About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / cpu-freq / core.txt




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 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	
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 drivers/cpufreq/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
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.