nixos-generate-config: detect CPU governor

* cpu-freq: Try powersave if ondemand is not available

* Revert "cpu-freq: Try powersave if ondemand is not available"

This reverts commit 4dc56db37e32dcfecd667ebbf88263e47b296097.
Consult available scaling governors; for freshly generated configs, this provides a better experience than relying on a default that might not work everywhere.

nixos/modules/config/power-management.nix

···
 
   config = mkIf cfg.enable {
 
-    # FIXME: Implement powersave governor for sandy bridge or later Intel CPUs
+    # Leftover for old setups, should be set by nixos-generate-config now
     powerManagement.cpuFreqGovernor = mkDefault "ondemand";
 
     systemd.targets.post-resume = {

nixos/modules/installer/tools/nixos-generate-config.pl

···
 my $cpus = scalar (grep {/^processor\s*:/} (split '\n', $cpuinfo));
 
 
+# Determine CPU governor to use
+if (-e "/sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors") {
+    my $governors = read_file("/sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors");
+    # ondemand governor is not available on sandy bridge or later Intel CPUs
+    my @desired_governors = ("ondemand", "powersave");
+    my $e;
+
+    foreach $e (@desired_governors) {
+        if (index($governors, $e) != -1) {
+            last if (push @attrs, "powerManagement.cpuFreqGovernor = \"$e\";");
+        }
+    }
+}
+
+
 # Virtualization support?
 push @kernelModules, "kvm-intel" if hasCPUFeature "vmx";
 push @kernelModules, "kvm-amd" if hasCPUFeature "svm";