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
+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*,
+ *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
+ described `above <s2idle_suspend_>`_.
+
+ 2. Freezing tasks.
+
+ This step is the same as step 2 of the suspend-to-idle suspend transision
+ described `above <s2idle_suspend_>`_.
+
+ 3. Suspending devices and reconfiguring IRQs.
+
+ This step is analogous to step 3 of the suspend-to-idle suspend transision
+ 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
+ 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
+ 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
+ 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
+ described `above <s2idle_resume_>`_.
+
+ 5. Thawing tasks.
+
+ This step is the same as step 3 of the suspend-to-idle suspend transision
+ 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
+ described `above <s2idle_resume_>`_.
Index: linux-pm/Documentation/admin-guide/pm/system-wide.rst
===================================================================
--- linux-pm.orig/Documentation/admin-guide/pm/system-wide.rst
+++ linux-pm/Documentation/admin-guide/pm/system-wide.rst
@@ -8,3 +8,4 @@ System-Wide Power Management
:maxdepth: 2
sleep-states
+ suspend-flows
