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 233. CPUFreq Table Generation with Operating Performance Point (OPP) 24 251. General Information 26======================= 27 28The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This 29cpufreq code offers a standardized interface for the CPUFreq 30architecture drivers (those pieces of code that do actual 31frequency transitions), as well as to "notifiers". These are device 32drivers or other part of the kernel that need to be informed of 33policy changes (ex. thermal modules like ACPI) or of all 34frequency changes (ex. timing code) or even need to force certain 35speed limits (like LCD drivers on ARM architecture). Additionally, the 36kernel "constant" loops_per_jiffy is updated on frequency changes 37here. 38 39Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu, 40which make sure that the cpufreq processor driver is correctly 41registered with the core, and will not be unloaded until 42cpufreq_put_cpu is called. 43 442. CPUFreq notifiers 45==================== 46 47CPUFreq notifiers conform to the standard kernel notifier interface. 48See linux/include/linux/notifier.h for details on notifiers. 49 50There are two different CPUFreq notifiers - policy notifiers and 51transition notifiers. 52 53 542.1 CPUFreq policy notifiers 55---------------------------- 56 57These are notified when a new policy is intended to be set. Each 58CPUFreq policy notifier is called three times for a policy transition: 59 601.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if 61 they see a need for this - may it be thermal considerations or 62 hardware limitations. 63 642.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid 65 hardware failure. 66 673.) 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 five values: cpu, min, max, policy and max_cpu_freq. min 76and max are the lower and upper frequencies (in kHz) of the new 77policy, policy the new policy, cpu the number of the affected CPU; and 78max_cpu_freq the maximum supported CPU frequency. This value is given 79for informational purposes only. 80 81 822.2 CPUFreq transition notifiers 83-------------------------------- 84 85These are notified twice when the CPUfreq driver switches the CPU core 86frequency and this change has any external implications. 87 88The second argument specifies the phase - CPUFREQ_PRECHANGE or 89CPUFREQ_POSTCHANGE. 90 91The third argument is a struct cpufreq_freqs with the following 92values: 93cpu - number of the affected CPU 94old - old frequency 95new - new frequency 96 973. CPUFreq Table Generation with Operating Performance Point (OPP) 98================================================================== 99For details about OPP, see Documentation/power/opp.txt 100 101dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with 102 cpufreq_frequency_table_cpuinfo which is provided with the list of 103 frequencies that are available for operation. This function provides 104 a ready to use conversion routine to translate the OPP layer's internal 105 information about the available frequencies into a format readily 106 providable to cpufreq. 107 108 WARNING: Do not use this function in interrupt context. 109 110 Example: 111 soc_pm_init() 112 { 113 /* Do things */ 114 r = dev_pm_opp_init_cpufreq_table(dev, &freq_table); 115 if (!r) 116 cpufreq_frequency_table_cpuinfo(policy, freq_table); 117 /* Do other things */ 118 } 119 120 NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in 121 addition to CONFIG_PM_OPP. 122 123dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table 124