From 57f0f512b273f60d52568b8c6b77e17f5636edc0 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Andr=C3=A9=20Fabian=20Silva=20Delgado?= Date: Wed, 5 Aug 2015 17:04:01 -0300 Subject: Initial import --- Documentation/cpu-freq/core.txt | 123 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 123 insertions(+) create mode 100644 Documentation/cpu-freq/core.txt (limited to 'Documentation/cpu-freq/core.txt') diff --git a/Documentation/cpu-freq/core.txt b/Documentation/cpu-freq/core.txt new file mode 100644 index 000000000..70933eadc --- /dev/null +++ b/Documentation/cpu-freq/core.txt @@ -0,0 +1,123 @@ + 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 + David Kimdon + + + + 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 is done by cpufreq_get_cpu and cpufreq_put_cpu, +which make sure that the cpufreq processor driver is correctly +registered with the core, and will not be unloaded until +cpufreq_put_cpu is called. + +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 three times 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.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid + hardware failure. + +3.) 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 five values: cpu, min, max, policy and max_cpu_freq. min +and max are the lower and upper frequencies (in kHz) of the new +policy, policy the new policy, cpu the number of the affected CPU; and +max_cpu_freq the maximum supported CPU frequency. This value is given +for informational purposes only. + + +2.2 CPUFreq transition notifiers +-------------------------------- + +These are notified twice when the CPUfreq driver switches the CPU core +frequency and this change has 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 + +3. CPUFreq Table Generation with Operating Performance Point (OPP) +================================================================== +For details about OPP, see Documentation/power/opp.txt + +dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with + cpufreq_frequency_table_cpuinfo which is provided with the list of + frequencies that are available for operation. 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) + cpufreq_frequency_table_cpuinfo(policy, 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 -- cgit v1.2.3