Date:   Thu, 17 Jan 2019 11:56:18 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Documentation <linux-doc@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Kevin Hilman <khilman@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH] Documentation: driver-api: PM: Add cpuidle document

On Wed, 16 Jan 2019 at 23:10, Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Jan 9, 2019 at 11:54 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > 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@...el.com>
>
> In the face of the lack of any feedback I'm assuming that this is fine
> by everyone.

I have look through it, looks good and sorry for the delay.

Feel free to add:

Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>

Kind regards
Uffe

>
> > ---
> >
> > On top of https://patchwork.kernel.org/patch/10747185/
> >
> > ---
> >  Documentation/cpuidle/driver.txt        |   37 ----
> >  Documentation/cpuidle/governor.txt      |   28 ---
> >  Documentation/driver-api/pm/cpuidle.rst |  282 ++++++++++++++++++++++++++++++++
> >  Documentation/driver-api/pm/index.rst   |    7
> >  MAINTAINERS                             |    1
> >  5 files changed, 287 insertions(+), 68 deletions(-)
> >
> > Index: linux-pm/Documentation/driver-api/pm/index.rst
> > ===================================================================
> > --- linux-pm.orig/Documentation/driver-api/pm/index.rst
> > +++ linux-pm/Documentation/driver-api/pm/index.rst
> > @@ -1,9 +1,10 @@
> > -=======================
> > -Device Power Management
> > -=======================
> > +===============================
> > +CPU and Device Power Management
> > +===============================
> >
> >  .. toctree::
> >
> > +   cpuidle
> >     devices
> >     notifiers
> >     types
> > Index: linux-pm/Documentation/driver-api/pm/cpuidle.rst
> > ===================================================================
> > --- /dev/null
> > +++ linux-pm/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@...el.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.
> > Index: linux-pm/Documentation/cpuidle/driver.txt
> > ===================================================================
> > --- linux-pm.orig/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);
> > Index: linux-pm/Documentation/cpuidle/governor.txt
> > ===================================================================
> > --- linux-pm.orig/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
> > Index: linux-pm/MAINTAINERS
> > ===================================================================
> > --- linux-pm.orig/MAINTAINERS
> > +++ linux-pm/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
> >
> >

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