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 .. SPDX-License-Identifier: GPL-2.0
 .. include:: <isonum.txt>
 
 ========================
 CPU Idle Time Management
 ========================
 
 :Copyright: |copy| 2019 Intel Corporation
 
 :Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 
 
 CPU Idle Time Management Subsystem
 ==================================
 
 Every time one of the logical CPUs in the system (the entities that appear to
 fetch and execute instructions: hardware threads, if present, or processor
 cores) is idle after an interrupt or equivalent wakeup event, which means that
 there are no tasks to run on it except for the special "idle" task associated
 with it, there is an opportunity to save energy for the processor that it
 belongs to.  That can be done by making the idle logical CPU stop fetching
 instructions from memory and putting some of the processor's functional units
 depended on by it into an idle state in which they will draw less power.
 
 However, there may be multiple different idle states that can be used in such a
 situation in principle, so it may be necessary to find the most suitable one
 (from the kernel perspective) and ask the processor to use (or "enter") that
 particular idle state.  That is the role of the CPU idle time management
 subsystem in the kernel, called ``CPUIdle``.
 
 The design of ``CPUIdle`` is modular and based on the code duplication avoidance
 principle, so the generic code that in principle need not depend on the hardware
 or platform design details in it is separate from the code that interacts with
 the hardware.  It generally is divided into three categories of functional
 units: *governors* responsible for selecting idle states to ask the processor
 to enter, *drivers* that pass the governors' decisions on to the hardware and
 the *core* providing a common framework for them.
 
 
 CPU Idle Time Governors
 =======================
 
 A CPU idle time (``CPUIdle``) governor is a bundle of policy code invoked when
 one of the logical CPUs in the system turns out to be idle.  Its role is to
 select an idle state to ask the processor to enter in order to save some energy.
 
 ``CPUIdle`` governors are generic and each of them can be used on any hardware
 platform that the Linux kernel can run on.  For this reason, data structures
 operated on by them cannot depend on any hardware architecture or platform
 design details as well.
 
 The governor itself is represented by a struct cpuidle_governor object
 containing four callback pointers, :c:member:`enable`, :c:member:`disable`,
 :c:member:`select`, :c:member:`reflect`, a :c:member:`rating` field described
 below, and a name (string) used for identifying it.
 
 For the governor to be available at all, that object needs to be registered
 with the ``CPUIdle`` core by calling :c:func:`cpuidle_register_governor()` with
 a pointer to it passed as the argument.  If successful, that causes the core to
 add the governor to the global list of available governors and, if it is the
 only one in the list (that is, the list was empty before) or the value of its
 :c:member:`rating` field is greater than the value of that field for the
 governor currently in use, or the name of the new governor was passed to the
 kernel as the value of the ``cpuidle.governor=`` command line parameter, the new
 governor will be used from that point on (there can be only one ``CPUIdle``
 governor in use at a time).  Also, user space can choose the ``CPUIdle``
 governor to use at run time via ``sysfs``.
 
 Once registered, ``CPUIdle`` governors cannot be unregistered, so it is not
 practical to put them into loadable kernel modules.
 
 The interface between ``CPUIdle`` governors and the core consists of four
 callbacks:
 
 :c:member:`enable`
         ::
 
           int (*enable) (struct cpuidle_driver *drv, struct cpuidle_device *dev);
 
         The role of this callback is to prepare the governor for handling the
         (logical) CPU represented by the struct cpuidle_device object   pointed
         to by the ``dev`` argument.  The struct cpuidle_driver object pointed
         to by the ``drv`` argument represents the ``CPUIdle`` driver to be used
         with that CPU (among other things, it should contain the list of
         struct cpuidle_state objects representing idle states that the
         processor holding the given CPU can be asked to enter).
 
         It may fail, in which case it is expected to return a negative error
         code, and that causes the kernel to run the architecture-specific
         default code for idle CPUs on the CPU in question instead of ``CPUIdle``
         until the ``->enable()`` governor callback is invoked for that CPU
         again.
 
 :c:member:`disable`
         ::
 
           void (*disable) (struct cpuidle_driver *drv, struct cpuidle_device *dev);
 
         Called to make the governor stop handling the (logical) CPU represented
         by the struct cpuidle_device object pointed to by the ``dev``
         argument.
 
         It is expected to reverse any changes made by the ``->enable()``
         callback when it was last invoked for the target CPU, free all memory
         allocated by that callback and so on.
 
 :c:member:`select`
         ::
 
           int (*select) (struct cpuidle_driver *drv, struct cpuidle_device *dev,
                          bool *stop_tick);
 
         Called to select an idle state for the processor holding the (logical)
         CPU represented by the struct cpuidle_device object pointed to by the
         ``dev`` argument.
 
         The list of idle states to take into consideration is represented by the
         :c:member:`states` array of struct cpuidle_state objects held by the
         struct cpuidle_driver object pointed to by the ``drv`` argument (which
         represents the ``CPUIdle`` driver to be used with the CPU at hand).  The
         value returned by this callback is interpreted as an index into that
         array (unless it is a negative error code).
 
         The ``stop_tick`` argument is used to indicate whether or not to stop
         the scheduler tick before asking the processor to enter the selected
         idle state.  When the ``bool`` variable pointed to by it (which is set
         to ``true`` before invoking this callback) is cleared to ``false``, the
         processor will be asked to enter the selected idle state without
         stopping the scheduler tick on the given CPU (if the tick has been
         stopped on that CPU already, however, it will not be restarted before
         asking the processor to enter the idle state).
 
         This callback is mandatory (i.e. the :c:member:`select` callback pointer
         in struct cpuidle_governor must not be ``NULL`` for the registration
         of the governor to succeed).
 
 :c:member:`reflect`
         ::
 
           void (*reflect) (struct cpuidle_device *dev, int index);
 
         Called to allow the governor to evaluate the accuracy of the idle state
         selection made by the ``->select()`` callback (when it was invoked last
         time) and possibly use the result of that to improve the accuracy of
         idle state selections in the future.
 
 In addition, ``CPUIdle`` governors are required to take power management
 quality of service (PM QoS) constraints on the processor wakeup latency into
 account when selecting idle states.  In order to obtain the current effective
 PM QoS wakeup latency constraint for a given CPU, a ``CPUIdle`` governor is
 expected to pass the number of the CPU to
 :c:func:`cpuidle_governor_latency_req()`.  Then, the governor's ``->select()``
 callback must not return the index of an indle state whose
 :c:member:`exit_latency` value is greater than the number returned by that
 function.
 
 
 CPU Idle Time Management Drivers
 ================================
 
 CPU idle time management (``CPUIdle``) drivers provide an interface between the
 other parts of ``CPUIdle`` and the hardware.
 
 First of all, a ``CPUIdle`` driver has to populate the :c:member:`states` array
 of struct cpuidle_state objects included in the struct cpuidle_driver object
 representing it.  Going forward this array will represent the list of available
 idle states that the processor hardware can be asked to enter shared by all of
 the logical CPUs handled by the given driver.
 
 The entries in the :c:member:`states` array are expected to be sorted by the
 value of the :c:member:`target_residency` field in struct cpuidle_state in
 the ascending order (that is, index 0 should correspond to the idle state with
 the minimum value of :c:member:`target_residency`).  [Since the
 :c:member:`target_residency` value is expected to reflect the "depth" of the
 idle state represented by the struct cpuidle_state object holding it, this
 sorting order should be the same as the ascending sorting order by the idle
 state "depth".]
 
 Three fields in struct cpuidle_state are used by the existing ``CPUIdle``
 governors for computations related to idle state selection:
 
 :c:member:`target_residency`
         Minimum time to spend in this idle state including the time needed to
         enter it (which may be substantial) to save more energy than could
         be saved by staying in a shallower idle state for the same amount of
         time, in microseconds.
 
 :c:member:`exit_latency`
         Maximum time it will take a CPU asking the processor to enter this idle
         state to start executing the first instruction after a wakeup from it,
         in microseconds.
 
 :c:member:`flags`
         Flags representing idle state properties.  Currently, governors only use
         the ``CPUIDLE_FLAG_POLLING`` flag which is set if the given object
         does not represent a real idle state, but an interface to a software
         "loop" that can be used in order to avoid asking the processor to enter
         any idle state at all.  [There are other flags used by the ``CPUIdle``
         core in special situations.]
 
 The :c:member:`enter` callback pointer in struct cpuidle_state, which must not
 be ``NULL``, points to the routine to execute in order to ask the processor to
 enter this particular idle state:
 
 ::
 
   void (*enter) (struct cpuidle_device *dev, struct cpuidle_driver *drv,
                  int index);
 
 The first two arguments of it point to the struct cpuidle_device object
 representing the logical CPU running this callback and the
 struct cpuidle_driver object representing the driver itself, respectively,
 and the last one is an index of the struct cpuidle_state entry in the driver's
 :c:member:`states` array representing the idle state to ask the processor to
 enter.
 
 The analogous ``->enter_s2idle()`` callback in struct cpuidle_state is used
 only for implementing the suspend-to-idle system-wide power management feature.
 The difference between in and ``->enter()`` is that it must not re-enable
 interrupts at any point (even temporarily) or attempt to change the states of
 clock event devices, which the ``->enter()`` callback may do sometimes.
 
 Once the :c:member:`states` array has been populated, the number of valid
 entries in it has to be stored in the :c:member:`state_count` field of the
 struct cpuidle_driver object representing the driver.  Moreover, if any
 entries in the :c:member:`states` array represent "coupled" idle states (that
 is, idle states that can only be asked for if multiple related logical CPUs are
 idle), the :c:member:`safe_state_index` field in struct cpuidle_driver needs
 to be the index of an idle state that is not "coupled" (that is, one that can be
 asked for if only one logical CPU is idle).
 
 In addition to that, if the given ``CPUIdle`` driver is only going to handle a
 subset of logical CPUs in the system, the :c:member:`cpumask` field in its
 struct cpuidle_driver object must point to the set (mask) of CPUs that will be
 handled by it.
 
 A ``CPUIdle`` driver can only be used after it has been registered.  If there
 are no "coupled" idle state entries in the driver's :c:member:`states` array,
 that can be accomplished by passing the driver's struct cpuidle_driver object
 to :c:func:`cpuidle_register_driver()`.  Otherwise, :c:func:`cpuidle_register()`
 should be used for this purpose.
 
 However, it also is necessary to register struct cpuidle_device objects for
 all of the logical CPUs to be handled by the given ``CPUIdle`` driver with the
 help of :c:func:`cpuidle_register_device()` after the driver has been registered
 and :c:func:`cpuidle_register_driver()`, unlike :c:func:`cpuidle_register()`,
 does not do that automatically.  For this reason, the drivers that use
 :c:func:`cpuidle_register_driver()` to register themselves must also take care
 of registering the struct cpuidle_device objects as needed, so it is generally
 recommended to use :c:func:`cpuidle_register()` for ``CPUIdle`` driver
 registration in all cases.
 
 The registration of a struct cpuidle_device object causes the ``CPUIdle``
 ``sysfs`` interface to be created and the governor's ``->enable()`` callback to
 be invoked for the logical CPU represented by it, so it must take place after
 registering the driver that will handle the CPU in question.
 
 ``CPUIdle`` drivers and struct cpuidle_device objects can be unregistered
 when they are not necessary any more which allows some resources associated with
 them to be released.  Due to dependencies between them, all of the
 struct cpuidle_device objects representing CPUs handled by the given
 ``CPUIdle`` driver must be unregistered, with the help of
 :c:func:`cpuidle_unregister_device()`, before calling
 :c:func:`cpuidle_unregister_driver()` to unregister the driver.  Alternatively,
 :c:func:`cpuidle_unregister()` can be called to unregister a ``CPUIdle`` driver
 along with all of the struct cpuidle_device objects representing CPUs handled
 by it.
 
 ``CPUIdle`` drivers can respond to runtime system configuration changes that
 lead to modifications of the list of available processor idle states (which can
 happen, for example, when the system's power source is switched from AC to
 battery or the other way around).  Upon a notification of such a change,
 a ``CPUIdle`` driver is expected to call :c:func:`cpuidle_pause_and_lock()` to
 turn ``CPUIdle`` off temporarily and then :c:func:`cpuidle_disable_device()` for
 all of the struct cpuidle_device objects representing CPUs affected by that
 change.  Next, it can update its :c:member:`states` array in accordance with
 the new configuration of the system, call :c:func:`cpuidle_enable_device()` for
 all of the relevant struct cpuidle_device objects and invoke
 :c:func:`cpuidle_resume_and_unlock()` to allow ``CPUIdle`` to be used again.

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