     CPU frequency and voltage scaling code in the Linux(TM) kernel


		         L i n u x    C P U F r e q

			     U S E R   G U I D E


		    Dominik Brodowski  <linux@brodo.de>


   Clock scaling allows you to change the clock speed of the CPUs on the
    fly. This is a nice method to save battery power, because the lower
            the clock speed, the less power the CPU consumes.


Contents:
---------
1. Supported Architectures and Processors
1.1 ARM and ARM64
1.2 x86
1.3 sparc64
1.4 ppc
1.5 SuperH
1.6 Blackfin

2. "Policy" / "Governor"?
2.1 Policy
2.2 Governor

3. How to change the CPU cpufreq policy and/or speed
3.1 Preferred interface: sysfs


1. Supported Architectures and Processors
=========================================

1.1 ARM and ARM64
-----------------

Almost all ARM and ARM64 platforms support CPU frequency scaling.

1.2 x86
-------

The following processors for the x86 architecture are supported by cpufreq:

AMD Elan - SC400, SC410
AMD mobile K6-2+
AMD mobile K6-3+
AMD mobile Duron
AMD mobile Athlon
AMD Opteron
AMD Athlon 64
Cyrix Media GXm
Intel mobile PIII and Intel mobile PIII-M on certain chipsets
Intel Pentium 4, Intel Xeon
Intel Pentium M (Centrino)
National Semiconductors Geode GX
Transmeta Crusoe
Transmeta Efficeon
VIA Cyrix 3 / C3
various processors on some ACPI 2.0-compatible systems [*]
And many more

[*] Only if "ACPI Processor Performance States" are available
to the ACPI<->BIOS interface.


1.3 sparc64
-----------

The following processors for the sparc64 architecture are supported by
cpufreq:

UltraSPARC-III


1.4 ppc
-------

Several "PowerBook" and "iBook2" notebooks are supported.
The following POWER processors are supported in powernv mode:
POWER8
POWER9

1.5 SuperH
----------

All SuperH processors supporting rate rounding through the clock
framework are supported by cpufreq.

1.6 Blackfin
------------

The following Blackfin processors are supported by cpufreq:

BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
BF531, BF532, BF533, Rev 0.3 or higher
BF534, BF536, BF537, Rev 0.2 or higher
BF561, Rev 0.3 or higher
BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher


2. "Policy" / "Governor" ?
==========================

Some CPU frequency scaling-capable processor switch between various
frequencies and operating voltages "on the fly" without any kernel or
user involvement. This guarantees very fast switching to a frequency
which is high enough to serve the user's needs, but low enough to save
power.


2.1 Policy
----------

On these systems, all you can do is select the lower and upper
frequency limit as well as whether you want more aggressive
power-saving or more instantly available processing power.


2.2 Governor
------------

On all other cpufreq implementations, these boundaries still need to
be set. Then, a "governor" must be selected. Such a "governor" decides
what speed the processor shall run within the boundaries. One such
"governor" is the "userspace" governor. This one allows the user - or
a yet-to-implement userspace program - to decide what specific speed
the processor shall run at.


3. How to change the CPU cpufreq policy and/or speed
====================================================

3.1 Preferred Interface: sysfs
------------------------------

The preferred interface is located in the sysfs filesystem. If you
mounted it at /sys, the cpufreq interface is located in a subdirectory
"cpufreq" within the cpu-device directory
(e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).

affected_cpus :			List of Online CPUs that require software
				coordination of frequency.

cpuinfo_cur_freq :		Current frequency of the CPU as obtained from
				the hardware, in KHz. This is the frequency
				the CPU actually runs at.

cpuinfo_min_freq :		this file shows the minimum operating
				frequency the processor can run at(in kHz) 

cpuinfo_max_freq :		this file shows the maximum operating
				frequency the processor can run at(in kHz) 

cpuinfo_transition_latency	The time it takes on this CPU to
				switch between two frequencies in nano
				seconds. If unknown or known to be
				that high that the driver does not
				work with the ondemand governor, -1
				(CPUFREQ_ETERNAL) will be returned.
				Using this information can be useful
				to choose an appropriate polling
				frequency for a kernel governor or
				userspace daemon. Make sure to not
				switch the frequency too often
				resulting in performance loss.

related_cpus :			List of Online + Offline CPUs that need software
				coordination of frequency.

scaling_available_frequencies : List of available frequencies, in KHz.

scaling_available_governors :	this file shows the CPUfreq governors
				available in this kernel. You can see the
				currently activated governor in

scaling_cur_freq :		Current frequency of the CPU as determined by
				the governor and cpufreq core, in KHz. This is
				the frequency the kernel thinks the CPU runs
				at.

scaling_driver :		this file shows what cpufreq driver is
				used to set the frequency on this CPU

scaling_governor,		and by "echoing" the name of another
				governor you can change it. Please note
				that some governors won't load - they only
				work on some specific architectures or
				processors.

scaling_min_freq and
scaling_max_freq		show the current "policy limits" (in
				kHz). By echoing new values into these
				files, you can change these limits.
				NOTE: when setting a policy you need to
				first set scaling_max_freq, then
				scaling_min_freq.

scaling_setspeed		This can be read to get the currently programmed
				value by the governor. This can be written to
				change the current frequency for a group of
				CPUs, represented by a policy. This is supported
				currently only by the userspace governor.

bios_limit :			If the BIOS tells the OS to limit a CPU to
				lower frequencies, the user can read out the
				maximum available frequency from this file.
				This typically can happen through (often not
				intended) BIOS settings, restrictions
				triggered through a service processor or other
				BIOS/HW based implementations.
				This does not cover thermal ACPI limitations
				which can be detected through the generic
				thermal driver.

If you have selected the "userspace" governor which allows you to
set the CPU operating frequency to a specific value, you can read out
the current frequency in

scaling_setspeed.		By "echoing" a new frequency into this
				you can change the speed of the CPU,
				but only within the limits of
				scaling_min_freq and scaling_max_freq.