linux/Documentation/cpu-freq/core.txt
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   1     CPU frequency and voltage scaling code in the Linux(TM) kernel
   2
   3
   4                         L i n u x    C P U F r e q
   5
   6                          C P U F r e q    C o r e
   7
   8
   9                    Dominik Brodowski  <linux@brodo.de>
  10                     David Kimdon <dwhedon@debian.org>
  11
  12
  13
  14   Clock scaling allows you to change the clock speed of the CPUs on the
  15    fly. This is a nice method to save battery power, because the lower
  16            the clock speed, the less power the CPU consumes.
  17
  18
  19Contents:
  20---------
  211.  CPUFreq core and interfaces
  222.  CPUFreq notifiers
  23
  241. General Information
  25=======================
  26
  27The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
  28cpufreq code offers a standardized interface for the CPUFreq
  29architecture drivers (those pieces of code that do actual
  30frequency transitions), as well as to "notifiers". These are device
  31drivers or other part of the kernel that need to be informed of
  32policy changes (ex. thermal modules like ACPI) or of all
  33frequency changes (ex. timing code) or even need to force certain
  34speed limits (like LCD drivers on ARM architecture). Additionally, the
  35kernel "constant" loops_per_jiffy is updated on frequency changes
  36here.
  37
  38Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu,
  39which make sure that the cpufreq processor driver is correctly
  40registered with the core, and will not be unloaded until
  41cpufreq_put_cpu is called.
  42
  432. CPUFreq notifiers
  44====================
  45
  46CPUFreq notifiers conform to the standard kernel notifier interface.
  47See linux/include/linux/notifier.h for details on notifiers.
  48
  49There are two different CPUFreq notifiers - policy notifiers and
  50transition notifiers.
  51
  52
  532.1 CPUFreq policy notifiers
  54----------------------------
  55
  56These are notified when a new policy is intended to be set. Each
  57CPUFreq policy notifier is called three times for a policy transition:
  58
  591.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
  60    they see a need for this - may it be thermal considerations or
  61    hardware limitations.
  62
  632.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid
  64    hardware failure.
  65
  663.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
  67   - if two hardware drivers failed to agree on a new policy before this
  68   stage, the incompatible hardware shall be shut down, and the user
  69   informed of this.
  70
  71The phase is specified in the second argument to the notifier.
  72
  73The third argument, a void *pointer, points to a struct cpufreq_policy
  74consisting of five values: cpu, min, max, policy and max_cpu_freq. min 
  75and max are the lower and upper frequencies (in kHz) of the new
  76policy, policy the new policy, cpu the number of the affected CPU; and 
  77max_cpu_freq the maximum supported CPU frequency. This value is given 
  78for informational purposes only.
  79
  80
  812.2 CPUFreq transition notifiers
  82--------------------------------
  83
  84These are notified twice when the CPUfreq driver switches the CPU core
  85frequency and this change has any external implications.
  86
  87The second argument specifies the phase - CPUFREQ_PRECHANGE or
  88CPUFREQ_POSTCHANGE.
  89
  90The third argument is a struct cpufreq_freqs with the following
  91values:
  92cpu     - number of the affected CPU
  93old     - old frequency
  94new     - new frequency
  95
  96If the cpufreq core detects the frequency has changed while the system
  97was suspended, these notifiers are called with CPUFREQ_RESUMECHANGE as
  98second argument.
  99
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