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>



   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 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

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_frequency_table_cpuinfo 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_frequency_table_cpuinfo(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