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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
-
- 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
-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
-ARM-SA1100
-ARM-SA1110
-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
-cpufreq:
-
-UltraSPARC-III
-
-
-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
-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).
-
-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
- processors.
-
-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
- scaling_min_freq.
-
-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
- at.
-
-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.