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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 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
12
13
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.
17
18
19Contents:
20---------
211. CPUFreq core and interfaces
222. CPUFreq notifiers
Nishanth Menona0dd7b72014-05-05 08:33:50 -0500233. CPUFreq Table Generation with Operating Performance Point (OPP)
Linus Torvalds1da177e2005-04-16 15:20:36 -070024
251. General Information
26=======================
27
Dominik Brodowskieff0df62006-10-02 19:26:47 -040028The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
Linus Torvalds1da177e2005-04-16 15:20:36 -070029cpufreq code offers a standardized interface for the CPUFreq
30architecture drivers (those pieces of code that do actual
31frequency transitions), as well as to "notifiers". These are device
32drivers or other part of the kernel that need to be informed of
33policy changes (ex. thermal modules like ACPI) or of all
34frequency changes (ex. timing code) or even need to force certain
35speed limits (like LCD drivers on ARM architecture). Additionally, the
36kernel "constant" loops_per_jiffy is updated on frequency changes
37here.
38
39Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu,
40which make sure that the cpufreq processor driver is correctly
41registered with the core, and will not be unloaded until
42cpufreq_put_cpu is called.
43
442. CPUFreq notifiers
45====================
46
47CPUFreq notifiers conform to the standard kernel notifier interface.
48See linux/include/linux/notifier.h for details on notifiers.
49
50There are two different CPUFreq notifiers - policy notifiers and
51transition notifiers.
52
53
542.1 CPUFreq policy notifiers
55----------------------------
56
57These are notified when a new policy is intended to be set. Each
Viresh Kumar6bfb7c72015-08-03 08:36:14 +053058CPUFreq policy notifier is called twice for a policy transition:
Linus Torvalds1da177e2005-04-16 15:20:36 -070059
601.) 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.
63
Viresh Kumar6bfb7c72015-08-03 08:36:14 +0530642.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
Linus Torvalds1da177e2005-04-16 15:20:36 -070065 - 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.
68
69The phase is specified in the second argument to the notifier.
70
71The third argument, a void *pointer, points to a struct cpufreq_policy
72consisting of five values: cpu, min, max, policy and max_cpu_freq. min
73and max are the lower and upper frequencies (in kHz) of the new
74policy, policy the new policy, cpu the number of the affected CPU; and
75max_cpu_freq the maximum supported CPU frequency. This value is given
76for informational purposes only.
77
78
792.2 CPUFreq transition notifiers
80--------------------------------
81
82These are notified twice when the CPUfreq driver switches the CPU core
83frequency and this change has any external implications.
84
85The second argument specifies the phase - CPUFREQ_PRECHANGE or
86CPUFREQ_POSTCHANGE.
87
88The third argument is a struct cpufreq_freqs with the following
89values:
90cpu - number of the affected CPU
91old - old frequency
92new - new frequency
Nishanth Menona0dd7b72014-05-05 08:33:50 -050093
943. CPUFreq Table Generation with Operating Performance Point (OPP)
95==================================================================
96For details about OPP, see Documentation/power/opp.txt
97
98dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
Viresh Kumar64bf55a2016-05-31 16:50:23 +053099 cpufreq_table_validate_and_show() which is provided with the list of
Nishanth Menona0dd7b72014-05-05 08:33:50 -0500100 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.
104
105 WARNING: Do not use this function in interrupt context.
106
107 Example:
108 soc_pm_init()
109 {
110 /* Do things */
111 r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
112 if (!r)
Viresh Kumar64bf55a2016-05-31 16:50:23 +0530113 cpufreq_table_validate_and_show(policy, freq_table);
Nishanth Menona0dd7b72014-05-05 08:33:50 -0500114 /* Do other things */
115 }
116
117 NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
118 addition to CONFIG_PM_OPP.
119
120dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table