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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:54 EST.

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>
11			Rafael J. Wysocki <rafael.j.wysocki@intel.com>
12			   Viresh Kumar <viresh.kumar@linaro.org>
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
25	3.  CPUFreq Table Generation with Operating Performance Point (OPP)
26	
27	1. General Information
28	=======================
29	
30	The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
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	
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.
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	
60	These are notified when a new policy is intended to be set. Each
61	CPUFreq policy notifier is called twice for a policy transition:
62	
63	1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
64	    they see a need for this - may it be thermal considerations or
65	    hardware limitations.
66	
67	2.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
68	   - if two hardware drivers failed to agree on a new policy before this
69	   stage, the incompatible hardware shall be shut down, and the user
70	   informed of this.
71	
72	The phase is specified in the second argument to the notifier.
73	
74	The third argument, a void *pointer, points to a struct cpufreq_policy
75	consisting of several values, including min, max (the lower and upper
76	frequencies (in kHz) of the new policy).
77	
78	
79	2.2 CPUFreq transition notifiers
80	--------------------------------
81	
82	These are notified twice for each online CPU in the policy, when the
83	CPUfreq driver switches the CPU core frequency and this change has no
84	any external implications.
85	
86	The second argument specifies the phase - CPUFREQ_PRECHANGE or
87	CPUFREQ_POSTCHANGE.
88	
89	The third argument is a struct cpufreq_freqs with the following
90	values:
91	cpu	- number of the affected CPU
92	old	- old frequency
93	new	- new frequency
94	flags	- flags of the cpufreq driver
95	
96	3. CPUFreq Table Generation with Operating Performance Point (OPP)
97	==================================================================
98	For details about OPP, see Documentation/power/opp.txt
99	
100	dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
101		cpufreq_table_validate_and_show() which is provided with the list of
102		frequencies that are available for operation. This function provides
103		a ready to use conversion routine to translate the OPP layer's internal
104		information about the available frequencies into a format readily
105		providable to cpufreq.
106	
107		WARNING: Do not use this function in interrupt context.
108	
109		Example:
110		 soc_pm_init()
111		 {
112			/* Do things */
113			r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
114			if (!r)
115				cpufreq_table_validate_and_show(policy, freq_table);
116			/* Do other things */
117		 }
118	
119		NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
120		addition to CONFIG_PM_OPP.
121	
122	dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table
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