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		Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 12		   Viresh Kumar <viresh.kumar@linaro.org>
 13
 14
 15
 16   Clock scaling allows you to change the clock speed of the CPUs on the
 17    fly. This is a nice method to save battery power, because the lower
 18            the clock speed, the less power the CPU consumes.
 19
 20
 21Contents:
 22---------
 231.  CPUFreq core and interfaces
 242.  CPUFreq notifiers
 253.  CPUFreq Table Generation with Operating Performance Point (OPP)
 26
 271. General Information
 28=======================
 29
 30The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
 31cpufreq code offers a standardized interface for the CPUFreq
 32architecture drivers (those pieces of code that do actual
 33frequency transitions), as well as to "notifiers". These are device
 34drivers or other part of the kernel that need to be informed of
 35policy changes (ex. thermal modules like ACPI) or of all
 36frequency changes (ex. timing code) or even need to force certain
 37speed limits (like LCD drivers on ARM architecture). Additionally, the
 38kernel "constant" loops_per_jiffy is updated on frequency changes
 39here.
 40
 41Reference counting of the cpufreq policies is done by cpufreq_cpu_get
 42and cpufreq_cpu_put, which make sure that the cpufreq driver is
 43correctly registered with the core, and will not be unloaded until
 44cpufreq_put_cpu is called. That also ensures that the respective cpufreq
 45policy doesn't get freed while being used.
 46
 472. CPUFreq notifiers
 48====================
 49
 50CPUFreq notifiers conform to the standard kernel notifier interface.
 51See linux/include/linux/notifier.h for details on notifiers.
 52
 53There are two different CPUFreq notifiers - policy notifiers and
 54transition notifiers.
 55
 56
 572.1 CPUFreq policy notifiers
 58----------------------------
 59
 60These are notified when a new policy is intended to be set. Each
 61CPUFreq policy notifier is called twice for a policy transition:
 62
 631.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
 64    they see a need for this - may it be thermal considerations or
 65    hardware limitations.
 66
 672.) 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
 72The phase is specified in the second argument to the notifier.
 73
 74The third argument, a void *pointer, points to a struct cpufreq_policy
 75consisting of several values, including min, max (the lower and upper
 76frequencies (in kHz) of the new policy).
 77
 78
 792.2 CPUFreq transition notifiers
 80--------------------------------
 81
 82These are notified twice for each online CPU in the policy, when the
 83CPUfreq driver switches the CPU core frequency and this change has no
 84any external implications.
 85
 86The second argument specifies the phase - CPUFREQ_PRECHANGE or
 87CPUFREQ_POSTCHANGE.
 88
 89The third argument is a struct cpufreq_freqs with the following
 90values:
 91cpu	- number of the affected CPU
 92old	- old frequency
 93new	- new frequency
 94flags	- flags of the cpufreq driver
 95
 963. CPUFreq Table Generation with Operating Performance Point (OPP)
 97==================================================================
 98For details about OPP, see Documentation/power/opp.txt
 99
100dev_pm_opp_init_cpufreq_table -
101	This function provides a ready to use conversion routine to translate
102	the OPP layer's internal information about the available frequencies
103	into a format readily providable to cpufreq.
104
105	WARNING: Do not use this function in interrupt context.
106
107	Example:
108	 soc_pm_init()
109	 {
110		/* Do things */
111		r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
112		if (!r)
113			policy->freq_table = freq_table;
114		/* Do other things */
115	 }
116
117	NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
118	addition to CONFIG_PM_OPP.
119
120dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table