99 lines
3.3 KiB
Plaintext
99 lines
3.3 KiB
Plaintext
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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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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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1. General Information
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=======================
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The CPUFreq core code is located in linux/kernel/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 is done by cpufreq_get_cpu and cpufreq_put_cpu,
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which make sure that the cpufreq processor driver is correctly
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registered with the core, and will not be unloaded until
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cpufreq_put_cpu is called.
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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 three times 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.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid
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hardware failure.
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3.) 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 five values: cpu, min, max, policy and max_cpu_freq. min
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and max are the lower and upper frequencies (in kHz) of the new
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policy, policy the new policy, cpu the number of the affected CPU; and
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max_cpu_freq the maximum supported CPU frequency. This value is given
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for informational purposes only.
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2.2 CPUFreq transition notifiers
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--------------------------------
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These are notified twice when the CPUfreq driver switches the CPU core
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frequency and this change has 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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If the cpufreq core detects the frequency has changed while the system
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was suspended, these notifiers are called with CPUFREQ_RESUMECHANGE as
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second argument.
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