In an effort to get the same performance from this laptop as I used when it ran windows (and I could use the excellent ThrottleStop program to undervolt), I did some investigation into how to undervolt while running Linux.

Linux PHC Project

The solution turns out to be the Linux PHC Project.  There wasn’t a patch for Linux 4.1.4, but looking in the forums a bit I found a patch for 3.1, which cleanly applied to 4.1.4.  I didn’t bother with their suggested way of building a module, I just patched the file in the Linux source tree directly and rebuilt the kernel.  Because the tree was already patched with grsecurity, the diff spat out a bunch of offset changes, but all the changes applied correctly which is all that matters.  I checked to ensure that nothing grsecurity/PaX related had been mangled.

Recompile, reboot and now I have the following sysfs interfaces:

/sys/devices/system/cpu/cpu*/cpufreq/phc_default_rawcontrol
/sys/devices/system/cpu/cpu*/cpufreq/phc_default_vids
/sys/devices/system/cpu/cpu*/cpufreq/phc_rawcontrols
/sys/devices/system/cpu/cpu*/cpufreq/phc_version
/sys/devices/system/cpu/cpu*/cpufreq/phc_vids

Writing same values there as I used to use under Windows and now my laptop is so much cooler.  Before when I built a kernel it was getting up towards 90c and it was limiting the CPU to keep itself getting hotter.  Now I can build a kernel, I get nowhere over 75c and it stays at the full 2.20Ghz the entire time.

Original VIDs: cat phc_default_vids 
47 41 28 18 11
New VIDs: cat phc_vids 
30 27 23 13 6

Excellent!  Thanks Linux PHC Project.