Documentation / cpu-freq / core.txt
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Based on kernel version 3.9. Page generated on 2013-05-02 22:56 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 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.
- [ cpu-freq ]
- amd-powernow.txt
- boost.txt
- core.txt
- cpu-drivers.txt
- cpufreq-nforce2.txt
- cpufreq-stats.txt
- governors.txt
- index.txt
- pcc-cpufreq.txt
- user-guide.txt
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