nixos/xserver: Implement configuration of NVIDIA Optimus via PRIME

This adds configuration options which automate the configuration of NVIDIA Optimus using PRIME. This allows using the NVIDIA proprietary driver on Optimus laptops, in order to render using the NVIDIA GPU while outputting to displays connected only to the integrated Intel GPU. It also adds an option for enabling kernel modesetting for the NVIDIA driver (via a kernel command line flag); this is particularly useful together with Optimus/PRIME because it fixes tearing on PRIME-connected screens.

The user still needs to enable the Optimus/PRIME feature and specify the bus IDs of the Intel and NVIDIA GPUs, but this is still much easier for users and more reliable. The implementation handles both the X configuration file as well as getting display managers to run certain necessary `xrandr` commands just after X has started.

Configuration of commands run after X startup is done using a new configuration option `services.xserver.displayManager.setupCommands`. Support for this option is implemented for LightDM, GDM and SDDM; all of these have been tested with this feature including logging into a Plasma session.

Note: support of `setupCommands` for GDM is implemented by making GDM run the session executable via a wrapper; the wrapper will run the `setupCommands` before execing. This seemed like the simplest and most reliable approach, and solves running these commands both for GDM's X server and user X servers (GDM starts separate X servers for itself and user sessions). An alternative approach would be with autostart files but that seems harder to set up and less reliable.

Note that some simple features for X configuration file generation (in `xserver.nix`) are added which are used in the implementation:
- `services.xserver.extraConfig`: Allows adding arbitrary new sections. This is used to add the Device section for the Intel GPU.
- `deviceSection` and `screenSection` within `services.xserver.drivers`. This allows the nvidia configuration module to add additional contents into the `Device` and `Screen` sections of the "nvidia" driver, and not into such sections for other drivers that may be enabled.

5 files changed, 42 insertions, 1 deletions

diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index 43ed21c95fee..e337f3af328f 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -227,6 +227,17 @@ in
         description = "List of arguments for the X server.";
       };
 
+      setupCommands = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Shell commands executed just after the X server has started.
+
+          This option is only effective for display managers for which this feature
+          is supported; currently these are LightDM, GDM and SDDM.
+        '';
+      };
+
       sessionCommands = mkOption {
         type = types.lines;
         default = "";
diff --git a/nixos/modules/services/x11/display-managers/gdm.nix b/nixos/modules/services/x11/display-managers/gdm.nix
index a6a38a21b617..0c6da2d564c6 100644
--- a/nixos/modules/services/x11/display-managers/gdm.nix
+++ b/nixos/modules/services/x11/display-managers/gdm.nix
@@ -7,6 +7,13 @@ let
   cfg = config.services.xserver.displayManager;
   gdm = pkgs.gnome3.gdm;
 
+  xSessionWrapper = if (cfg.setupCommands == "") then null else
+    pkgs.writeScript "gdm-x-session-wrapper" ''
+      #!${pkgs.bash}/bin/bash
+      ${cfg.setupCommands}
+      exec "$@"
+    '';
+
 in
 
 {
@@ -112,6 +119,11 @@ in
           GDM_SESSIONS_DIR = "${cfg.session.desktops}";
           # Find the mouse
           XCURSOR_PATH = "~/.icons:${pkgs.gnome3.adwaita-icon-theme}/share/icons";
+        } // optionalAttrs (xSessionWrapper != null) {
+          # Make GDM use this wrapper before running the session, which runs the
+          # configured setupCommands. This relies on a patched GDM which supports
+          # this environment variable.
+          GDM_X_SESSION_WRAPPER = "${xSessionWrapper}";
         };
         execCmd = "exec ${gdm}/bin/gdm";
       };
diff --git a/nixos/modules/services/x11/display-managers/lightdm.nix b/nixos/modules/services/x11/display-managers/lightdm.nix
index 5beadacdfa93..028d655f8456 100644
--- a/nixos/modules/services/x11/display-managers/lightdm.nix
+++ b/nixos/modules/services/x11/display-managers/lightdm.nix
@@ -61,6 +61,12 @@ let
       ${optionalString hasDefaultUserSession ''
         user-session=${defaultSessionName}
       ''}
+      ${optionalString (dmcfg.setupCommands != "") ''
+        display-setup-script=${pkgs.writeScript "lightdm-display-setup" ''
+          #!${pkgs.bash}/bin/bash
+          ${dmcfg.setupCommands}
+        ''}
+      ''}
       ${cfg.extraSeatDefaults}
     '';
 
diff --git a/nixos/modules/services/x11/display-managers/sddm.nix b/nixos/modules/services/x11/display-managers/sddm.nix
index df782e82ed15..fe97666dc242 100644
--- a/nixos/modules/services/x11/display-managers/sddm.nix
+++ b/nixos/modules/services/x11/display-managers/sddm.nix
@@ -31,6 +31,7 @@ let
     rm -fr /var/lib/sddm/.cache/sddm-greeter/qmlcache
 
     ${cfg.setupScript}
+    ${dmcfg.setupCommands}
   '';
 
   Xstop = pkgs.writeScript "Xstop" ''
@@ -148,7 +149,8 @@ in
           xrandr --auto
         '';
         description = ''
-          A script to execute when starting the display server.
+          A script to execute when starting the display server. DEPRECATED, please
+          use <option>services.xserver.displayManager.setupCommands</option>.
         '';
       };
 
diff --git a/nixos/modules/services/x11/xserver.nix b/nixos/modules/services/x11/xserver.nix
index 3048cd02683f..accdf3a5cf8a 100644
--- a/nixos/modules/services/x11/xserver.nix
+++ b/nixos/modules/services/x11/xserver.nix
@@ -374,6 +374,12 @@ in
         description = "Contents of the first Monitor section of the X server configuration file.";
       };
 
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = "Additional contents (sections) included in the X server configuration file";
+      };
+
       xrandrHeads = mkOption {
         default = [];
         example = [
@@ -741,6 +747,7 @@ in
             Driver "${driver.driverName or driver.name}"
             ${if cfg.useGlamor then ''Option "AccelMethod" "glamor"'' else ""}
             ${cfg.deviceSection}
+            ${driver.deviceSection or ""}
             ${xrandrDeviceSection}
           EndSection
 
@@ -752,6 +759,7 @@ in
             ''}
 
             ${cfg.screenSection}
+            ${driver.screenSection or ""}
 
             ${optionalString (cfg.defaultDepth != 0) ''
               DefaultDepth ${toString cfg.defaultDepth}
@@ -781,6 +789,8 @@ in
         '')}
 
         ${xrandrMonitorSections}
+
+        ${cfg.extraConfig}
       '';
 
     fonts.enableDefaultFonts = mkDefault true;