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authorRafael J. Wysocki <rafael.j.wysocki@intel.com>2019-01-09 13:53:50 +0300
committerRafael J. Wysocki <rafael.j.wysocki@intel.com>2019-01-18 00:15:20 +0300
commit3b7357663a47092576c2802dd1dc74d35c8c8721 (patch)
treef91419c3fbb9589e945b2831e981e013f66c483a
parentb26bf6ab716f27955e2a503ffca1691582127cbb (diff)
downloadlinux-3b7357663a47092576c2802dd1dc74d35c8c8721.tar.xz
Documentation: driver-api: PM: Add cpuidle document
Replace the remaining documents under Documentation/cpuidle/ with one more complete governor and driver API document for cpuidle under Documentation/driver-api/pm/. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
-rw-r--r--Documentation/cpuidle/driver.txt37
-rw-r--r--Documentation/cpuidle/governor.txt28
-rw-r--r--Documentation/driver-api/pm/cpuidle.rst282
-rw-r--r--Documentation/driver-api/pm/index.rst7
-rw-r--r--MAINTAINERS1
5 files changed, 287 insertions, 68 deletions
diff --git a/Documentation/cpuidle/driver.txt b/Documentation/cpuidle/driver.txt
deleted file mode 100644
index 1b0d81d92583..000000000000
--- a/Documentation/cpuidle/driver.txt
+++ /dev/null
@@ -1,37 +0,0 @@
-
-
- Supporting multiple CPU idle levels in kernel
-
- cpuidle drivers
-
-
-
-
-cpuidle driver hooks into the cpuidle infrastructure and handles the
-architecture/platform dependent part of CPU idle states. Driver
-provides the platform idle state detection capability and also
-has mechanisms in place to support actual entry-exit into CPU idle states.
-
-cpuidle driver initializes the cpuidle_device structure for each CPU device
-and registers with cpuidle using cpuidle_register_device.
-
-If all the idle states are the same, the wrapper function cpuidle_register
-could be used instead.
-
-It can also support the dynamic changes (like battery <-> AC), by using
-cpuidle_pause_and_lock, cpuidle_disable_device and cpuidle_enable_device,
-cpuidle_resume_and_unlock.
-
-Interfaces:
-extern int cpuidle_register(struct cpuidle_driver *drv,
- const struct cpumask *const coupled_cpus);
-extern int cpuidle_unregister(struct cpuidle_driver *drv);
-extern int cpuidle_register_driver(struct cpuidle_driver *drv);
-extern void cpuidle_unregister_driver(struct cpuidle_driver *drv);
-extern int cpuidle_register_device(struct cpuidle_device *dev);
-extern void cpuidle_unregister_device(struct cpuidle_device *dev);
-
-extern void cpuidle_pause_and_lock(void);
-extern void cpuidle_resume_and_unlock(void);
-extern int cpuidle_enable_device(struct cpuidle_device *dev);
-extern void cpuidle_disable_device(struct cpuidle_device *dev);
diff --git a/Documentation/cpuidle/governor.txt b/Documentation/cpuidle/governor.txt
deleted file mode 100644
index d9020f5e847b..000000000000
--- a/Documentation/cpuidle/governor.txt
+++ /dev/null
@@ -1,28 +0,0 @@
-
-
-
- Supporting multiple CPU idle levels in kernel
-
- cpuidle governors
-
-
-
-
-cpuidle governor is policy routine that decides what idle state to enter at
-any given time. cpuidle core uses different callbacks to the governor.
-
-* enable() to enable governor for a particular device
-* disable() to disable governor for a particular device
-* select() to select an idle state to enter
-* reflect() called after returning from the idle state, which can be used
- by the governor for some record keeping.
-
-More than one governor can be registered at the same time and
-users can switch between drivers using /sysfs interface (when enabled).
-More than one governor part is supported for developers to easily experiment
-with different governors. By default, most optimal governor based on your
-kernel configuration and platform will be selected by cpuidle.
-
-Interfaces:
-extern int cpuidle_register_governor(struct cpuidle_governor *gov);
-struct cpuidle_governor
diff --git a/Documentation/driver-api/pm/cpuidle.rst b/Documentation/driver-api/pm/cpuidle.rst
new file mode 100644
index 000000000000..5842ab621a58
--- /dev/null
+++ b/Documentation/driver-api/pm/cpuidle.rst
@@ -0,0 +1,282 @@
+.. |struct cpuidle_governor| replace:: :c:type:`struct cpuidle_governor <cpuidle_governor>`
+.. |struct cpuidle_device| replace:: :c:type:`struct cpuidle_device <cpuidle_device>`
+.. |struct cpuidle_driver| replace:: :c:type:`struct cpuidle_driver <cpuidle_driver>`
+.. |struct cpuidle_state| replace:: :c:type:`struct cpuidle_state <cpuidle_state>`
+
+========================
+CPU Idle Time Management
+========================
+
+::
+
+ Copyright (c) 2019 Intel Corp., 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, if ``cpuidle_sysfs_switch`` is passed to the
+kernel in the command line, 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.
diff --git a/Documentation/driver-api/pm/index.rst b/Documentation/driver-api/pm/index.rst
index 2f6d0e9cf6b7..56975c6bc789 100644
--- a/Documentation/driver-api/pm/index.rst
+++ b/Documentation/driver-api/pm/index.rst
@@ -1,9 +1,10 @@
-=======================
-Device Power Management
-=======================
+===============================
+CPU and Device Power Management
+===============================
.. toctree::
+ cpuidle
devices
notifiers
types
diff --git a/MAINTAINERS b/MAINTAINERS
index 4d04cebb4a71..750aa2df031f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4016,6 +4016,7 @@ S: Maintained
T: git git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git
B: https://bugzilla.kernel.org
F: Documentation/admin-guide/pm/cpuidle.rst
+F: Documentation/driver-api/pm/cpuidle.rst
F: drivers/cpuidle/*
F: include/linux/cpuidle.h