Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

     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  

                     David Kimdon 

   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.