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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Randy Dunlap <rdunlap@infradead.org>
Cc:     Linux PM <linux-pm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Documentation <linux-doc@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>
Subject: Re: [PATCH] Documentation: PM: sleep: Document system-wide suspend code flows
Date:   Thu, 02 Apr 2020 14:46:36 +0200
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On Thursday, April 2, 2020 9:03:06 AM CEST Randy Dunlap wrote:
> Hi--
> 
> Please see edits below:
> 
> 
> On 4/1/20 10:59 AM, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > 
> > Add a document describing high-level system-wide suspend code flows
> > in Linux.
> > 
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > ---
> >  Documentation/admin-guide/pm/suspend-flows.rst |  270 +++++++++++++++++++++++++
> >  Documentation/admin-guide/pm/system-wide.rst   |    1 
> >  2 files changed, 271 insertions(+)
> > 
> > Index: linux-pm/Documentation/admin-guide/pm/suspend-flows.rst
> > ===================================================================
> > --- /dev/null
> > +++ linux-pm/Documentation/admin-guide/pm/suspend-flows.rst
> > @@ -0,0 +1,270 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +.. include:: <isonum.txt>
> > +
> > +=========================
> > +System Suspend Code Flows
> > +=========================
> > +
> > +:Copyright: |copy| 2020 Intel Corporation
> > +
> > +:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > +
> > +At least one global system-wide transition needs to be carried out for the
> > +system to get from the working state into one of the supported
> > +:doc:`sleep states <sleep-states>`.  Hibernation requires more than one
> > +transition to occur for this purpose, but the other sleep states, commonly
> > +referred to as *system-wide suspend* (or simply *system suspend*) states, need
> > +only one.
> > +
> > +For those sleep states, the transition from the working state of the system into
> > +the target sleep state is referred to as *system suspend* too (in the majority
> > +of cases, whether this means a transition or a sleep state of the system should
> > +be clear from the context) and the transition back from the sleep state into the
> > +working state is referred to as *system resume*.
> > +
> > +The kernel code flows associated with the syspend and resume transitions for
> 
>                                              suspend
> 
> > +different sleep states of the system are quite similar, but there are some
> > +significant differences between the :ref:`suspend-to-idle <s2idle>` code flows
> > +and the code flows related to the :ref:`suspend-to-RAM <s2ram>` and
> > +:ref:`standby <standby>` sleep states.
> > +
> > +The :ref:`suspend-to-RAM <s2ram>` and :ref:`standby <standby>` sleep states
> > +cannot be implemented without platform support and the difference between them
> > +boils down to the platform-specific actions carried out by the suspend and
> > +resume hooks that need to be provided by the platform driver to make them
> > +available.  Apart from that, the suspend and resume code flows for these sleep
> > +states are mostly identical, so they both together will be referred to as
> > +*platform-dependent suspend* states in what follows.
> > +
> > +
> > +.. _s2idle_suspend:
> > +
> > +Suspend-to-idle Suspend Code Flow
> > +=================================
> > +
> > +The following steps are taken in order to transition the system from the working
> > +state to the :ref:`suspend-to-idle <s2idle>` sleep state:
> > +
> > + 1. Invoking system-wide suspend notifiers.
> > +
> > +    Kernel subsystems can register callbacks to be invoked when the suspend
> > +    transition is about to occur and when the resume transition has finished.
> > +
> > +    That allows them to prepare for the change of the system state and to clean
> > +    up after getting back to the working state.
> > +
> > + 2. Freezing tasks.
> > +
> > +    Tasks are frozen primarily in order to avoid unchecked hardware accesses
> > +    from user space through MMIO regions or I/O registers exposed directly to
> > +    it and to prevent user space from entering the kernel while the next step
> > +    of the transition is in progress (which might have been problematic for
> > +    various reasons).
> > +
> > +    All user space tasks are intercepted as though they were sent a signal and
> > +    put into uninterruptible sleep until the end of the subsequent system resume
> > +    transition.
> > +
> > +    The kernel threads that choose to be frozen during system suspend for
> > +    specific reasons are frozen subsequently, but they are not intercepted.
> > +    Instead, they are expected to periodically check whether or not they need
> > +    to be frozen and to put themselves into uninterruptible sleep if so.  [Note,
> > +    however, that kernel threads can use locking and other concurrency controls
> > +    available in kernel space to synchronize themselves with system suspend and
> > +    resume, which can be much more precise than the freezing, so the latter is
> > +    not a recommended option for kernel threads.]
> > +
> > + 3. Suspending devices and reconfiguring IRQs.
> > +
> > +    Devices are suspended in four phases called *prepare*, *suspend*,
> > +    *late suspend* and *noirq suspend* (see :ref:`driverapi_pm_devices` for more
> > +    information on what exactly happens in each phase).
> > +
> > +    Every device is visited in each phase, but typically it is not physically
> > +    accessed in more than two of them.
> > +
> > +    The runtime PM API is disabled for every device during the *late* suspend
> > +    phase and high-level ("action") interrupt handlers are prevented from being
> > +    invoked before the *noirq* suspend phase.
> > +
> > +    Interrupts are still handled after that, but they are only acknowledged to
> > +    interrupt controllers without performing any device-specific actions that
> > +    would be triggered in the working state of the system (those actions are
> > +    deferred till the subsequent system resume transition as described
> > +    `below <s2idle_resume_>`_).
> > +
> > +    IRQs associated with system wakeup devices are "armed" so that the resume
> > +    transition of the system is started when one of them signals an event.
> > +
> > + 4. Freezing the scheduler tick and suspending timekeeping.
> > +
> > +    When all devices have been suspended, CPUs enter the idle loop and are put
> > +    into the deepest available idle state.  While doing that, each of them
> > +    "freezes" its own scheduler tick so that the timer events associated with
> > +    the tick do not occur until the CPU is woken up by another interrupt source.
> > +
> > +    The last CPU to enter the idle state also stops the timekeeping which
> > +    (among other things) prevents high resolution timers from triggering going
> > +    forward until the first CPU that is woken up restarts the timekeeping.
> > +    That allows the CPUs to stay in the deep idle state relatively long in one
> > +    go.
> > +
> > +    From this point on, the CPUs can only be woken up by non-timer hardware
> > +    interrupts.  If that happens, they go back to the idle state unless the
> > +    interrupt that woke up one of them comes from an IRQ that has been armed for
> > +    system wakeup, in which case the system resume transition is started.
> > +
> > +
> > +.. _s2idle_resume:
> > +
> > +Suspend-to-idle Resume Code Flow
> > +================================
> > +
> > +The following steps are taken in order to transition the system from the
> > +:ref:`suspend-to-idle <s2idle>` sleep state into the working state:
> > +
> > + 1. Resuming timekeeping and unfreezing the scheduler tick.
> > +
> > +    When one of the CPUs is woken up (by a non-timer hardware interrupt), it
> > +    leaves the idle state entered in the last step of the preceding suspend
> > +    transition, restarts the timekeeping (unless it has been restarted already
> > +    by another CPU that woke up earlier) and the scheduler tick on that CPU is
> > +    unfrozen.
> > +
> > +    If the interrupt that has woken up the CPU was armed for system wakeup,
> > +    the system resume transition begins.
> > +
> > + 2. Resuming devices and restoring the working-state configuration of IRQs.
> > +
> > +    Devices are resumeed in four phases called *noirq resume*, *early resume*,
> 
>                    resumed
> 
> > +    *resume* and *complete* (see :ref:`driverapi_pm_devices` for more
> > +    information on what exactly happens in each phase).
> > +
> > +    Every device is visited in each phase, but typically it is not physically
> > +    accessed in more than two of them.
> > +
> > +    The working-state configuration of IRQs is restored after the *noirq* resume
> > +    phase and the runtime PM API is re-enabled for every device whose driver
> > +    supports it during the *early* resume phase.
> > +
> > + 3. Thawing tasks.
> > +
> > +    Tasks frozen in step 2 of the preceding `suspend <s2idle_suspend_>`_
> > +    transition are "thawed", which means that they are woken up from the
> > +    uninterruptible sleep that they went into at that time and user space tasks
> > +    are allowed to exit the kernel.
> > +
> > + 4. Invoking system-wide resume notifiers.
> > +
> > +    This is analogous to step 1 of the `suspend <s2idle_suspend_>`_ transition
> > +    and the same set of callbacks is invoked at this point, but a different
> > +    "notification type" parameter value is passed to them.
> > +
> > +
> > +Platform-dependent Suspend Code Flow
> > +====================================
> > +
> > +The following steps are taken in order to transition the system from the working
> > +state to platform-dependent suspend state:
> > +
> > + 1. Invoking system-wide suspend notifiers.
> > +
> > +    This step is the same as step 1 of the suspend-to-idle suspend transision
> 
>                                                                       transition
> 
> > +    described `above <s2idle_suspend_>`_.
> > +
> > + 2. Freezing tasks.
> > +
> > +    This step is the same as step 2 of the suspend-to-idle suspend transision
> 
>                                                                       transition
> 
> > +    described `above <s2idle_suspend_>`_.
> > +
> > + 3. Suspending devices and reconfiguring IRQs.
> > +
> > +    This step is analogous to step 3 of the suspend-to-idle suspend transision
> 
>                                                                        transition
> 
> > +    described `above <s2idle_suspend_>`_, but the arming of IRQs for system
> > +    wakeup generally does not have any effect on the platform.
> > +
> > +    There are platforms that can go into a very deep low-power state internally
> > +    when all CPUs in them are in sufficiently deep idle states and all I/O
> > +    devices have been put into low-power states.  On those platforms,
> > +    suspend-to-idle can reduce system power very effectively.
> > +
> > +    On the other platforms, however, low-level components (like interrupt
> > +    controllers) need to be turned off in a platform-specific way (implemented
> > +    in the hooks provided by the platform driver) to achieve comparable power
> > +    reduction.
> > +
> > +    That usually prevents in-band hardware interrupts from waking up the system,
> > +    which must be done in a special platform-dependent way.  Then, the
> > +    configuration of system wakeup sources usually starts when system wakeup
> > +    devices are suspended and is finalized by the platform suspend hooks later
> > +    on.
> > +
> > + 4. Disabling non-boot CPUs.
> > +
> > +    On some platforms the suspend hooks mentioned above must run in a one-CPU
> > +    configuration of the system (in particular, the herware cannot be accessed
> 
>                                                        hardware
> 
> > +    by any code running in parallel with the platform suspend hooks that may,
> > +    and often do, trap into the platform firmware in order to finalize the
> > +    suspend transition).
> > +
> > +    For this reason, the CPU offline/online (CPU hotplug) framework is used
> > +    to take all of the CPUs in the system, except for one (the boot CPU),
> > +    offline (typially, the CPUs that have been taken offline go into deep idle
> 
>                 typically
> 
> > +    states).
> > +
> > +    This means that all tasks are migrated away from those CPUs and all IRQs are
> > +    rerouted to the only CPU that remains online.
> > +
> > + 5. Suspending core system components.
> > +
> > +    This prepares the core system components for (possibly) losing power going
> > +    forward and suspends the timekeeping.
> > +
> > + 6. Platform-specific power removal.
> > +
> > +    This is expected to remove power from all of the system components except
> > +    for the mamory controller and RAM (in order to preserve the contents of the
> 
>                memory
> 
> > +    latter) and some devices designated for system wakeup.
> > +
> > +    In many cases control is passed to the platform firmware which is expected
> > +    to finalize the suspend transition as needed.
> > +
> > +
> > +Platform-dependent Resume Code Flow
> > +===================================
> > +
> > +The following steps are taken in order to transition the system from a
> > +platform-dependent suspend state into the working state:
> > +
> > + 1. Platform-specific system wakeup.
> > +
> > +    The platform is woken up by a signal from one of the designated system
> > +    wakeup devices (which need not be an in-band hardware interrupt)  and
> > +    control is passed back to the kernel (the working configuration of the
> > +    platform may need to be restored by the platform firmware before the
> > +    kernel gets control again).
> > +
> > + 2. Resuming core system components.
> > +
> > +    The suspend-time configuration of the core system components is restored and
> > +    the timekeeping is resumed.
> > +
> > + 3. Re-enabling non-boot CPUs.
> > +
> > +    The CPUs disabled in step 4 of the preceding suspend transition are taken
> > +    back online and their suspend-time configuration is restored.
> > +
> > + 4. Resuming devices and restoring the working-state configuration of IRQs.
> > +
> > +    This step is the same as step 2 of the suspend-to-idle suspend transision
> 
>                                                                       transition
> 
> > +    described `above <s2idle_resume_>`_.
> > +
> > + 5. Thawing tasks.
> > +
> > +    This step is the same as step 3 of the suspend-to-idle suspend transision
> 
>                                                                       transition
> 
> > +    described `above <s2idle_resume_>`_.
> > +
> > + 6. Invoking system-wide resume notifiers.
> > +
> > +    This step is the same as step 4 of the suspend-to-idle suspend transision
> 
>                                                                       transition
> 
> > +    described `above <s2idle_resume_>`_.

Thanks a lot, all of the comments above should be addressed in the v2 of the
patch that has just been posted.

Cheers!