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 Rafael J. Wysocki <rafael.j.wysocki@intel.com> 12 Viresh Kumar <viresh.kumar@linaro.org> 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 21Contents: 22--------- 231. CPUFreq core and interfaces 242. CPUFreq notifiers 253. CPUFreq Table Generation with Operating Performance Point (OPP) 26 271. General Information 28======================= 29 30The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This 31cpufreq code offers a standardized interface for the CPUFreq 32architecture drivers (those pieces of code that do actual 33frequency transitions), as well as to "notifiers". These are device 34drivers or other part of the kernel that need to be informed of 35policy changes (ex. thermal modules like ACPI) or of all 36frequency changes (ex. timing code) or even need to force certain 37speed limits (like LCD drivers on ARM architecture). Additionally, the 38kernel "constant" loops_per_jiffy is updated on frequency changes 39here. 40 41Reference counting of the cpufreq policies is done by cpufreq_cpu_get 42and cpufreq_cpu_put, which make sure that the cpufreq driver is 43correctly registered with the core, and will not be unloaded until 44cpufreq_put_cpu is called. That also ensures that the respective cpufreq 45policy doesn't get freed while being used. 46 472. CPUFreq notifiers 48==================== 49 50CPUFreq notifiers conform to the standard kernel notifier interface. 51See linux/include/linux/notifier.h for details on notifiers. 52 53There are two different CPUFreq notifiers - policy notifiers and 54transition notifiers. 55 56 572.1 CPUFreq policy notifiers 58---------------------------- 59 60These are notified when a new policy is intended to be set. Each 61CPUFreq policy notifier is called twice for a policy transition: 62 631.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if 64 they see a need for this - may it be thermal considerations or 65 hardware limitations. 66 672.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy 68 - if two hardware drivers failed to agree on a new policy before this 69 stage, the incompatible hardware shall be shut down, and the user 70 informed of this. 71 72The phase is specified in the second argument to the notifier. 73 74The third argument, a void *pointer, points to a struct cpufreq_policy 75consisting of several values, including min, max (the lower and upper 76frequencies (in kHz) of the new policy). 77 78 792.2 CPUFreq transition notifiers 80-------------------------------- 81 82These are notified twice for each online CPU in the policy, when the 83CPUfreq driver switches the CPU core frequency and this change has no 84any external implications. 85 86The second argument specifies the phase - CPUFREQ_PRECHANGE or 87CPUFREQ_POSTCHANGE. 88 89The third argument is a struct cpufreq_freqs with the following 90values: 91cpu - number of the affected CPU 92old - old frequency 93new - new frequency 94flags - flags of the cpufreq driver 95 963. CPUFreq Table Generation with Operating Performance Point (OPP) 97================================================================== 98For details about OPP, see Documentation/power/opp.txt 99 100dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with 101 cpufreq_table_validate_and_show() which is provided with the list of 102 frequencies that are available for operation. This function provides 103 a ready to use conversion routine to translate the OPP layer's internal 104 information about the available frequencies into a format readily 105 providable to cpufreq. 106 107 WARNING: Do not use this function in interrupt context. 108 109 Example: 110 soc_pm_init() 111 { 112 /* Do things */ 113 r = dev_pm_opp_init_cpufreq_table(dev, &freq_table); 114 if (!r) 115 cpufreq_table_validate_and_show(policy, freq_table); 116 /* Do other things */ 117 } 118 119 NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in 120 addition to CONFIG_PM_OPP. 121 122dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table 123