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.

1 files changed, 116 insertions, 4 deletions

diff --git a/nixos/modules/hardware/video/nvidia.nix b/nixos/modules/hardware/video/nvidia.nix
index eb1952280331..6944d1a4f76b 100644
--- a/nixos/modules/hardware/video/nvidia.nix
+++ b/nixos/modules/hardware/video/nvidia.nix
@@ -26,9 +26,73 @@ let
   nvidia_libs32 = (nvidiaForKernel pkgs_i686.linuxPackages).override { libsOnly = true; kernel = null; };
 
   enabled = nvidia_x11 != null;
+
+  cfg = config.hardware.nvidia;
+  optimusCfg = cfg.optimus_prime;
 in
 
 {
+  options = {
+    hardware.nvidia.modesetting.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = ''
+        Enable kernel modesetting when using the NVIDIA proprietary driver.
+
+        Enabling this fixes screen tearing when using Optimus via PRIME (see
+        <option>hardware.nvidia.optimus_prime.enable</option>. This is not enabled
+        by default because it is not officially supported by NVIDIA and would not
+        work with SLI.
+      '';
+    };
+
+    hardware.nvidia.optimus_prime.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = ''
+        Enable NVIDIA Optimus support using the NVIDIA proprietary driver via PRIME.
+        If enabled, the NVIDIA GPU will be always on and used for all rendering,
+        while enabling output to displays attached only to the integrated Intel GPU
+        without a multiplexer.
+
+        Note that this option only has any effect if the "nvidia" driver is specified
+        in <option>services.xserver.videoDrivers</option>, and it should preferably
+        be the only driver there.
+
+        If this is enabled, then the bus IDs of the NVIDIA and Intel GPUs have to be
+        specified (<option>hardware.nvidia.optimus_prime.nvidiaBusId</option> and
+        <option>hardware.nvidia.optimus_prime.intelBusId</option>).
+
+        If you enable this, you may want to also enable kernel modesetting for the
+        NVIDIA driver (<option>hardware.nvidia.modesetting.enable</option>) in order
+        to prevent tearing.
+
+        Note that this configuration will only be successful when a display manager
+        for which the <option>services.xserver.displayManager.setupCommands</option>
+        option is supported is used; notably, SLiM is not supported.
+      '';
+    };
+
+    hardware.nvidia.optimus_prime.nvidiaBusId = lib.mkOption {
+      type = lib.types.string;
+      default = "";
+      example = "PCI:1:0:0";
+      description = ''
+        Bus ID of the NVIDIA GPU. You can find it using lspci; for example if lspci
+        shows the NVIDIA GPU at "01:00.0", set this option to "PCI:1:0:0".
+      '';
+    };
+
+    hardware.nvidia.optimus_prime.intelBusId = lib.mkOption {
+      type = lib.types.string;
+      default = "";
+      example = "PCI:0:2:0";
+      description = ''
+        Bus ID of the Intel GPU. You can find it using lspci; for example if lspci
+        shows the Intel GPU at "00:02.0", set this option to "PCI:0:2:0".
+      '';
+    };
+  };
 
   config = mkIf enabled {
     assertions = [
@@ -36,16 +100,62 @@ in
         assertion = config.services.xserver.displayManager.gdm.wayland;
         message = "NVidia drivers don't support wayland";
       }
+      {
+        assertion = !optimusCfg.enable ||
+          (optimusCfg.nvidiaBusId != "" && optimusCfg.intelBusId != "");
+        message = ''
+          When NVIDIA Optimus via PRIME is enabled, the GPU bus IDs must configured.
+        '';
+      }
     ];
 
-    services.xserver.drivers = singleton
-      { name = "nvidia"; modules = [ nvidia_x11.bin ]; libPath = [ nvidia_x11 ]; };
+    # If Optimus/PRIME is enabled, we:
+    # - Specify the configured NVIDIA GPU bus ID in the Device section for the
+    #   "nvidia" driver.
+    # - Add the AllowEmptyInitialConfiguration option to the Screen section for the
+    #   "nvidia" driver, in order to allow the X server to start without any outputs.
+    # - Add a separate Device section for the Intel GPU, using the "modesetting"
+    #   driver and with the configured BusID.
+    # - Reference that Device section from the ServerLayout section as an inactive
+    #   device.
+    # - Configure the display manager to run specific `xrandr` commands which will
+    #   configure/enable displays connected to the Intel GPU.
+
+    services.xserver.drivers = singleton {
+      name = "nvidia";
+      modules = [ nvidia_x11.bin ];
+      libPath = [ nvidia_x11 ];
+      deviceSection = optionalString optimusCfg.enable
+        ''
+          BusID "${optimusCfg.nvidiaBusId}"
+        '';
+      screenSection =
+        ''
+          Option "RandRRotation" "on"
+          ${optionalString optimusCfg.enable "Option \"AllowEmptyInitialConfiguration\""}
+        '';
+    };
 
-    services.xserver.screenSection =
+    services.xserver.extraConfig = optionalString optimusCfg.enable
+      ''
+        Section "Device"
+          Identifier "nvidia-optimus-intel"
+          Driver "modesetting"
+          BusID  "${optimusCfg.intelBusId}"
+          Option "AccelMethod" "none"
+        EndSection
+      '';
+    services.xserver.serverLayoutSection = optionalString optimusCfg.enable
       ''
-        Option "RandRRotation" "on"
+        Inactive "nvidia-optimus-intel"
       '';
 
+    services.xserver.displayManager.setupCommands = optionalString optimusCfg.enable ''
+      # Added by nvidia configuration module for Optimus/PRIME.
+      ${pkgs.xorg.xrandr}/bin/xrandr --setprovideroutputsource modesetting NVIDIA-0
+      ${pkgs.xorg.xrandr}/bin/xrandr --auto
+    '';
+
     environment.etc."nvidia/nvidia-application-profiles-rc" = mkIf nvidia_x11.useProfiles {
       source = "${nvidia_x11.bin}/share/nvidia/nvidia-application-profiles-rc";
     };
@@ -62,6 +172,8 @@ in
     boot.kernelModules = [ "nvidia-uvm" ] ++
       lib.optionals config.services.xserver.enable [ "nvidia" "nvidia_modeset" "nvidia_drm" ];
 
+    # If requested enable modesetting via kernel parameter.
+    boot.kernelParams = optional cfg.modesetting.enable "nvidia-drm.modeset=1";
 
     # Create /dev/nvidia-uvm when the nvidia-uvm module is loaded.
     services.udev.extraRules =