CPU frequency and voltage scaling code in the Linux(TM) kernel
  
  
  		         L i n u x    C P U F r e q
  
  			  C P U F r e q    C o r e
  
  
  		    Dominik Brodowski  <linux@brodo.de>
  		     David Kimdon <dwhedon@debian.org>
  
  
  
     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.  CPUFreq core and interfaces
  2.  CPUFreq notifiers
  
  1. General Information
  =======================

  The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This

  cpufreq code offers a standardized interface for the CPUFreq
  architecture drivers (those pieces of code that do actual
  frequency transitions), as well as to "notifiers". These are device
  drivers or other part of the kernel that need to be informed of
  policy changes (ex. thermal modules like ACPI) or of all
  frequency changes (ex. timing code) or even need to force certain
  speed limits (like LCD drivers on ARM architecture). Additionally, the
  kernel "constant" loops_per_jiffy is updated on frequency changes
  here.
  
  Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu,
  which make sure that the cpufreq processor driver is correctly
  registered with the core, and will not be unloaded until
  cpufreq_put_cpu is called.
  
  2. CPUFreq notifiers
  ====================
  
  CPUFreq notifiers conform to the standard kernel notifier interface.
  See linux/include/linux/notifier.h for details on notifiers.
  
  There are two different CPUFreq notifiers - policy notifiers and
  transition notifiers.
  
  
  2.1 CPUFreq policy notifiers
  ----------------------------
  
  These are notified when a new policy is intended to be set. Each
  CPUFreq policy notifier is called three times for a policy transition:
  
  1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
      they see a need for this - may it be thermal considerations or
      hardware limitations.
  
  2.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid
      hardware failure.
  
  3.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
     - if two hardware drivers failed to agree on a new policy before this
     stage, the incompatible hardware shall be shut down, and the user
     informed of this.
  
  The phase is specified in the second argument to the notifier.
  
  The third argument, a void *pointer, points to a struct cpufreq_policy
  consisting of five values: cpu, min, max, policy and max_cpu_freq. min 
  and max are the lower and upper frequencies (in kHz) of the new
  policy, policy the new policy, cpu the number of the affected CPU; and 
  max_cpu_freq the maximum supported CPU frequency. This value is given 
  for informational purposes only.
  
  
  2.2 CPUFreq transition notifiers
  --------------------------------
  
  These are notified twice when the CPUfreq driver switches the CPU core
  frequency and this change has any external implications.
  
  The second argument specifies the phase - CPUFREQ_PRECHANGE or
  CPUFREQ_POSTCHANGE.
  
  The third argument is a struct cpufreq_freqs with the following
  values:
  cpu	- number of the affected CPU
  old	- old frequency
  new	- new frequency
  
  If the cpufreq core detects the frequency has changed while the system
  was suspended, these notifiers are called with CPUFREQ_RESUMECHANGE as
  second argument.