depot/nixos/modules/config/power-management.nix

{ config, lib, ... }:
let

  cfg = config.powerManagement;

in

{

  ###### interface

  options = {

    powerManagement = {

      enable = lib.mkOption {
        type = lib.types.bool;
        default = true;
        description = ''
          Whether to enable power management.  This includes support
          for suspend-to-RAM and powersave features on laptops.
        '';
      };

      resumeCommands = lib.mkOption {
        type = lib.types.lines;
        default = "";
        description = "Commands executed after the system resumes from suspend-to-RAM.";
      };

      powerUpCommands = lib.mkOption {
        type = lib.types.lines;
        default = "";
        example = lib.literalExpression ''
          "''${pkgs.hdparm}/sbin/hdparm -B 255 /dev/sda"
        '';
        description = ''
          Commands executed when the machine powers up.  That is,
          they're executed both when the system first boots and when
          it resumes from suspend or hibernation.
        '';
      };

      powerDownCommands = lib.mkOption {
        type = lib.types.lines;
        default = "";
        example = lib.literalExpression ''
          "''${pkgs.hdparm}/sbin/hdparm -B 255 /dev/sda"
        '';
        description = ''
          Commands executed when the machine powers down.  That is,
          they're executed both when the system shuts down and when
          it goes to suspend or hibernation.
        '';
      };

    };

  };

  ###### implementation

  config = lib.mkIf cfg.enable {

    systemd.targets.post-resume = {
      description = "Post-Resume Actions";
      requires = [ "post-resume.service" ];
      after = [ "post-resume.service" ];
      wantedBy = [ "sleep.target" ];
      unitConfig.StopWhenUnneeded = true;
    };

    # Service executed before suspending/hibernating.
    systemd.services.pre-sleep = {
      description = "Pre-Sleep Actions";
      wantedBy = [ "sleep.target" ];
      before = [ "sleep.target" ];
      script = ''
        ${cfg.powerDownCommands}
      '';
      serviceConfig.Type = "oneshot";
    };

    systemd.services.post-resume = {
      description = "Post-Resume Actions";
      after = [
        "suspend.target"
        "hibernate.target"
        "hybrid-sleep.target"
        "suspend-then-hibernate.target"
      ];
      script = ''
        /run/current-system/systemd/bin/systemctl try-restart --no-block post-resume.target
        ${cfg.resumeCommands}
        ${cfg.powerUpCommands}
      '';
      serviceConfig.Type = "oneshot";
    };

  };

}