1 .. SPDX-License-Identifier: GPL-2.0
2
3 =============================================================
4 General description of the CPUFreq core and CPUFreq notifiers
5 =============================================================
6
7 Authors:
8 - Dominik Brodowski <linux@brodo.de>
9 - David Kimdon <dwhedon@debian.org>
10 - Rafael J. Wysocki <rafael.j.wysocki@intel.com>
11 - Viresh Kumar <viresh.kumar@linaro.org>
12
13 .. Contents:
14
15 1. CPUFreq core and interfaces
16 2. CPUFreq notifiers
17 3. CPUFreq Table Generation with Operating Performance Point (OPP)
18
19 1. General Information
20 ======================
21
22 The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
23 cpufreq code offers a standardized interface for the CPUFreq
24 architecture drivers (those pieces of code that do actual
25 frequency transitions), as well as to "notifiers". These are device
26 drivers or other part of the kernel that need to be informed of
27 policy changes (ex. thermal modules like ACPI) or of all
28 frequency changes (ex. timing code) or even need to force certain
29 speed limits (like LCD drivers on ARM architecture). Additionally, the
30 kernel "constant" loops_per_jiffy is updated on frequency changes
31 here.
32
33 Reference counting of the cpufreq policies is done by cpufreq_cpu_get
34 and cpufreq_cpu_put, which make sure that the cpufreq driver is
35 correctly registered with the core, and will not be unloaded until
36 cpufreq_put_cpu is called. That also ensures that the respective cpufreq
37 policy doesn't get freed while being used.
38
39 2. CPUFreq notifiers
40 ====================
41
42 CPUFreq notifiers conform to the standard kernel notifier interface.
43 See linux/include/linux/notifier.h for details on notifiers.
44
45 There are two different CPUFreq notifiers - policy notifiers and
46 transition notifiers.
47
48
49 2.1 CPUFreq policy notifiers
50 ----------------------------
51
52 These are notified when a new policy is created or removed.
53
54 The phase is specified in the second argument to the notifier. The phase is
55 CPUFREQ_CREATE_POLICY when the policy is first created and it is
56 CPUFREQ_REMOVE_POLICY when the policy is removed.
57
58 The third argument, a ``void *pointer``, points to a struct cpufreq_policy
59 consisting of several values, including min, max (the lower and upper
60 frequencies (in kHz) of the new policy).
61
62
63 2.2 CPUFreq transition notifiers
64 --------------------------------
65
66 These are notified twice for each online CPU in the policy, when the
67 CPUfreq driver switches the CPU core frequency and this change has no
68 any external implications.
69
70 The second argument specifies the phase - CPUFREQ_PRECHANGE or
71 CPUFREQ_POSTCHANGE.
72
73 The third argument is a struct cpufreq_freqs with the following
74 values:
75
76 ====== ======================================
77 policy a pointer to the struct cpufreq_policy
78 old old frequency
79 new new frequency
80 flags flags of the cpufreq driver
81 ====== ======================================
82
83 3. CPUFreq Table Generation with Operating Performance Point (OPP)
84 ==================================================================
85 For details about OPP, see Documentation/power/opp.rst
86
87 dev_pm_opp_init_cpufreq_table -
88 This function provides a ready to use conversion routine to translate
89 the OPP layer's internal information about the available frequencies
90 into a format readily providable to cpufreq.
91
92 .. Warning::
93
94 Do not use this function in interrupt context.
95
96 Example::
97
98 soc_pm_init()
99 {
100 /* Do things */
101 r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
102 if (!r)
103 policy->freq_table = freq_table;
104 /* Do other things */
105 }
106
107 .. note::
108
109 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
113 Free up the table allocated by dev_pm_opp_init_cpufreq_table
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