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+.. SPDX-License-Identifier: GPL-2.0
+
+=============================================================
+General description of the CPUFreq core and CPUFreq notifiers
+=============================================================
+
+Authors:
+ - Dominik Brodowski <linux@brodo.de>
+ - David Kimdon <dwhedon@debian.org>
+ - Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+ - Viresh Kumar <viresh.kumar@linaro.org>
+
+.. Contents:
+
+ 1. CPUFreq core and interfaces
+ 2. CPUFreq notifiers
+ 3. CPUFreq Table Generation with Operating Performance Point (OPP)
+
+1. General Information
+======================
+
+The CPUFreq core code is located in drivers/cpufreq/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
+here.
+
+Reference counting of the cpufreq policies is done by cpufreq_cpu_get
+and cpufreq_cpu_put, which make sure that the cpufreq driver is
+correctly registered with the core, and will not be unloaded until
+cpufreq_put_cpu is called. That also ensures that the respective cpufreq
+policy doesn't get freed while being used.
+
+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 created or removed.
+
+The phase is specified in the second argument to the notifier. The phase is
+CPUFREQ_CREATE_POLICY when the policy is first created and it is
+CPUFREQ_REMOVE_POLICY when the policy is removed.
+
+The third argument, a ``void *pointer``, points to a struct cpufreq_policy
+consisting of several values, including min, max (the lower and upper
+frequencies (in kHz) of the new policy).
+
+
+2.2 CPUFreq transition notifiers
+--------------------------------
+
+These are notified twice for each online CPU in the policy, when the
+CPUfreq driver switches the CPU core frequency and this change has no
+any external implications.
+
+The second argument specifies the phase - CPUFREQ_PRECHANGE or
+CPUFREQ_POSTCHANGE.
+
+The third argument is a struct cpufreq_freqs with the following
+values:
+
+====== ======================================
+policy a pointer to the struct cpufreq_policy
+old old frequency
+new new frequency
+flags flags of the cpufreq driver
+====== ======================================
+
+3. CPUFreq Table Generation with Operating Performance Point (OPP)
+==================================================================
+For details about OPP, see Documentation/power/opp.rst
+
+dev_pm_opp_init_cpufreq_table -
+ This function provides a ready to use conversion routine to translate
+ the OPP layer's internal information about the available frequencies
+ into a format readily providable to cpufreq.
+
+ .. Warning::
+
+ Do not use this function in interrupt context.
+
+ Example::
+
+ soc_pm_init()
+ {
+ /* Do things */
+ r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
+ if (!r)
+ policy->freq_table = freq_table;
+ /* Do other things */
+ }
+
+ .. note::
+
+ This function is available only if CONFIG_CPU_FREQ is enabled in
+ addition to CONFIG_PM_OPP.
+
+dev_pm_opp_free_cpufreq_table
+ Free up the table allocated by dev_pm_opp_init_cpufreq_table