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CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
Dominik Brodowski <>
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.
1. Supported Architectures and Processors
1.1 ARM
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
The following ARM processors are supported by cpufreq:
ARM Integrator
Intel PXA
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 [*]
[*] 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
1.4 ppc
Several "PowerBook" and "iBook2" notebooks are supported.
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
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).
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.
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
cpuinfo_cur_freq : Current frequency of the CPU as obtained from
the hardware, in KHz. This is the frequency
the CPU actually runs at.
scaling_available_frequencies : List of available frequencies, in KHz.
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
affected_cpus : List of CPUs that require software coordination
of frequency.
related_cpus : List of CPUs that need some sort of frequency
coordination, whether software or hardware.
scaling_driver : Hardware driver for cpufreq.
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
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.