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1da177e4 LT |
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 | ||
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 | ||
19 | Contents: | |
20 | --------- | |
21 | 1. CPUFreq core and interfaces | |
22 | 2. CPUFreq notifiers | |
23 | ||
24 | 1. General Information | |
25 | ======================= | |
26 | ||
27 | The CPUFreq core code is located in linux/kernel/cpufreq.c. This | |
28 | cpufreq code offers a standardized interface for the CPUFreq | |
29 | architecture drivers (those pieces of code that do actual | |
30 | frequency transitions), as well as to "notifiers". These are device | |
31 | drivers or other part of the kernel that need to be informed of | |
32 | policy changes (ex. thermal modules like ACPI) or of all | |
33 | frequency changes (ex. timing code) or even need to force certain | |
34 | speed limits (like LCD drivers on ARM architecture). Additionally, the | |
35 | kernel "constant" loops_per_jiffy is updated on frequency changes | |
36 | here. | |
37 | ||
38 | Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu, | |
39 | which make sure that the cpufreq processor driver is correctly | |
40 | registered with the core, and will not be unloaded until | |
41 | cpufreq_put_cpu is called. | |
42 | ||
43 | 2. CPUFreq notifiers | |
44 | ==================== | |
45 | ||
46 | CPUFreq notifiers conform to the standard kernel notifier interface. | |
47 | See linux/include/linux/notifier.h for details on notifiers. | |
48 | ||
49 | There are two different CPUFreq notifiers - policy notifiers and | |
50 | transition notifiers. | |
51 | ||
52 | ||
53 | 2.1 CPUFreq policy notifiers | |
54 | ---------------------------- | |
55 | ||
56 | These are notified when a new policy is intended to be set. Each | |
57 | CPUFreq policy notifier is called three times for a policy transition: | |
58 | ||
59 | 1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if | |
60 | they see a need for this - may it be thermal considerations or | |
61 | hardware limitations. | |
62 | ||
63 | 2.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid | |
64 | hardware failure. | |
65 | ||
66 | 3.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy | |
67 | - if two hardware drivers failed to agree on a new policy before this | |
68 | stage, the incompatible hardware shall be shut down, and the user | |
69 | informed of this. | |
70 | ||
71 | The phase is specified in the second argument to the notifier. | |
72 | ||
73 | The third argument, a void *pointer, points to a struct cpufreq_policy | |
74 | consisting of five values: cpu, min, max, policy and max_cpu_freq. min | |
75 | and max are the lower and upper frequencies (in kHz) of the new | |
76 | policy, policy the new policy, cpu the number of the affected CPU; and | |
77 | max_cpu_freq the maximum supported CPU frequency. This value is given | |
78 | for informational purposes only. | |
79 | ||
80 | ||
81 | 2.2 CPUFreq transition notifiers | |
82 | -------------------------------- | |
83 | ||
84 | These are notified twice when the CPUfreq driver switches the CPU core | |
85 | frequency and this change has any external implications. | |
86 | ||
87 | The second argument specifies the phase - CPUFREQ_PRECHANGE or | |
88 | CPUFREQ_POSTCHANGE. | |
89 | ||
90 | The third argument is a struct cpufreq_freqs with the following | |
91 | values: | |
92 | cpu - number of the affected CPU | |
93 | old - old frequency | |
94 | new - new frequency | |
95 | ||
96 | If the cpufreq core detects the frequency has changed while the system | |
97 | was suspended, these notifiers are called with CPUFREQ_RESUMECHANGE as | |
98 | second argument. |