nixos/sddm: add setupScript and stopScript options

These options allow setting the start and stop scripts for the display
manager. Making these configurable is necessary to allow some hardware
configurations. Upstream ships empty scripts by default, anyway.

nixos/modules/services/x11/display-managers/sddm.nix

@@ -17,6 +17,16 @@ let
     exec ${dmcfg.xserverBin} ${dmcfg.xserverArgs} "$@"
   '';
 
+  Xsetup = pkgs.writeScript "Xsetup" ''
+    #!/bin/sh
+    ${cfg.setupScript}
+  '';
+
+  Xstop = pkgs.writeScript "Xstop" ''
+    #!/bin/sh
+    ${cfg.stopScript}
+  '';
+
   cfgFile = pkgs.writeText "sddm.conf" ''
     [General]
     HaltCommand=${pkgs.systemd}/bin/systemctl poweroff
@@ -39,6 +49,8 @@ let
     SessionCommand=${dmcfg.session.script}
     SessionDir=${dmcfg.session.desktops}
     XauthPath=${pkgs.xorg.xauth}/bin/xauth
+    DisplayCommand=${Xsetup}
+    DisplayStopCommand=${Xstop}
 
     ${optionalString cfg.autoLogin.enable ''
     [Autologin]
@@ -98,6 +110,27 @@ in
         '';
       };
 
+      setupScript = mkOption {
+        type = types.str;
+        default = "";
+        example = ''
+          # workaround for using NVIDIA Optimus without Bumblebee
+          xrandr --setprovideroutputsource modesetting NVIDIA-0
+          xrandr --auto
+        '';
+        description = ''
+          A script to execute when starting the display server.
+        '';
+      };
+
+      stopScript = mkOption {
+        type = types.str;
+        default = "";
+        description = ''
+          A script to execute when stopping the display server.
+        '';
+      };
+
       autoLogin = mkOption {
         default = {};
         description = ''