cpufreq: add option for setting the cpu max and min frequencies

This adds a NixOS option for setting the CPU max and min frequencies
with `cpufreq`.  The two options that have been added are:

- `powerManagement.cpufreq.max`
- `powerManagement.cpufreq.min`

(cherry picked from commit b0f10d2)
(cherry picked from commit 46ecec8)

nixos/modules/services/hardware/tlp.nix

@@ -56,6 +56,8 @@ in
 
     powerManagement.scsiLinkPolicy = null;
     powerManagement.cpuFreqGovernor = null;
+    powerManagement.cpufreq.max = null;
+    powerManagement.cpufreq.min = null;
 
     systemd.sockets."systemd-rfkill".enable = false;
 

nixos/modules/tasks/cpu-freq.nix

@@ -10,43 +10,79 @@ in
 {
   ###### interface
 
-  options = {
+  options.powerManagement = {
 
-    powerManagement.cpuFreqGovernor = mkOption {
+    cpuFreqGovernor = mkOption {
       type = types.nullOr types.str;
       default = null;
       example = "ondemand";
       description = ''
         Configure the governor used to regulate the frequence of the
         available CPUs. By default, the kernel configures the
-        performance governor.
+        performance governor, although this may be overwritten in your
+        hardware-configuration.nix file.
+
+        Often used values: "ondemand", "powersave", "performance"
       '';
     };
 
+    cpufreq = {
+
+      max = mkOption {
+        type = types.nullOr types.ints.unsigned;
+        default = null;
+        example = 2200000;
+        description = ''
+          The maximum frequency the CPU will use.  Defaults to the maximum possible.
+        '';
+      };
+
+      min = mkOption {
+        type = types.nullOr types.ints.unsigned;
+        default = null;
+        example = 800000;
+        description = ''
+          The minimum frequency the CPU will use.
+        '';
+      };
+    };
+
   };
 
 
   ###### implementation
 
-  config = mkIf (!config.boot.isContainer && config.powerManagement.cpuFreqGovernor != null) {
+  config =
+    let
+      governorEnable = cfg.cpuFreqGovernor != null;
+      maxEnable = cfg.cpufreq.max != null;
+      minEnable = cfg.cpufreq.min != null;
+      enable =
+        !config.boot.isContainer &&
+        (governorEnable || maxEnable || minEnable);
+    in
+    mkIf enable {
 
-    boot.kernelModules = [ "cpufreq_${cfg.cpuFreqGovernor}" ];
+      boot.kernelModules = optional governorEnable "cpufreq_${cfg.cpuFreqGovernor}";
 
-    environment.systemPackages = [ cpupower ];
+      environment.systemPackages = [ cpupower ];
 
-    systemd.services.cpufreq = {
-      description = "CPU Frequency Governor Setup";
-      after = [ "systemd-modules-load.service" ];
-      wantedBy = [ "multi-user.target" ];
-      path = [ cpupower pkgs.kmod ];
-      unitConfig.ConditionVirtualization = false;
-      serviceConfig = {
-        Type = "oneshot";
-        RemainAfterExit = "yes";
-        ExecStart = "${cpupower}/bin/cpupower frequency-set -g ${cfg.cpuFreqGovernor}";
-        SuccessExitStatus = "0 237";
+      systemd.services.cpufreq = {
+        description = "CPU Frequency Setup";
+        after = [ "systemd-modules-load.service" ];
+        wantedBy = [ "multi-user.target" ];
+        path = [ cpupower pkgs.kmod ];
+        unitConfig.ConditionVirtualization = false;
+        serviceConfig = {
+          Type = "oneshot";
+          RemainAfterExit = "yes";
+          ExecStart = "${cpupower}/bin/cpupower frequency-set " +
+            optionalString governorEnable "--governor ${cfg.cpuFreqGovernor} " +
+            optionalString maxEnable "--max ${toString cfg.cpufreq.max} " +
+            optionalString minEnable "--min ${toString cfg.cpufreq.min} ";
+          SuccessExitStatus = "0 237";
+        };
       };
-    };
 
   };
 }