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>
  		Rafael J. Wysocki <rafael.j.wysocki@intel.com>
  		   Viresh Kumar <viresh.kumar@linaro.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
  3.  CPUFreq Table Generation with Operating Performance Point (OPP)
  
  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 of the cpufreq policies is done by cpufreq_cpu_get
  and cpufreq_cpu_put, which make sure that the cpufreq driver is
  correctly registered with the core, and will not be unloaded until
  cpufreq_put_cpu is called. That also ensures that the respective cpufreq
  policy doesn't get freed while being used.
  
  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 twice 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.) 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 several values, including min, max (the lower and upper
  frequencies (in kHz) of the new policy).
  
  
  2.2 CPUFreq transition notifiers
  --------------------------------
  
  These are notified twice for each online CPU in the policy, when the
  CPUfreq driver switches the CPU core frequency and this change has no
  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
  flags	- flags of the cpufreq driver
  
  3. CPUFreq Table Generation with Operating Performance Point (OPP)
  ==================================================================
  For details about OPP, see Documentation/power/opp.txt
  
  dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
  	cpufreq_table_validate_and_show() which is provided with the list of
  	frequencies that are available for operation. This function provides
  	a ready to use conversion routine to translate the OPP layer's internal
  	information about the available frequencies into a format readily
  	providable to cpufreq.
  
  	WARNING: Do not use this function in interrupt context.
  
  	Example:
  	 soc_pm_init()
  	 {
  		/* Do things */
  		r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
  		if (!r)
  			cpufreq_table_validate_and_show(policy, freq_table);
  		/* Do other things */
  	 }
  
  	NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
  	addition to CONFIG_PM_OPP.
  
  dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table