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| -rw-r--r-- | Documentation/cpu-freq/core.txt | 120 |
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diff --git a/Documentation/cpu-freq/core.txt b/Documentation/cpu-freq/core.txt new file mode 100644 index 000000000..073f128af --- /dev/null +++ b/Documentation/cpu-freq/core.txt @@ -0,0 +1,120 @@ + CPU frequency and voltage scaling code in the Linux(TM) kernel + + + L i n u x C P U F r e q + + C P U F r e q C o r e + + + Dominik Brodowski <linux@brodo.de> + David Kimdon <dwhedon@debian.org> + Rafael J. Wysocki <rafael.j.wysocki@intel.com> + Viresh Kumar <viresh.kumar@linaro.org> + + + + Clock scaling allows you to change the clock speed of the CPUs on the + fly. This is a nice method to save battery power, because the lower + the clock speed, the less power the CPU consumes. + + +Contents: +--------- +1. CPUFreq core and interfaces +2. CPUFreq notifiers +3. CPUFreq Table Generation with Operating Performance Point (OPP) + +1. General Information +======================= + +The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This +cpufreq code offers a standardized interface for the CPUFreq +architecture drivers (those pieces of code that do actual +frequency transitions), as well as to "notifiers". These are device +drivers or other part of the kernel that need to be informed of +policy changes (ex. thermal modules like ACPI) or of all +frequency changes (ex. timing code) or even need to force certain +speed limits (like LCD drivers on ARM architecture). Additionally, the +kernel "constant" loops_per_jiffy is updated on frequency changes +here. + +Reference counting of the cpufreq policies is done by cpufreq_cpu_get +and cpufreq_cpu_put, which make sure that the cpufreq driver is +correctly registered with the core, and will not be unloaded until +cpufreq_put_cpu is called. That also ensures that the respective cpufreq +policy doesn't get freed while being used. + +2. CPUFreq notifiers +==================== + +CPUFreq notifiers conform to the standard kernel notifier interface. +See linux/include/linux/notifier.h for details on notifiers. + +There are two different CPUFreq notifiers - policy notifiers and +transition notifiers. + + +2.1 CPUFreq policy notifiers +---------------------------- + +These are notified when a new policy is intended to be set. Each +CPUFreq policy notifier is called twice for a policy transition: + +1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if + they see a need for this - may it be thermal considerations or + hardware limitations. + +2.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy + - if two hardware drivers failed to agree on a new policy before this + stage, the incompatible hardware shall be shut down, and the user + informed of this. + +The phase is specified in the second argument to the notifier. + +The third argument, a void *pointer, points to a struct cpufreq_policy +consisting of several values, including min, max (the lower and upper +frequencies (in kHz) of the new policy). + + +2.2 CPUFreq transition notifiers +-------------------------------- + +These are notified twice for each online CPU in the policy, when the +CPUfreq driver switches the CPU core frequency and this change has no +any external implications. + +The second argument specifies the phase - CPUFREQ_PRECHANGE or +CPUFREQ_POSTCHANGE. + +The third argument is a struct cpufreq_freqs with the following +values: +cpu - number of the affected CPU +old - old frequency +new - new frequency +flags - flags of the cpufreq driver + +3. CPUFreq Table Generation with Operating Performance Point (OPP) +================================================================== +For details about OPP, see Documentation/power/opp.txt + +dev_pm_opp_init_cpufreq_table - + This function provides a ready to use conversion routine to translate + the OPP layer's internal information about the available frequencies + into a format readily providable to cpufreq. + + WARNING: Do not use this function in interrupt context. + + Example: + soc_pm_init() + { + /* Do things */ + r = dev_pm_opp_init_cpufreq_table(dev, &freq_table); + if (!r) + policy->freq_table = freq_table; + /* Do other things */ + } + + NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in + addition to CONFIG_PM_OPP. + +dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table |