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
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  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
 233.  CPUFreq Table Generation with Operating Performance Point (OPP)
 24
 251. General Information
 26=======================
 27
 28The CPUFreq core code is located in drivers/cpufreq/cpufreq.c. This
 29cpufreq code offers a standardized interface for the CPUFreq
 30architecture drivers (those pieces of code that do actual
 31frequency transitions), as well as to "notifiers". These are device
 32drivers or other part of the kernel that need to be informed of
 33policy changes (ex. thermal modules like ACPI) or of all
 34frequency changes (ex. timing code) or even need to force certain
 35speed limits (like LCD drivers on ARM architecture). Additionally, the
 36kernel "constant" loops_per_jiffy is updated on frequency changes
 37here.
 38
 39Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu,
 40which make sure that the cpufreq processor driver is correctly
 41registered with the core, and will not be unloaded until
 42cpufreq_put_cpu is called.
 43
 442. CPUFreq notifiers
 45====================
 46
 47CPUFreq notifiers conform to the standard kernel notifier interface.
 48See linux/include/linux/notifier.h for details on notifiers.
 49
 50There are two different CPUFreq notifiers - policy notifiers and
 51transition notifiers.
 52
 53
 542.1 CPUFreq policy notifiers
 55----------------------------
 56
 57These are notified when a new policy is intended to be set. Each
 58CPUFreq policy notifier is called twice for a policy transition:
 59
 601.) 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
 642.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
 65   - if two hardware drivers failed to agree on a new policy before this
 66   stage, the incompatible hardware shall be shut down, and the user
 67   informed of this.
 68
 69The phase is specified in the second argument to the notifier.
 70
 71The third argument, a void *pointer, points to a struct cpufreq_policy
 72consisting of five values: cpu, min, max, policy and max_cpu_freq. min 
 73and max are the lower and upper frequencies (in kHz) of the new
 74policy, policy the new policy, cpu the number of the affected CPU; and 
 75max_cpu_freq the maximum supported CPU frequency. This value is given 
 76for informational purposes only.
 77
 78
 792.2 CPUFreq transition notifiers
 80--------------------------------
 81
 82These are notified twice when the CPUfreq driver switches the CPU core
 83frequency and this change has any external implications.
 84
 85The second argument specifies the phase - CPUFREQ_PRECHANGE or
 86CPUFREQ_POSTCHANGE.
 87
 88The third argument is a struct cpufreq_freqs with the following
 89values:
 90cpu	- number of the affected CPU
 91old	- old frequency
 92new	- new frequency
 93
 943. CPUFreq Table Generation with Operating Performance Point (OPP)
 95==================================================================
 96For details about OPP, see Documentation/power/opp.txt
 97
 98dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
 99	cpufreq_frequency_table_cpuinfo which is provided with the list of
100	frequencies that are available for operation. This function provides
101	a ready to use conversion routine to translate the OPP layer's internal
102	information about the available frequencies into a format readily
103	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			cpufreq_frequency_table_cpuinfo(policy, 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