name}
+        ln -s /dev/null $out/${name}
+      '' else if unit.linkTarget != null then ''
+        mkdir -p $out
+        ln -s ${unit.linkTarget} $out/${name}
       '' else ''
         mkdir -p $out
-        ln -s /dev/null $out/${name}
+        echo -n "$text" > $out/${name}
       '');
 
   upstreamUnits =
@@ -338,7 +341,7 @@ let
       done
 
       for i in ${toString (mapAttrsToList (n: v: v.unit) cfg.units)}; do
-        ln -s $i/* $out/
+        ln -fs $i/* $out/
       done
 
       for i in ${toString cfg.packages}; do
@@ -362,7 +365,7 @@ let
       ln -s rescue.target $out/kbrequest.target
 
       mkdir -p $out/getty.target.wants/
-      ln -s ../getty@tty1.service $out/getty.target.wants/
+      ln -s ../autovt@tty1.service $out/getty.target.wants/
 
       ln -s ../local-fs.target ../remote-fs.target ../network.target ../nss-lookup.target \
             ../nss-user-lookup.target ../swap.target $out/multi-user.target.wants/
@@ -416,6 +419,11 @@ in
               internal = true;
               description = "The generated unit.";
             };
+            linkTarget = mkOption {
+              default = null;
+              description = "The file to symlink this target to.";
+              type = types.nullOr types.path;
+            };
           };
           config = {
             unit = makeUnit name config;