About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / cpu-freq / core.txt

Custom Search

Based on kernel version 4.9. Page generated on 2016-12-21 14:28 EST.

1	     CPU frequency and voltage scaling code in the Linux(TM) kernel
4			         L i n u x    C P U F r e q
6				  C P U F r e q    C o r e
9			    Dominik Brodowski  <linux@brodo.de>
10			     David Kimdon <dwhedon@debian.org>
14	   Clock scaling allows you to change the clock speed of the CPUs on the
15	    fly. This is a nice method to save battery power, because the lower
16	            the clock speed, the less power the CPU consumes.
19	Contents:
20	---------
21	1.  CPUFreq core and interfaces
22	2.  CPUFreq notifiers
23	3.  CPUFreq Table Generation with Operating Performance Point (OPP)
25	1. General Information
26	=======================
28	The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
29	cpufreq code offers a standardized interface for the CPUFreq
30	architecture drivers (those pieces of code that do actual
31	frequency transitions), as well as to "notifiers". These are device
32	drivers or other part of the kernel that need to be informed of
33	policy changes (ex. thermal modules like ACPI) or of all
34	frequency changes (ex. timing code) or even need to force certain
35	speed limits (like LCD drivers on ARM architecture). Additionally, the
36	kernel "constant" loops_per_jiffy is updated on frequency changes
37	here.
39	Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu,
40	which make sure that the cpufreq processor driver is correctly
41	registered with the core, and will not be unloaded until
42	cpufreq_put_cpu is called.
44	2. CPUFreq notifiers
45	====================
47	CPUFreq notifiers conform to the standard kernel notifier interface.
48	See linux/include/linux/notifier.h for details on notifiers.
50	There are two different CPUFreq notifiers - policy notifiers and
51	transition notifiers.
54	2.1 CPUFreq policy notifiers
55	----------------------------
57	These are notified when a new policy is intended to be set. Each
58	CPUFreq policy notifier is called twice for a policy transition:
60	1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
61	    they see a need for this - may it be thermal considerations or
62	    hardware limitations.
64	2.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
65	   - if two hardware drivers failed to agree on a new policy before this
66	   stage, the incompatible hardware shall be shut down, and the user
67	   informed of this.
69	The phase is specified in the second argument to the notifier.
71	The third argument, a void *pointer, points to a struct cpufreq_policy
72	consisting of five values: cpu, min, max, policy and max_cpu_freq. min 
73	and max are the lower and upper frequencies (in kHz) of the new
74	policy, policy the new policy, cpu the number of the affected CPU; and 
75	max_cpu_freq the maximum supported CPU frequency. This value is given 
76	for informational purposes only.
79	2.2 CPUFreq transition notifiers
80	--------------------------------
82	These are notified twice when the CPUfreq driver switches the CPU core
83	frequency and this change has any external implications.
85	The second argument specifies the phase - CPUFREQ_PRECHANGE or
88	The third argument is a struct cpufreq_freqs with the following
89	values:
90	cpu	- number of the affected CPU
91	old	- old frequency
92	new	- new frequency
94	3. CPUFreq Table Generation with Operating Performance Point (OPP)
95	==================================================================
96	For details about OPP, see Documentation/power/opp.txt
98	dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
99		cpufreq_table_validate_and_show() which is provided with the list of
100		frequencies that are available for operation. This function provides
101		a ready to use conversion routine to translate the OPP layer's internal
102		information about the available frequencies into a format readily
103		providable to cpufreq.
105		WARNING: Do not use this function in interrupt context.
107		Example:
108		 soc_pm_init()
109		 {
110			/* Do things */
111			r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
112			if (!r)
113				cpufreq_table_validate_and_show(policy, freq_table);
114			/* Do other things */
115		 }
117		NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
118		addition to CONFIG_PM_OPP.
120	dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.