[linux-block.git] / 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>
		Rafael J. Wysocki <rafael.j.wysocki@intel.com>
		   Viresh Kumar <viresh.kumar@linaro.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 of the cpufreq policies is done by cpufreq_cpu_get
and cpufreq_cpu_put, which make sure that the cpufreq driver is
correctly registered with the core, and will not be unloaded until
cpufreq_put_cpu is called. That also ensures that the respective cpufreq
policy doesn't get freed while being used.

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 created or removed.

The phase is specified in the second argument to the notifier.  The phase is
CPUFREQ_CREATE_POLICY when the policy is first created and it is
CPUFREQ_REMOVE_POLICY when the policy is removed.

The third argument, a void *pointer, points to a struct cpufreq_policy
consisting of several values, including min, max (the lower and upper
frequencies (in kHz) of the new policy).


2.2 CPUFreq transition notifiers
--------------------------------

These are notified twice for each online CPU in the policy, when the
CPUfreq driver switches the CPU core frequency and this change has no
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
flags	- flags of the cpufreq driver

3. CPUFreq Table Generation with Operating Performance Point (OPP)
==================================================================
For details about OPP, see Documentation/power/opp.rst

dev_pm_opp_init_cpufreq_table -
	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)
			policy->freq_table = 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
Commit	Line	Data
1da177e4 LT	1	CPU frequency and voltage scaling code in the Linux(TM) kernel
	2
	3
	4	L i n u x C P U F r e q
	5
	6	C P U F r e q C o r e
	7
	8
	9	Dominik Brodowski <linux@brodo.de>
	10	David Kimdon <dwhedon@debian.org>
7de962c0 VK	11	Rafael J. Wysocki <rafael.j.wysocki@intel.com>
7de962c0 VK	12	Viresh Kumar <viresh.kumar@linaro.org>
1da177e4 LT	13
	14
	15
	16	Clock scaling allows you to change the clock speed of the CPUs on the
	17	fly. This is a nice method to save battery power, because the lower
	18	the clock speed, the less power the CPU consumes.
	19
	20
	21	Contents:
	22	---------
	23	1. CPUFreq core and interfaces
	24	2. CPUFreq notifiers
a0dd7b79	25	3. CPUFreq Table Generation with Operating Performance Point (OPP)
1da177e4 LT	26
	27	1. General Information
	28	=======================
	29
eff0df65	30	The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
1da177e4 LT	31	cpufreq code offers a standardized interface for the CPUFreq
	32	architecture drivers (those pieces of code that do actual
	33	frequency transitions), as well as to "notifiers". These are device
	34	drivers or other part of the kernel that need to be informed of
	35	policy changes (ex. thermal modules like ACPI) or of all
	36	frequency changes (ex. timing code) or even need to force certain
	37	speed limits (like LCD drivers on ARM architecture). Additionally, the
	38	kernel "constant" loops_per_jiffy is updated on frequency changes
	39	here.
	40
7de962c0 VK	41	Reference counting of the cpufreq policies is done by cpufreq_cpu_get
	42	and cpufreq_cpu_put, which make sure that the cpufreq driver is
	43	correctly registered with the core, and will not be unloaded until
	44	cpufreq_put_cpu is called. That also ensures that the respective cpufreq
	45	policy doesn't get freed while being used.
1da177e4 LT	46
	47	2. CPUFreq notifiers
	48	====================
	49
	50	CPUFreq notifiers conform to the standard kernel notifier interface.
	51	See linux/include/linux/notifier.h for details on notifiers.
	52
	53	There are two different CPUFreq notifiers - policy notifiers and
	54	transition notifiers.
	55
	56
	57	2.1 CPUFreq policy notifiers
	58	----------------------------
	59
c27c38af	60	These are notified when a new policy is created or removed.
1da177e4	61
c27c38af VK	62	The phase is specified in the second argument to the notifier. The phase is
	63	CPUFREQ_CREATE_POLICY when the policy is first created and it is
	64	CPUFREQ_REMOVE_POLICY when the policy is removed.
1da177e4 LT	65
1da177e4 LT	66	The third argument, a void *pointer, points to a struct cpufreq_policy
7de962c0 VK	67	consisting of several values, including min, max (the lower and upper
7de962c0 VK	68	frequencies (in kHz) of the new policy).
1da177e4 LT	69
	70
	71	2.2 CPUFreq transition notifiers
	72	--------------------------------
	73
7de962c0 VK	74	These are notified twice for each online CPU in the policy, when the
	75	CPUfreq driver switches the CPU core frequency and this change has no
	76	any external implications.
1da177e4 LT	77
	78	The second argument specifies the phase - CPUFREQ_PRECHANGE or
	79	CPUFREQ_POSTCHANGE.
	80
	81	The third argument is a struct cpufreq_freqs with the following
	82	values:
	83	cpu - number of the affected CPU
	84	old - old frequency
	85	new - new frequency
7de962c0	86	flags - flags of the cpufreq driver
a0dd7b79 NM	87
	88	3. CPUFreq Table Generation with Operating Performance Point (OPP)
	89	==================================================================
151f4e2b	90	For details about OPP, see Documentation/power/opp.rst
a0dd7b79	91
2dd0df84 VK	92	dev_pm_opp_init_cpufreq_table -
	93	This function provides a ready to use conversion routine to translate
	94	the OPP layer's internal information about the available frequencies
	95	into a format readily providable to cpufreq.
a0dd7b79 NM	96
	97	WARNING: Do not use this function in interrupt context.
	98
	99	Example:
	100	soc_pm_init()
	101	{
	102	/* Do things */
	103	r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
	104	if (!r)
2dd0df84	105	policy->freq_table = freq_table;
a0dd7b79 NM	106	/* Do other things */
	107	}
	108
	109	NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
	110	addition to CONFIG_PM_OPP.
	111
	112	dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table