Documentation/cpu-freq/cpu-drivers.txt - arm/linux-arm64-legacy - Git at Google

      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  <linux@brodo.de>


    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/target_index or setpolicy?
 1.5  target/target_index
 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.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/
 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.


 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
 				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/target_index 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


 1.4. 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.

 Deprecated:
 ----------
 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.


 1.5 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 drivers/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_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
 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.

 The following macros can be used as iterators over cpufreq_frequency_table:

 cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
 table.

 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 = ...
 	}
	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 <linux@brodo.de>



	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/target_index or setpolicy?
	1.5 target/target_index
	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.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/
	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.


	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
	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/target_index 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


	1.4. 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.

	Deprecated:
	----------
	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.


	1.5 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 drivers/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_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
	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.

	The following macros can be used as iterators over cpufreq_frequency_table:

	cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
	table.

	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 = ...
	}