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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> |
12 | Viresh Kumar <viresh.kumar@linaro.org> | |
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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) |
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26 | |
27 | 1. General Information | |
28 | ======================= | |
29 | ||
eff0df65 | 30 | The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This |
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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 | ||
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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. | |
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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. | |
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65 | |
66 | The third argument, a void *pointer, points to a struct cpufreq_policy | |
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67 | consisting of several values, including min, max (the lower and upper |
68 | frequencies (in kHz) of the new policy). | |
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69 | |
70 | ||
71 | 2.2 CPUFreq transition notifiers | |
72 | -------------------------------- | |
73 | ||
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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. | |
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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 |
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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 | |
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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. | |
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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; |
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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 |