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.