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     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   D r i v e r s

                       - information for developers -

                    Dominik Brodowski  

   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.   What To Do?

1.1  Initialization

1.2  Per-CPU Initialization

1.3  verify

1.4  target or setpolicy?

1.5  target

1.6  setpolicy

2.   Frequency Table Helpers

1. What To Do?

==============

So, you just got a brand-new CPU / chipset with datasheets and want to

add cpufreq support for this CPU / chipset? Great. Here are some hints

on what is necessary:

1.1 Initialization

------------------

First of all, in an __initcall level 7 (module_init()) or later

function check whether this kernel runs on the right CPU and the right

chipset. If so, register a struct cpufreq_driver with the CPUfreq core

using cpufreq_register_driver()

What shall this struct cpufreq_driver contain?

cpufreq_driver.name -           The name of this driver.

cpufreq_driver.owner -          THIS_MODULE;

cpufreq_driver.init -           A pointer to the per-CPU initialization

                                function.

cpufreq_driver.verify -         A pointer to a "verification" function.

cpufreq_driver.setpolicy _or_

cpufreq_driver.target -         See below on the differences.

And optionally

cpufreq_driver.exit -           A pointer to a per-CPU cleanup function.

cpufreq_driver.resume -         A pointer to a per-CPU resume function

                                which is called with interrupts disabled

                                and _before_ the pre-suspend frequency

                                and/or policy is restored by a call to

                                ->target or ->setpolicy.

cpufreq_driver.attr -           A pointer to a NULL-terminated list of

                                "struct freq_attr" which allow to

                                export values to sysfs.

1.2 Per-CPU Initialization

--------------------------

Whenever a new CPU is registered with the device model, or after the

cpufreq driver registers itself, the per-CPU initialization function

cpufreq_driver.init is called. It takes a struct cpufreq_policy

*policy as argument. What to do now?

If necessary, activate the CPUfreq support on your CPU.

Then, the driver must fill in the following values:

policy->cpuinfo.min_freq _and_

policy->cpuinfo.max_freq -      the minimum and maximum frequency

                                (in kHz) which is supported by

                                this CPU

policy->cpuinfo.transition_latency   the time it takes on this CPU to

                                switch between two frequencies (if

                                appropriate, else specify

                                CPUFREQ_ETERNAL)

policy->cur                     The current operating frequency of

                                this CPU (if appropriate)

policy->min,

policy->max,

policy->policy and, if necessary,

policy->governor                must contain the "default policy" for

                                this CPU. A few moments later,

                                cpufreq_driver.verify and either

                                cpufreq_driver.setpolicy or

                                cpufreq_driver.target is called with

                                these values.

For setting some of these values, the frequency table helpers might be

helpful. See the section 2 for more information on them.

1.3 verify

------------

When the user decides a new policy (consisting of

"policy,governor,min,max") shall be set, this policy must be validated

so that incompatible values can be corrected. For verifying these

values, a frequency table helper and/or the

cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned

int min_freq, unsigned int max_freq) function might be helpful. See

section 2 for details on frequency table helpers.

You need to make sure that at least one valid frequency (or operating

range) is within policy->min and policy->max. If necessary, increase

policy->max first, and only if this is no solution, decrease policy->min.

1.4 target or setpolicy?

----------------------------

Most cpufreq drivers or even most cpu frequency scaling algorithms

only allow the CPU to be set to one frequency. For these, you use the

->target call.

Some cpufreq-capable processors switch the frequency between certain

limits on their own. These shall use the ->setpolicy call

1.4. target

-------------

The target call has three arguments: struct cpufreq_policy *policy,

unsigned int target_frequency, unsigned int relation.

The CPUfreq driver must set the new frequency when called here. The

actual frequency must be determined using the following rules:

- keep close to "target_freq"

- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)

- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal

  target_freq. ("L for lowest, but no lower than")

- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal

  target_freq. ("H for highest, but no higher than")

Here again the frequency table helper might assist you - see section 3

for details.

1.5 setpolicy

---------------

The setpolicy call only takes a struct cpufreq_policy *policy as

argument. You need to set the lower limit of the in-processor or

in-chipset dynamic frequency switching to policy->min, the upper limit

to policy->max, and -if supported- select a performance-oriented

setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a

powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check

the reference implementation in arch/i386/kernel/cpu/cpufreq/longrun.c

2. Frequency Table Helpers

==========================

As most cpufreq processors only allow for being set to a few specific

frequencies, a "frequency table" with some functions might assist in

some work of the processor driver. Such a "frequency table" consists

of an array of struct cpufreq_freq_table entries, with any value in

"index" you want to use, and the corresponding frequency in

"frequency". At the end of the table, you need to add a

cpufreq_freq_table entry with frequency set to CPUFREQ_TABLE_END. And

if you want to skip one entry in the table, set the frequency to

CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending

order.

By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,

                                        struct cpufreq_frequency_table *table);

the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and

policy->min and policy->max are set to the same values. This is

helpful for the per-CPU initialization stage.

int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,

                                   struct cpufreq_frequency_table *table);

assures that at least one valid frequency is within policy->min and

policy->max, and all other criteria are met. This is helpful for the

->verify call.

int cpufreq_frequency_table_target(struct cpufreq_policy *policy,

                                   struct cpufreq_frequency_table *table,

                                   unsigned int target_freq,

                                   unsigned int relation,

                                   unsigned int *index);

is the corresponding frequency table helper for the ->target

stage. Just pass the values to this function, and the unsigned int

index returns the number of the frequency table entry which contains

the frequency the CPU shall be set to. PLEASE NOTE: This is not the

"index" which is in this cpufreq_table_entry.index, but instead

cpufreq_table[index]. So, the new frequency is

cpufreq_table[index].frequency, and the value you stored into the

frequency table "index" field is

cpufreq_table[index].index.