linux/Documentation/cpu-freq/core.txt

     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.rst

dev_pm_opp_init_cpufreq_table -
	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)
			policy->freq_table = 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