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