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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.
1. What To Do?
1.1 Initialization
1.2 Per-CPU Initialization
1.3 verify
1.4 target/target_index or setpolicy?
1.5 target/target_index
1.6 setpolicy
1.7 get_intermediate and target_intermediate
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? - The name of this driver.
cpufreq_driver.init - A pointer to the per-CPU initialization
cpufreq_driver.verify - A pointer to a "verification" function.
cpufreq_driver.setpolicy _or_
target_index - See below on the differences.
And optionally
cpufreq_driver.exit - A pointer to a per-CPU cleanup
function called during CPU_POST_DEAD
phase of cpu hotplug process.
cpufreq_driver.stop_cpu - A pointer to a per-CPU stop function
called during CPU_DOWN_PREPARE phase of
cpu hotplug process.
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/target_index or ->setpolicy.
cpufreq_driver.attr - A pointer to a NULL-terminated list of
"struct freq_attr" which allow to
export values to sysfs.
and target_intermediate Used to switch to stable frequency while
changing CPU frequency.
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 in
nanoseconds (if appropriate, else
policy->cur The current operating frequency of
this CPU (if appropriate)
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 is called
with these values.
For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
frequency table helpers might be helpful. See the section 2 for more information
on them.
SMP systems normally have same clock source for a group of cpus. For these the
.init() would be called only once for the first online cpu. Here the .init()
routine must initialize policy->cpus with mask of all possible cpus (Online +
Offline) that share the clock. Then the core would copy this mask onto
policy->related_cpus and will reset policy->cpus to carry only online cpus.
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/target_index 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/target_index call.
Some cpufreq-capable processors switch the frequency between certain
limits on their own. These shall use the ->setpolicy call
1.5. target/target_index
The target_index call has two arguments: struct cpufreq_policy *policy,
and unsigned int index (into the exposed frequency table).
The CPUfreq driver must set the new frequency when called here. The
actual frequency must be determined by freq_table[index].frequency.
It should always restore to earlier frequency (i.e. policy->restore_freq) in
case of errors, even if we switched to intermediate frequency earlier.
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 2
for details.
1.6 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 drivers/cpufreq/longrun.c
1.7 get_intermediate and target_intermediate
Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.
get_intermediate should return a stable intermediate frequency platform wants to
switch to, and target_intermediate() should set CPU to to that frequency, before
jumping to the frequency corresponding to 'index'. Core will take care of
sending notifications and driver doesn't have to handle them in
target_intermediate() or target_index().
Drivers can return '0' from get_intermediate() in case they don't wish to switch
to intermediate frequency for some target frequency. In that case core will
directly call ->target_index().
NOTE: ->target_index() should restore to policy->restore_freq in case of
failures as core would send notifications for that.
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_frequency_table entries, with any value in
"driver_data" you want to use, and the corresponding frequency in
"frequency". At the end of the table, you need to add a
cpufreq_frequency_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
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.
The following macros can be used as iterators over cpufreq_frequency_table:
cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
cpufreq-for_each_valid_entry(pos, table) - iterates over all entries,
excluding CPUFREQ_ENTRY_INVALID frequencies.
Use arguments "pos" - a cpufreq_frequency_table * as a loop cursor and
"table" - the cpufreq_frequency_table * you want to iterate over.
For example:
struct cpufreq_frequency_table *pos, *driver_freq_table;
cpufreq_for_each_entry(pos, driver_freq_table) {
/* Do something with pos */
pos->frequency = ...