Documentation/cpu-freq/core.txt

   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
  19 Contents:
  20 ---------
  21 1.  CPUFreq core and interfaces
  22 2.  CPUFreq notifiers
  23 3.  CPUFreq Table Generation with Operating Performance Point (OPP)
  24
  25 1. General Information
  26 =======================
  27
  28 The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
  29 cpufreq code offers a standardized interface for the CPUFreq
  30 architecture drivers (those pieces of code that do actual
  31 frequency transitions), as well as to "notifiers". These are device
  32 drivers or other part of the kernel that need to be informed of
  33 policy changes (ex. thermal modules like ACPI) or of all
  34 frequency changes (ex. timing code) or even need to force certain
  35 speed limits (like LCD drivers on ARM architecture). Additionally, the
  36 kernel "constant" loops_per_jiffy is updated on frequency changes
  37 here.
  38
  39 Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu,
  40 which make sure that the cpufreq processor driver is correctly
  41 registered with the core, and will not be unloaded until
  42 cpufreq_put_cpu is called.
  43
  44 2. CPUFreq notifiers
  45 ====================
  46
  47 CPUFreq notifiers conform to the standard kernel notifier interface.
  48 See linux/include/linux/notifier.h for details on notifiers.
  49
  50 There are two different CPUFreq notifiers - policy notifiers and
  51 transition notifiers.
  52
  53
  54 2.1 CPUFreq policy notifiers
  55 ----------------------------
  56
  57 These are notified when a new policy is intended to be set. Each
  58 CPUFreq policy notifier is called three times for a policy transition:
  59
  60 1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
  61     they see a need for this - may it be thermal considerations or
  62     hardware limitations.
  63
  64 2.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid
  65     hardware failure.
  66
  67 3.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
  68    - if two hardware drivers failed to agree on a new policy before this
  69    stage, the incompatible hardware shall be shut down, and the user
  70    informed of this.
  71
  72 The phase is specified in the second argument to the notifier.
  73
  74 The third argument, a void *pointer, points to a struct cpufreq_policy
  75 consisting of five values: cpu, min, max, policy and max_cpu_freq. min
  76 and max are the lower and upper frequencies (in kHz) of the new
  77 policy, policy the new policy, cpu the number of the affected CPU; and
  78 max_cpu_freq the maximum supported CPU frequency. This value is given
  79 for informational purposes only.
  80
  81
  82 2.2 CPUFreq transition notifiers
  83 --------------------------------
  84
  85 These are notified twice when the CPUfreq driver switches the CPU core
  86 frequency and this change has any external implications.
  87
  88 The second argument specifies the phase - CPUFREQ_PRECHANGE or
  89 CPUFREQ_POSTCHANGE.
  90
  91 The third argument is a struct cpufreq_freqs with the following
  92 values:
  93 cpu     - number of the affected CPU
  94 old     - old frequency
  95 new     - new frequency
  96
  97 3. CPUFreq Table Generation with Operating Performance Point (OPP)
  98 ==================================================================
  99 For details about OPP, see Documentation/power/opp.txt
 100
 101 dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
 102         cpufreq_frequency_table_cpuinfo which is provided with the list of
 103         frequencies that are available for operation. This function provides
 104         a ready to use conversion routine to translate the OPP layer's internal
 105         information about the available frequencies into a format readily
 106         providable to cpufreq.
 107
 108         WARNING: Do not use this function in interrupt context.
 109
 110         Example:
 111          soc_pm_init()
 112          {
 113                 /* Do things */
 114                 r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
 115                 if (!r)
 116                         cpufreq_frequency_table_cpuinfo(policy, freq_table);
 117                 /* Do other things */
 118          }
 119
 120         NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
 121         addition to CONFIG_PM_OPP.
 122
 123 dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table