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+ 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>
+ 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.
+1. CPUFreq core and interfaces
+2. CPUFreq notifiers
+1. General Information
+The CPUFreq core code is located in linux/kernel/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
+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
+The third argument is a struct cpufreq_freqs with the following
+cpu - number of the affected CPU
+old - old frequency
+new - new frequency
+If the cpufreq core detects the frequency has changed while the system
+was suspended, these notifiers are called with CPUFREQ_RESUMECHANGE as
+second argument.