     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
1.2 x86
1.3 sparc64

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
3.2 Deprecated interfaces



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

1.1 ARM
-------

The following ARM processors are supported by cpufreq:

ARM Integrator
ARM-SA1100
ARM-SA1110


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
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
VIA Cyrix 3 / C3
various processors on some ACPI 2.0-compatible systems [*]

[*] 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



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/class/cpu/cpu0/cpufreq/ for the first CPU).

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

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

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.


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.
				

3.2 Deprecated Interfaces
-------------------------

Depending on your kernel configuration, you might find the following 
cpufreq-related files:
/proc/cpufreq
/proc/sys/cpu/*/speed
/proc/sys/cpu/*/speed-min
/proc/sys/cpu/*/speed-max

These are files for deprecated interfaces to cpufreq, which offer far
less functionality. Because of this, these interfaces aren't described
here.

