nix-config/modules/nixos/desktop/gaming.nix

{
  pkgs,
  config,
  lib,
  ...
}:
with lib;
let
  cfg = config.modules.desktop.gaming;
in
{
  options.modules.desktop = {
    gaming = {
      enable = mkEnableOption "desktop gaming support";
    };
  };

  config = mkIf cfg.enable {
    # ==========================================================================
    # Gaming on Linux
    #
    #   <https://www.protondb.com/> can give you an idea what works where and how.
    #   Begineer Guide: <https://www.reddit.com/r/linux_gaming/wiki/faq/>
    # ==========================================================================

    # Steam runs in a nixpak sandbox (hardening/nixpaks/steam.nix) that the
    # home manager installs, so `programs.steam` is off: its `package` option
    # calls `.override` on whatever it is given, which a sandbox script cannot
    # answer, and it would install a second, unsandboxed steam plus `steam-run`.
    # What is kept from it: 32-bit graphics, which the game sandboxes mount as
    # /run/opengl-driver-32, and the controller/VR udev rules. Dropped with it:
    # `extest` (needs /dev/uinput, which no sandbox gets) and the host
    # protontricks (it would run Wine on game-writable prefixes; the umu
    # sandbox carries one instead).
    hardware.graphics.enable32Bit = true;
    hardware.steam-hardware.enable = true;

    # Expose DW-Proton to umu-run: umu ignores Steam's
    # STEAM_EXTRA_COMPAT_TOOLS_PATHS, and a store path baked into a generated
    # launcher goes stale on every dwproton-bin update. The nix-config-umu-game
    # installer reads this on the host; the umu sandbox
    # (hardening/nixpaks/umu.nix) sets the same path for the run/exec scripts.
    environment.variables.UMU_PROTONPATH = "${pkgs.dwproton-bin.steamcompattool}";

    # GameScope: run a game in its own nested compositor and let it renice
    # itself for steadier frame pacing. GPU-specific PRIME render offload belongs
    # to the host, not here (see programs.gamescope.env in
    # hosts/idols-ai/hardware-nvidia.nix).
    # https://github.com/ValveSoftware/gamescope
    programs.gamescope = {
      enable = true;
      capSysNice = true;
    };

    # The sysctls SteamOS ships in steamos-customizations-jupiter (file names
    # below), as carried by nix-gaming's platformOptimizations module.
    boot.kernel.sysctl = {
      # 20-shed.conf
      "kernel.sched_cfs_bandwidth_slice_us" = 3000;
      # 20-net-timeout.conf: lets a game that was killed and restarted quickly
      # rebind its TCP ports.
      "net.ipv4.tcp_fin_timeout" = 5;
      # 30-splitlock.conf: no deliberate slowdown when a game hits a split lock.
      "kernel.split_lock_mitigate" = 0;
      # 30-vm.conf: MAX_INT - MAPCOUNT_ELF_CORE_MARGIN, see include/linux/mm.h.
      "vm.max_map_count" = 2147483642;
    };

    # Low-latency PipeWire: a 64/48000 minimum quantum plus realtime scheduling,
    # as in nix-gaming's pipewireLowLatency module.
    services.pipewire.extraConfig = {
      pipewire."99-lowlatency" = {
        "context.properties"."default.clock.min-quantum" = 64;
        "context.modules" = [
          {
            name = "libpipewire-module-rt";
            flags = [
              "ifexists"
              "nofail"
            ];
            args = {
              "nice.level" = -15;
              "rt.prio" = 88;
              "rt.time.soft" = 200000;
              "rt.time.hard" = 200000;
            };
          }
        ];
      };
      pipewire-pulse."99-lowlatency"."pulse.properties" = {
        "server.address" = [ "unix:native" ];
        "pulse.min.req" = "64/48000";
        "pulse.min.quantum" = "64/48000";
        "pulse.min.frag" = "64/48000";
      };
      client."99-lowlatency"."stream.properties" = {
        "node.latency" = "64/48000";
        "resample.quality" = 1;
      };
    };

    # Optimise Linux system performance on demand
    # https://github.com/FeralInteractive/GameMode
    # https://wiki.archlinux.org/title/Gamemode
    #
    # Usage:
    #   1. For games/launchers which integrate GameMode support:
    #      https://github.com/FeralInteractive/GameMode#apps-with-gamemode-integration
    #      simply running the game will automatically activate GameMode.
    programs.gamemode.enable = true;
  };
}