Hi--
Please see edits below:
On 4/1/20 10:59 AM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> Add a document describing high-level system-wide suspend code flows
> in Linux.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
> 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@xxxxxxxxx>
> +
> +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_>`_.
--
~Randy
