[DEV][KERNEL] Tegra3 power management notes and tricks

---

This thread contains findings on how to view and set kernel parameters related to power management. Although there is no kernel source for One X, some information can be obtained from Transformer Prime kernel sources (also a Tegra 3 device), and from the device itself via sysfs interface.

sysfs files related to CPU power management:

/sys/kernel/debug/tegra_hotplug/max_cpus - maximum CPUs to use. User-configurable, default 4. This is not touched by the Android OS, and will remain in whatever state we set until reboot.
Set this to "1" to make the phone a lot less power-hungry and overheating.
At the cost of performance, of course.
*NOTE: second core will activate sometimes if this is set to "1", but 3rd and 4th cores are disabled for sure.

/sys/kernel/debug/tegra_hotplug/min_cpus - minimum number of CPUs to use.
This _is_ touched by the OS, and can not be set reliably.

/sys/kernel/debug/tegra_hotplug/stats - time each core spent active. Including 5-th "low-power" core.
e.g.:

cpu:            G0         G1         G2         G3         LP
transitions:    26         12         0          0          25
time plugged:   3110       1222       0          0          95122
time-stamp:     4296183486

/sys/kernel/debug/cpuidle/lp2 - another stats interface.

/sys/module/cpu_tegra/parameters/cpu_user_cap - Max frequency cap for all cores. Default 1500000 (1.5GHz), can be set to any lower value to underclock.
But it is often set by the OS different values (most frequently, back to it's maximum). Maybe, changing this string in libhtc-opt2.so via hex editor can prevent system from changing it?

/sys/kernel/cluster/active - currently active CPU cluster. "LP" for low-power core, "G" for generic (normal) cores.
Can be changed manually, but is also modified by the system at will.

All above values can be changed from root shell via "echo 'value' > /sys/path/to/file"

Maybe, this will come handy to some ROM or CPU-monitoring app developers.

References:
Asus Transformer kernel source
[DEV] Enable 2D GPU rendering in HTC One X & about build.prop tweaks
some findings by phirenz

is there any way to request more use of the companion core?

In case anyone's wondering, I just found out that /sys/kernel/cluster/active must be opened as root even for reading; opening the file as any other user will not succeed.

The other files seem to allow reading as a standard user, however.

It woulod be great is someone created an app for setting number of CPUs. It is useful sometimes to set it to 1 when you are traveling and want to get max possible battery.
It would be even better if it was possible to disable all 4 cores and force system to use LP core only. It would be brilliant max power saving mode.

I've modified OS Monitor to support Tegra 3's stuff (temperature, number of cores and determining whether it is low power/general CPU in use) and my experience is that there's no or very little battery gains to be had because the other 3 cores are off most of the time anyway (when idle)

I'm using stock 1.28. The low power core doesn't seem to like being activated for more than a few seconds at a time, too.

I've modified OS Monitor to support Tegra 3's stuff

Can u share the apk here...xD???

thanks to the OP.....I don't have a One X but this info helped me to force all 4 cores of my Asus TF300T go online and offline as i wish.

Originally Posted by schriss

It woulod be great is someone created an app for setting number of CPUs. It is useful sometimes to set it to 1 when you are traveling and want to get max possible battery.
It would be even better if it was possible to disable all 4 cores and force system to use LP core only. It would be brilliant max power saving mode.

use core control or francos kernel updater works for every custom kernel also stock kernel and you can choose how many cores...but battery life is not better with deactivated cores and soemetimes even worser -.-