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 19Contents: 20--------- 211. CPUFreq core and interfaces 222. CPUFreq notifiers 23 241. General Information 25======================= 26 27The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This 28cpufreq code offers a standardized interface for the CPUFreq 29architecture drivers (those pieces of code that do actual 30frequency transitions), as well as to "notifiers". These are device 31drivers or other part of the kernel that need to be informed of 32policy changes (ex. thermal modules like ACPI) or of all 33frequency changes (ex. timing code) or even need to force certain 34speed limits (like LCD drivers on ARM architecture). Additionally, the 35kernel "constant" loops_per_jiffy is updated on frequency changes 36here. 37 38Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu, 39which make sure that the cpufreq processor driver is correctly 40registered with the core, and will not be unloaded until 41cpufreq_put_cpu is called. 42 432. CPUFreq notifiers 44==================== 45 46CPUFreq notifiers conform to the standard kernel notifier interface. 47See linux/include/linux/notifier.h for details on notifiers. 48 49There are two different CPUFreq notifiers - policy notifiers and 50transition notifiers. 51 52 532.1 CPUFreq policy notifiers 54---------------------------- 55 56These are notified when a new policy is intended to be set. Each 57CPUFreq policy notifier is called three times for a policy transition: 58 591.) 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. 62 632.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid 64 hardware failure. 65 663.) 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 71The phase is specified in the second argument to the notifier. 72 73The third argument, a void *pointer, points to a struct cpufreq_policy 74consisting of five values: cpu, min, max, policy and max_cpu_freq. min 75and max are the lower and upper frequencies (in kHz) of the new 76policy, policy the new policy, cpu the number of the affected CPU; and 77max_cpu_freq the maximum supported CPU frequency. This value is given 78for informational purposes only. 79 80 812.2 CPUFreq transition notifiers 82-------------------------------- 83 84These are notified twice when the CPUfreq driver switches the CPU core 85frequency and this change has any external implications. 86 87The second argument specifies the phase - CPUFREQ_PRECHANGE or 88CPUFREQ_POSTCHANGE. 89 90The third argument is a struct cpufreq_freqs with the following 91values: 92cpu - number of the affected CPU 93old - old frequency 94new - new frequency 95 96If the cpufreq core detects the frequency has changed while the system 97was suspended, these notifiers are called with CPUFREQ_RESUMECHANGE as 98second argument. 99