code:
===================================
Regulator Consumer Driver Interface
===================================

This text describes the regulator interface for consumer device drivers.
Please see overview.txt for a description of the terms used in this text.

1. Consumer Regulator Access (static & dynamic drivers)
=======================================================

A consumer driver can get access to its supply regulator by calling ::

	regulator = regulator_get(dev, "Vcc");

The consumer passes in its struct device pointer and power supply ID. The core
then finds the correct regulator by consulting a machine specific lookup table.
If the lookup is successful then this call will return a pointer to the struct
regulator that supplies this consumer.

To release the regulator the consumer driver should call ::

	regulator_put(regulator);

Consumers can be supplied by more than one regulator e.g. codec consumer with
analog and digital supplies ::

	digital = regulator_get(dev, "Vcc");  /* digital core */
	analog = regulator_get(dev, "Avdd");  /* analog */

The regulator access functions regulator_get() and regulator_put() will
usually be called in your device drivers probe() and remove() respectively.

2. Regulator Output Enable & Disable (static & dynamic drivers)
===============================================================

A consumer can enable its power supply by calling::

	int regulator_enable(regulator);

NOTE:
  The supply may already be enabled before regulator_enabled() is called.
  This may happen if the consumer shares the regulator or the regulator has been
  previously enabled by bootloader or kernel board initialization code.

A consumer can determine if a regulator is enabled by calling::

	int regulator_is_enabled(regulator);

This will return > zero when the regulator is enabled.

A consumer can disable its supply when no longer needed by calling::

	int regulator_disable(regulator);

NOTE:
  This may not disable the supply if it's shared with other consumers. The
  regulator will only be disabled when the enabled reference count is zero.

Finally, a regulator can be forcefully disabled in the case of an emergency::

	int regulator_force_disable(regulator);

NOTE:
  this will immediately and forcefully shutdown the regulator output. All
  consumers will be powered off.

3. Regulator Voltage Control & Status (dynamic drivers)
=======================================================

Some consumer drivers need to be able to dynamically change their supply
voltage to match system operating points. e.g. CPUfreq drivers can scale
voltage along with frequency to save power, SD drivers may need to select the
correct card voltage, etc.

Consumers can control their supply voltage by calling::

	int regulator_set_voltage(regulator, min_uV, max_uV);

Where min_uV and max_uV are the minimum and maximum acceptable voltages in
microvolts.

NOTE: this can be called when the regulator is enabled or disabled. If called
when enabled, then the voltage changes instantly, otherwise the voltage
configuration changes and the voltage is physically set when the regulator is
next enabled.

The regulators configured voltage output can be found by calling::

	int regulator_get_voltage(regulator);

NOTE:
  get_voltage() will return the configured output voltage whether the
  regulator is enabled or disabled and should NOT be used to determine regulator
  output state. However this can be used in conjunction with is_enabled() to
  determine the regulator physical output voltage.

4. Regulator Current Limit Control & Status (dynamic drivers)
=============================================================

Some consumer drivers need to be able to dynamically change their supply
current limit to match system operating points. e.g. LCD backlight driver can
change the current limit to vary the backlight brightness, USB drivers may want
to set the limit to 500mA when supplying power.

Consumers can control their supply current limit by calling::

	int regulator_set_current_limit(regulator, min_uA, max_uA);

Where min_uA and max_uA are the minimum and maximum acceptable current limit in
microamps.

NOTE:
  this can be called when the regulator is enabled or disabled. If called
  when enabled, then the current limit changes instantly, otherwise the current
  limit configuration changes and the current limit is physically set when the
  regulator is next enabled.

A regulators current limit can be found by calling::

	int regulator_get_current_limit(regulator);

NOTE:
  get_current_limit() will return the current limit whether the regulator
  is enabled or disabled and should not be used to determine regulator current
  load.

5. Regulator Operating Mode Control & Status (dynamic drivers)
==============================================================

Some consumers can further save system power by changing the operating mode of
their supply regulator to be more efficient when the consumers operating state
changes. e.g. consumer driver is idle and subsequently draws less current

Regulator operating mode can be changed indirectly or directly.

Indirect operating mode control.
--------------------------------
Consumer drivers can request a change in their supply regulator operating mode
by calling::

	int regulator_set_load(struct regulator *regulator, int load_uA);

This will cause the core to recalculate the total load on the regulator (based
on all its consumers) and change operating mode (if necessary and permitted)
to best match the current operating load.

The load_uA value can be determined from the consumer's datasheet. e.g. most
datasheets have tables showing the maximum current consumed in certain
situations.

Most consumers will use indirect operating mode control since they have no
knowledge of the regulator or whether the regulator is shared with other
consumers.

Direct operating mode control.
------------------------------

Bespoke or tightly coupled drivers may want to directly control regulator
operating mode depending on their operating point. This can be achieved by
calling::

	int regulator_set_mode(struct regulator *regulator, unsigned int mode);
	unsigned int regulator_get_mode(struct regulator *regulator);

Direct mode will only be used by consumers that *know* about the regulator and
are not sharing the regulator with other consumers.

6. Regulator Events
===================

Regulators can notify consumers of external events. Events could be received by
consumers under regulator stress or failure conditions.

Consumers can register interest in regulator events by calling::

	int regulator_register_notifier(struct regulator *regulator,
					struct notifier_block *nb);

Consumers can unregister interest by calling::

	int regulator_unregister_notifier(struct regulator *regulator,
					  struct notifier_block *nb);

Regulators use the kernel notifier framework to send event to their interested
consumers.

7. Regulator Direct Register Access
===================================

Some kinds of power management hardware or firmware are designed such that
they need to do low-level hardware access to regulators, with no involvement
from the kernel. Examples of such devices are:

- clocksource with a voltage-controlled oscillator and control logic to change
  the supply voltage over I2C to achieve a desired output clock rate
- thermal management firmware that can issue an arbitrary I2C transaction to
  perform system poweroff during overtemperature conditions

To set up such a device/firmware, various parameters like I2C address of the
regulator, addresses of various regulator registers etc. need to be configured
to it. The regulator framework provides the following helpers for querying
these details.

Bus-specific details, like I2C addresses or transfer rates are handled by the
regmap framework. To get the regulator's regmap (if supported), use::

	struct regmap *regulator_get_regmap(struct regulator *regulator);

To obtain the hardware register offset and bitmask for the regulator's voltage
selector register, use::

	int regulator_get_hardware_vsel_register(struct regulator *regulator,
						 unsigned *vsel_reg,
						 unsigned *vsel_mask);

To convert a regulator framework voltage selector code (used by
regulator_list_voltage) to a hardware-specific voltage selector that can be
directly written to the voltage selector register, use::

	int regulator_list_hardware_vsel(struct regulator *regulator,
					 unsigned selector);


code:
==========================
Regulator API design notes
==========================

This document provides a brief, partially structured, overview of some
of the design considerations which impact the regulator API design.

Safety
------

 - Errors in regulator configuration can have very serious consequences
   for the system, potentially including lasting hardware damage.
 - It is not possible to automatically determine the power configuration
   of the system - software-equivalent variants of the same chip may
   have different power requirements, and not all components with power
   requirements are visible to software.

.. note::

     The API should make no changes to the hardware state unless it has
     specific knowledge that these changes are safe to perform on this
     particular system.

Consumer use cases
------------------

 - The overwhelming majority of devices in a system will have no
   requirement to do any runtime configuration of their power beyond
   being able to turn it on or off.

 - Many of the power supplies in the system will be shared between many
   different consumers.

.. note::

     The consumer API should be structured so that these use cases are
     very easy to handle and so that consumers will work with shared
     supplies without any additional effort.

repo_name: angery/linux


code:
==================================
Regulator Machine Driver Interface
==================================

The regulator machine driver interface is intended for board/machine specific
initialisation code to configure the regulator subsystem.

Consider the following machine::

  Regulator-1 -+-> Regulator-2 --> [Consumer A @ 1.8 - 2.0V]
               |
               +-> [Consumer B @ 3.3V]

The drivers for consumers A & B must be mapped to the correct regulator in
order to control their power supplies. This mapping can be achieved in machine
initialisation code by creating a struct regulator_consumer_supply for
each regulator::

  struct regulator_consumer_supply {
	const char *dev_name;	/* consumer dev_name() */
	const char *supply;	/* consumer supply - e.g. "vcc" */
  };

e.g. for the machine above::

  static struct regulator_consumer_supply regulator1_consumers[] = {
	REGULATOR_SUPPLY("Vcc", "consumer B"),
  };

  static struct regulator_consumer_supply regulator2_consumers[] = {
	REGULATOR_SUPPLY("Vcc", "consumer A"),
  };

This maps Regulator-1 to the 'Vcc' supply for Consumer B and maps Regulator-2
to the 'Vcc' supply for Consumer A.

Constraints can now be registered by defining a struct regulator_init_data
for each regulator power domain. This structure also maps the consumers
to their supply regulators::

  static struct regulator_init_data regulator1_data = {
	.constraints = {
		.name = "Regulator-1",
		.min_uV = 3300000,
		.max_uV = 3300000,
		.valid_modes_mask = REGULATOR_MODE_NORMAL,
	},
	.num_consumer_supplies = ARRAY_SIZE(regulator1_consumers),
	.consumer_supplies = regulator1_consumers,
  };

The name field should be set to something that is usefully descriptive
for the board for configuration of supplies for other regulators and
for use in logging and other diagnostic output.  Normally the name
used for the supply rail in the schematic is a good choice.  If no
name is provided then the subsystem will choose one.

Regulator-1 supplies power to Regulator-2. This relationship must be registered
with the core so that Regulator-1 is also enabled when Consumer A enables its
supply (Regulator-2). The supply regulator is set by the supply_regulator
field below and co::

  static struct regulator_init_data regulator2_data = {
	.supply_regulator = "Regulator-1",
	.constraints = {
		.min_uV = 1800000,
		.max_uV = 2000000,
		.valid_ops_mask = REGULATOR_CHANGE_VOLTAGE,
		.valid_modes_mask = REGULATOR_MODE_NORMAL,
	},
	.num_consumer_supplies = ARRAY_SIZE(regulator2_consumers),
	.consumer_supplies = regulator2_consumers,
  };

Finally the regulator devices must be registered in the usual manner::

  static struct platform_device regulator_devices[] = {
	{
		.name = "regulator",
		.id = DCDC_1,
		.dev = {
			.platform_data = &regulator1_data,
		},
	},
	{
		.name = "regulator",
		.id = DCDC_2,
		.dev = {
			.platform_data = &regulator2_data,
		},
	},
  };
  /* register regulator 1 device */
  platform_device_register(&regulator_devices[0]);

  /* register regulator 2 device */
  platform_device_register(&regulator_devices[1]);


code:
=============================================
Linux voltage and current regulator framework
=============================================

About
=====

This framework is designed to provide a standard kernel interface to control
voltage and current regulators.

The intention is to allow systems to dynamically control regulator power output
in order to save power and prolong battery life. This applies to both voltage
regulators (where voltage output is controllable) and current sinks (where
current limit is controllable).

(C) 2008  Wolfson Microelectronics PLC.

Author: Liam Girdwood <lrg@slimlogic.co.uk>

Nomenclature
============

Some terms used in this document:

  - Regulator
                 - Electronic device that supplies power to other devices.
                   Most regulators can enable and disable their output while
                   some can control their output voltage and or current.

                   Input Voltage -> Regulator -> Output Voltage

  - PMIC
                 - Power Management IC. An IC that contains numerous
                   regulators and often contains other subsystems.

  - Consumer
                 - Electronic device that is supplied power by a regulator.
                   Consumers can be classified into two types:-

                   Static: consumer does not change its supply voltage or
                   current limit. It only needs to enable or disable its
                   power supply. Its supply voltage is set by the hardware,
                   bootloader, firmware or kernel board initialisation code.

                   Dynamic: consumer needs to change its supply voltage or
                   current limit to meet operation demands.

  - Power Domain
                 - Electronic circuit that is supplied its input power by the
                   output power of a regulator, switch or by another power
                   domain.

                   The supply regulator may be behind a switch(s). i.e.::

                     Regulator -+-> Switch-1 -+-> Switch-2 --> [Consumer A]
                                |             |
                                |             +-> [Consumer B], [Consumer C]
                                |
                                +-> [Consumer D], [Consumer E]

                   That is one regulator and three power domains:

                   - Domain 1: Switch-1, Consumers D & E.
                   - Domain 2: Switch-2, Consumers B & C.
                   - Domain 3: Consumer A.

                   and this represents a "supplies" relationship:

                   Domain-1 --> Domain-2 --> Domain-3.

                   A power domain may have regulators that are supplied power
                   by other regulators. i.e.::

                     Regulator-1 -+-> Regulator-2 -+-> [Consumer A]
                                  |
                                  +-> [Consumer B]

                   This gives us two regulators and two power domains:

                   - Domain 1: Regulator-2, Consumer B.
                   - Domain 2: Consumer A.

                   and a "supplies" relationship:

                   Domain-1 --> Domain-2

  - Constraints
                 - Constraints are used to define power levels for performance
                   and hardware protection. Constraints exist at three levels:

                   Regulator Level: This is defined by the regulator hardware
                   operating parameters and is specified in the regulator
                   datasheet. i.e.

                     - voltage output is in the range 800mV -> 3500mV.
                     - regulator current output limit is 20mA @ 5V but is
                       10mA @ 10V.

                   Power Domain Level: This is defined in software by kernel
                   level board initialisation code. It is used to constrain a
                   power domain to a particular power range. i.e.

                     - Domain-1 voltage is 3300mV
                     - Domain-2 voltage is 1400mV -> 1600mV
                     - Domain-3 current limit is 0mA -> 20mA.

                   Consumer Level: This is defined by consumer drivers
                   dynamically setting voltage or current limit levels.

                   e.g. a consumer backlight driver asks for a current increase
                   from 5mA to 10mA to increase LCD illumination. This passes
                   to through the levels as follows :-

                   Consumer: need to increase LCD brightness. Lookup and
                   request next current mA value in brightness table (the
                   consumer driver could be used on several different
                   personalities based upon the same reference device).

                   Power Domain: is the new current limit within the domain
                   operating limits for this domain and system state (e.g.
                   battery power, USB power)

                   Regulator Domains: is the new current limit within the
                   regulator operating parameters for input/output voltage.

                   If the regulator request passes all the constraint tests
                   then the new regulator value is applied.

Design
======

The framework is designed and targeted at SoC based devices but may also be
relevant to non SoC devices and is split into the following four interfaces:-

   1. Consumer driver interface.

      This uses a similar API to the kernel clock interface in that consumer
      drivers can get and put a regulator (like they can with clocks atm) and
      get/set voltage, current limit, mode, enable and disable. This should
      allow consumers complete control over their supply voltage and current
      limit. This also compiles out if not in use so drivers can be reused in
      systems with no regulator based power control.

        See Documentation/power/regulator/consumer.rst

   2. Regulator driver interface.

      This allows regulator drivers to register their regulators and provide
      operations to the core. It also has a notifier call chain for propagating
      regulator events to clients.

        See Documentation/power/regulator/regulator.rst

   3. Machine interface.

      This interface is for machine specific code and allows the creation of
      voltage/current domains (with constraints) for each regulator. It can
      provide regulator constraints that will prevent device damage through
      overvoltage or overcurrent caused by buggy client drivers. It also
      allows the creation of a regulator tree whereby some regulators are
      supplied by others (similar to a clock tree).

        See Documentation/power/regulator/machine.rst

   4. Userspace ABI.

      The framework also exports a lot of useful voltage/current/opmode data to
      userspace via sysfs. This could be used to help monitor device power
      consumption and status.

        See Documentation/ABI/testing/sysfs-class-regulator


code:
==========================
Regulator Driver Interface
==========================

The regulator driver interface is relatively simple and designed to allow
regulator drivers to register their services with the core framework.

Registration
============

Drivers can register a regulator by calling::

  struct regulator_dev *regulator_register(struct regulator_desc *regulator_desc,
					   const struct regulator_config *config);

This will register the regulator's capabilities and operations to the regulator
core.

Regulators can be unregistered by calling::

  void regulator_unregister(struct regulator_dev *rdev);

Regulator Events
================

Regulators can send events (e.g. overtemperature, undervoltage, etc) to
consumer drivers by calling::

  int regulator_notifier_call_chain(struct regulator_dev *rdev,
				    unsigned long event, void *data);


code:
==================================================
Runtime Power Management Framework for I/O Devices
==================================================

(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.

(C) 2010 Alan Stern <stern@rowland.harvard.edu>

(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1. Introduction
===============

Support for runtime power management (runtime PM) of I/O devices is provided
at the power management core (PM core) level by means of:

* The power management workqueue pm_wq in which bus types and device drivers can
  put their PM-related work items.  It is strongly recommended that pm_wq be
  used for queuing all work items related to runtime PM, because this allows
  them to be synchronized with system-wide power transitions (suspend to RAM,
  hibernation and resume from system sleep states).  pm_wq is declared in
  include/linux/pm_runtime.h and defined in kernel/power/main.c.

* A number of runtime PM fields in the 'power' member of 'struct device' (which
  is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
  be used for synchronizing runtime PM operations with one another.

* Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in
  include/linux/pm.h).

* A set of helper functions defined in drivers/base/power/runtime.c that can be
  used for carrying out runtime PM operations in such a way that the
  synchronization between them is taken care of by the PM core.  Bus types and
  device drivers are encouraged to use these functions.

The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
fields of 'struct dev_pm_info' and the core helper functions provided for
runtime PM are described below.

2. Device Runtime PM Callbacks
==============================

There are three device runtime PM callbacks defined in 'struct dev_pm_ops'::

  struct dev_pm_ops {
	...
	int (*runtime_suspend)(struct device *dev);
	int (*runtime_resume)(struct device *dev);
	int (*runtime_idle)(struct device *dev);
	...
  };

The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks
are executed by the PM core for the device's subsystem that may be either of
the following:

  1. PM domain of the device, if the device's PM domain object, dev->pm_domain,
     is present.

  2. Device type of the device, if both dev->type and dev->type->pm are present.

  3. Device class of the device, if both dev->class and dev->class->pm are
     present.

  4. Bus type of the device, if both dev->bus and dev->bus->pm are present.

If the subsystem chosen by applying the above rules doesn't provide the relevant
callback, the PM core will invoke the corresponding driver callback stored in
dev->driver->pm directly (if present).

The PM core always checks which callback to use in the order given above, so the
priority order of callbacks from high to low is: PM domain, device type, class
and bus type.  Moreover, the high-priority one will always take precedence over
a low-priority one.  The PM domain, bus type, device type and class callbacks
are referred to as subsystem-level callbacks in what follows.

By default, the callbacks are always invoked in process context with interrupts
enabled.  However, the pm_runtime_irq_safe() helper function can be used to tell
the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
and ->runtime_idle() callbacks for the given device in atomic context with
interrupts disabled.  This implies that the callback routines in question must
not block or sleep, but it also means that the synchronous helper functions
listed at the end of Section 4 may be used for that device within an interrupt
handler or generally in an atomic context.

The subsystem-level suspend callback, if present, is _entirely_ _responsible_
for handling the suspend of the device as appropriate, which may, but need not
include executing the device driver's own ->runtime_suspend() callback (from the
PM core's point of view it is not necessary to implement a ->runtime_suspend()
callback in a device driver as long as the subsystem-level suspend callback
knows what to do to handle the device).

  * Once the subsystem-level suspend callback (or the driver suspend callback,
    if invoked directly) has completed successfully for the given device, the PM
    core regards the device as suspended, which need not mean that it has been
    put into a low power state.  It is supposed to mean, however, that the
    device will not process data and will not communicate with the CPU(s) and
    RAM until the appropriate resume callback is executed for it.  The runtime
    PM status of a device after successful execution of the suspend callback is
    'suspended'.

  * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM
    status remains 'active', which means that the device _must_ be fully
    operational afterwards.

  * If the suspend callback returns an error code different from -EBUSY and
    -EAGAIN, the PM core regards this as a fatal error and will refuse to run
    the helper functions described in Section 4 for the device until its status
    is directly set to  either 'active', or 'suspended' (the PM core provides
    special helper functions for this purpose).

In particular, if the driver requires remote wakeup capability (i.e. hardware
mechanism allowing the device to request a change of its power state, such as
PCI PME) for proper functioning and device_can_wakeup() returns 'false' for the
device, then ->runtime_suspend() should return -EBUSY.  On the other hand, if
device_can_wakeup() returns 'true' for the device and the device is put into a
low-power state during the execution of the suspend callback, it is expected
that remote wakeup will be enabled for the device.  Generally, remote wakeup
should be enabled for all input devices put into low-power states at run time.

The subsystem-level resume callback, if present, is **entirely responsible** for
handling the resume of the device as appropriate, which may, but need not
include executing the device driver's own ->runtime_resume() callback (from the
PM core's point of view it is not necessary to implement a ->runtime_resume()
callback in a device driver as long as the subsystem-level resume callback knows
what to do to handle the device).

  * Once the subsystem-level resume callback (or the driver resume callback, if
    invoked directly) has completed successfully, the PM core regards the device
    as fully operational, which means that the device _must_ be able to complete
    I/O operations as needed.  The runtime PM status of the device is then
    'active'.

  * If the resume callback returns an error code, the PM core regards this as a
    fatal error and will refuse to run the helper functions described in Section
    4 for the device, until its status is directly set to either 'active', or
    'suspended' (by means of special helper functions provided by the PM core
    for this purpose).

The idle callback (a subsystem-level one, if present, or the driver one) is
executed by the PM core whenever the device appears to be idle, which is
indicated to the PM core by two counters, the device's usage counter and the
counter of 'active' children of the device.

  * If any of these counters is decreased using a helper function provided by
    the PM core and it turns out to be equal to zero, the other counter is
    checked.  If that counter also is equal to zero, the PM core executes the
    idle callback with the device as its argument.

The action performed by the idle callback is totally dependent on the subsystem
(or driver) in question, but the expected and recommended action is to check
if the device can be suspended (i.e. if all of the conditions necessary for
suspending the device are satisfied) and to queue up a suspend request for the
device in that case.  If there is no idle callback, or if the callback returns
0, then the PM core will attempt to carry out a runtime suspend of the device,
also respecting devices configured for autosuspend.  In essence this means a
call to pm_runtime_autosuspend() (do note that drivers needs to update the
device last busy mark, pm_runtime_mark_last_busy(), to control the delay under
this circumstance).  To prevent this (for example, if the callback routine has
started a delayed suspend), the routine must return a non-zero value.  Negative
error return codes are ignored by the PM core.

The helper functions provided by the PM core, described in Section 4, guarantee
that the following constraints are met with respect to runtime PM callbacks for
one device:

(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
    ->runtime_suspend() in parallel with ->runtime_resume() or with another
    instance of ->runtime_suspend() for the same device) with the exception that
    ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
    ->runtime_idle() (although ->runtime_idle() will not be started while any
    of the other callbacks is being executed for the same device).

(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
    devices (i.e. the PM core will only execute ->runtime_idle() or
    ->runtime_suspend() for the devices the runtime PM status of which is
    'active').

(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
    the usage counter of which is equal to zero _and_ either the counter of
    'active' children of which is equal to zero, or the 'power.ignore_children'
    flag of which is set.

(4) ->runtime_resume() can only be executed for 'suspended' devices  (i.e. the
    PM core will only execute ->runtime_resume() for the devices the runtime
    PM status of which is 'suspended').

Additionally, the helper functions provided by the PM core obey the following
rules:

  * If ->runtime_suspend() is about to be executed or there's a pending request
    to execute it, ->runtime_idle() will not be executed for the same device.

  * A request to execute or to schedule the execution of ->runtime_suspend()
    will cancel any pending requests to execute ->runtime_idle() for the same
    device.

  * If ->runtime_resume() is about to be executed or there's a pending request
    to execute it, the other callbacks will not be executed for the same device.

  * A request to execute ->runtime_resume() will cancel any pending or
    scheduled requests to execute the other callbacks for the same device,
    except for scheduled autosuspends.

3. Runtime PM Device Fields
===========================

The following device runtime PM fields are present in 'struct dev_pm_info', as
defined in include/linux/pm.h:

  `struct timer_list suspend_timer;`
    - timer used for scheduling (delayed) suspend and autosuspend requests

  `unsigned long timer_expires;`
    - timer expiration time, in jiffies (if this is different from zero, the
      timer is running and will expire at that time, otherwise the timer is not
      running)

  `struct work_struct work;`
    - work structure used for queuing up requests (i.e. work items in pm_wq)

  `wait_queue_head_t wait_queue;`
    - wait queue used if any of the helper functions needs to wait for another
      one to complete

  `spinlock_t lock;`
    - lock used for synchronization

  `atomic_t usage_count;`
    - the usage counter of the device

  `atomic_t child_count;`
    - the count of 'active' children of the device

  `unsigned int ignore_children;`
    - if set, the value of child_count is ignored (but still updated)

  `unsigned int disable_depth;`
    - used for disabling the helper functions (they work normally if this is
      equal to zero); the initial value of it is 1 (i.e. runtime PM is
      initially disabled for all devices)

  `int runtime_error;`
    - if set, there was a fatal error (one of the callbacks returned error code
      as described in Section 2), so the helper functions will not work until
      this flag is cleared; this is the error code returned by the failing
      callback

  `unsigned int idle_notification;`
    - if set, ->runtime_idle() is being executed

  `unsigned int request_pending;`
    - if set, there's a pending request (i.e. a work item queued up into pm_wq)

  `enum rpm_request request;`
    - type of request that's pending (valid if request_pending is set)

  `unsigned int deferred_resume;`
    - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
      being executed for that device and it is not practical to wait for the
      suspend to complete; means "start a resume as soon as you've suspended"

  `enum rpm_status runtime_status;`
    - the runtime PM status of the device; this field's initial value is
      RPM_SUSPENDED, which means that each device is initially regarded by the
      PM core as 'suspended', regardless of its real hardware status

  `enum rpm_status last_status;`
    - the last runtime PM status of the device captured before disabling runtime
      PM for it (invalid initially and when disable_depth is 0)

  `unsigned int runtime_auto;`
    - if set, indicates that the user space has allowed the device driver to
      power manage the device at run time via the /sys/devices/.../power/control
      `interface;` it may only be modified with the help of the
      pm_runtime_allow() and pm_runtime_forbid() helper functions

  `unsigned int no_callbacks;`
    - indicates that the device does not use the runtime PM callbacks (see
      Section 8); it may be modified only by the pm_runtime_no_callbacks()
      helper function

  `unsigned int irq_safe;`
    - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
      will be invoked with the spinlock held and interrupts disabled

  `unsigned int use_autosuspend;`
    - indicates that the device's driver supports delayed autosuspend (see
      Section 9); it may be modified only by the
      pm_runtime{_dont}_use_autosuspend() helper functions

  `unsigned int timer_autosuspends;`
    - indicates that the PM core should attempt to carry out an autosuspend
      when the timer expires rather than a normal suspend

  `int autosuspend_delay;`
    - the delay time (in milliseconds) to be used for autosuspend

  `unsigned long last_busy;`
    - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
      function was last called for this device; used in calculating inactivity
      periods for autosuspend

All of the above fields are members of the 'power' member of 'struct device'.

4. Runtime PM Device Helper Functions
=====================================

The following runtime PM helper functions are defined in
drivers/base/power/runtime.c and include/linux/pm_runtime.h:

  `void pm_runtime_init(struct device *dev);`
    - initialize the device runtime PM fields in 'struct dev_pm_info'

  `void pm_runtime_remove(struct device *dev);`
    - make sure that the runtime PM of the device will be disabled after
      removing the device from device hierarchy

  `int pm_runtime_idle(struct device *dev);`
    - execute the subsystem-level idle callback for the device; returns an
      error code on failure, where -EINPROGRESS means that ->runtime_idle() is
      already being executed; if there is no callback or the callback returns 0
      then run pm_runtime_autosuspend(dev) and return its result

  `int pm_runtime_suspend(struct device *dev);`
    - execute the subsystem-level suspend callback for the device; returns 0 on
      success, 1 if the device's runtime PM status was already 'suspended', or
      error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
      to suspend the device again in future and -EACCES means that
      'power.disable_depth' is different from 0

  `int pm_runtime_autosuspend(struct device *dev);`
    - same as pm_runtime_suspend() except that the autosuspend delay is taken
      `into account;` if pm_runtime_autosuspend_expiration() says the delay has
      not yet expired then an autosuspend is scheduled for the appropriate time
      and 0 is returned

  `int pm_runtime_resume(struct device *dev);`
    - execute the subsystem-level resume callback for the device; returns 0 on
      success, 1 if the device's runtime PM status is already 'active' (also if
      'power.disable_depth' is nonzero, but the status was 'active' when it was
      changing from 0 to 1) or error code on failure, where -EAGAIN means it may
      be safe to attempt to resume the device again in future, but
      'power.runtime_error' should be checked additionally, and -EACCES means
      that the callback could not be run, because 'power.disable_depth' was
      different from 0

  `int pm_runtime_resume_and_get(struct device *dev);`
    - run pm_runtime_resume(dev) and if successful, increment the device's
      usage counter; return the result of pm_runtime_resume

  `int pm_request_idle(struct device *dev);`
    - submit a request to execute the subsystem-level idle callback for the
      device (the request is represented by a work item in pm_wq); returns 0 on
      success or error code if the request has not been queued up

  `int pm_request_autosuspend(struct device *dev);`
    - schedule the execution of the subsystem-level suspend callback for the
      device when the autosuspend delay has expired; if the delay has already
      expired then the work item is queued up immediately

  `int pm_schedule_suspend(struct device *dev, unsigned int delay);`
    - schedule the execution of the subsystem-level suspend callback for the
      device in future, where 'delay' is the time to wait before queuing up a
      suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
      item is queued up immediately); returns 0 on success, 1 if the device's PM
      runtime status was already 'suspended', or error code if the request
      hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
      ->runtime_suspend() is already scheduled and not yet expired, the new
      value of 'delay' will be used as the time to wait

  `int pm_request_resume(struct device *dev);`
    - submit a request to execute the subsystem-level resume callback for the
      device (the request is represented by a work item in pm_wq); returns 0 on
      success, 1 if the device's runtime PM status was already 'active', or
      error code if the request hasn't been queued up

  `void pm_runtime_get_noresume(struct device *dev);`
    - increment the device's usage counter

  `int pm_runtime_get(struct device *dev);`
    - increment the device's usage counter, run pm_request_resume(dev) and
      return its result

  `int pm_runtime_get_sync(struct device *dev);`
    - increment the device's usage counter, run pm_runtime_resume(dev) and
      return its result;
      note that it does not drop the device's usage counter on errors, so
      consider using pm_runtime_resume_and_get() instead of it, especially
      if its return value is checked by the caller, as this is likely to
      result in cleaner code.

  `int pm_runtime_get_if_in_use(struct device *dev);`
    - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
      runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
      nonzero, increment the counter and return 1; otherwise return 0 without
      changing the counter

  `int pm_runtime_get_if_active(struct device *dev, bool ign_usage_count);`
    - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
      runtime PM status is RPM_ACTIVE, and either ign_usage_count is true
      or the device's usage_count is non-zero, increment the counter and
      return 1; otherwise return 0 without changing the counter

  `void pm_runtime_put_noidle(struct device *dev);`
    - decrement the device's usage counter

  `int pm_runtime_put(struct device *dev);`
    - decrement the device's usage counter; if the result is 0 then run
      pm_request_idle(dev) and return its result

  `int pm_runtime_put_autosuspend(struct device *dev);`
    - decrement the device's usage counter; if the result is 0 then run
      pm_request_autosuspend(dev) and return its result

  `int pm_runtime_put_sync(struct device *dev);`
    - decrement the device's usage counter; if the result is 0 then run
      pm_runtime_idle(dev) and return its result

  `int pm_runtime_put_sync_suspend(struct device *dev);`
    - decrement the device's usage counter; if the result is 0 then run
      pm_runtime_suspend(dev) and return its result

  `int pm_runtime_put_sync_autosuspend(struct device *dev);`
    - decrement the device's usage counter; if the result is 0 then run
      pm_runtime_autosuspend(dev) and return its result

  `void pm_runtime_enable(struct device *dev);`
    - decrement the device's 'power.disable_depth' field; if that field is equal
      to zero, the runtime PM helper functions can execute subsystem-level
      callbacks described in Section 2 for the device

  `int pm_runtime_disable(struct device *dev);`
    - increment the device's 'power.disable_depth' field (if the value of that
      field was previously zero, this prevents subsystem-level runtime PM
      callbacks from being run for the device), make sure that all of the
      pending runtime PM operations on the device are either completed or
      canceled; returns 1 if there was a resume request pending and it was
      necessary to execute the subsystem-level resume callback for the device
      to satisfy that request, otherwise 0 is returned

  `int pm_runtime_barrier(struct device *dev);`
    - check if there's a resume request pending for the device and resume it
      (synchronously) in that case, cancel any other pending runtime PM requests
      regarding it and wait for all runtime PM operations on it in progress to
      complete; returns 1 if there was a resume request pending and it was
      necessary to execute the subsystem-level resume callback for the device to
      satisfy that request, otherwise 0 is returned

  `void pm_suspend_ignore_children(struct device *dev, bool enable);`
    - set/unset the power.ignore_children flag of the device

  `int pm_runtime_set_active(struct device *dev);`
    - clear the device's 'power.runtime_error' flag, set the device's runtime
      PM status to 'active' and update its parent's counter of 'active'
      children as appropriate (it is only valid to use this function if
      'power.runtime_error' is set or 'power.disable_depth' is greater than
      zero); it will fail and return error code if the device has a parent
      which is not active and the 'power.ignore_children' flag of which is unset

  `void pm_runtime_set_suspended(struct device *dev);`
    - clear the device's 'power.runtime_error' flag, set the device's runtime
      PM status to 'suspended' and update its parent's counter of 'active'
      children as appropriate (it is only valid to use this function if
      'power.runtime_error' is set or 'power.disable_depth' is greater than
      zero)

  `bool pm_runtime_active(struct device *dev);`
    - return true if the device's runtime PM status is 'active' or its
      'power.disable_depth' field is not equal to zero, or false otherwise

  `bool pm_runtime_suspended(struct device *dev);`
    - return true if the device's runtime PM status is 'suspended' and its
      'power.disable_depth' field is equal to zero, or false otherwise

  `bool pm_runtime_status_suspended(struct device *dev);`
    - return true if the device's runtime PM status is 'suspended'

  `void pm_runtime_allow(struct device *dev);`
    - set the power.runtime_auto flag for the device and decrease its usage
      counter (used by the /sys/devices/.../power/control interface to
      effectively allow the device to be power managed at run time)

  `void pm_runtime_forbid(struct device *dev);`
    - unset the power.runtime_auto flag for the device and increase its usage
      counter (used by the /sys/devices/.../power/control interface to
      effectively prevent the device from being power managed at run time)

  `void pm_runtime_no_callbacks(struct device *dev);`
    - set the power.no_callbacks flag for the device and remove the runtime
      PM attributes from /sys/devices/.../power (or prevent them from being
      added when the device is registered)

  `void pm_runtime_irq_safe(struct device *dev);`
    - set the power.irq_safe flag for the device, causing the runtime-PM
      callbacks to be invoked with interrupts off

  `bool pm_runtime_is_irq_safe(struct device *dev);`
    - return true if power.irq_safe flag was set for the device, causing
      the runtime-PM callbacks to be invoked with interrupts off

  `void pm_runtime_mark_last_busy(struct device *dev);`
    - set the power.last_busy field to the current time

  `void pm_runtime_use_autosuspend(struct device *dev);`
    - set the power.use_autosuspend flag, enabling autosuspend delays; call
      pm_runtime_get_sync if the flag was previously cleared and
      power.autosuspend_delay is negative

  `void pm_runtime_dont_use_autosuspend(struct device *dev);`
    - clear the power.use_autosuspend flag, disabling autosuspend delays;
      decrement the device's usage counter if the flag was previously set and
      power.autosuspend_delay is negative; call pm_runtime_idle

  `void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);`
    - set the power.autosuspend_delay value to 'delay' (expressed in
      milliseconds); if 'delay' is negative then runtime suspends are
      prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be
      called or the device's usage counter may be decremented and
      pm_runtime_idle called depending on if power.autosuspend_delay is
      changed to or from a negative value; if power.use_autosuspend is clear,
      pm_runtime_idle is called

  `unsigned long pm_runtime_autosuspend_expiration(struct device *dev);`
    - calculate the time when the current autosuspend delay period will expire,
      based on power.last_busy and power.autosuspend_delay; if the delay time
      is 1000 ms or larger then the expiration time is rounded up to the
      nearest second; returns 0 if the delay period has already expired or
      power.use_autosuspend isn't set, otherwise returns the expiration time
      in jiffies

It is safe to execute the following helper functions from interrupt context:

- pm_request_idle()
- pm_request_autosuspend()
- pm_schedule_suspend()
- pm_request_resume()
- pm_runtime_get_noresume()
- pm_runtime_get()
- pm_runtime_put_noidle()
- pm_runtime_put()
- pm_runtime_put_autosuspend()
- pm_runtime_enable()
- pm_suspend_ignore_children()
- pm_runtime_set_active()
- pm_runtime_set_suspended()
- pm_runtime_suspended()
- pm_runtime_mark_last_busy()
- pm_runtime_autosuspend_expiration()

If pm_runtime_irq_safe() has been called for a device then the following helper
functions may also be used in interrupt context:

- pm_runtime_idle()
- pm_runtime_suspend()
- pm_runtime_autosuspend()
- pm_runtime_resume()
- pm_runtime_get_sync()
- pm_runtime_put_sync()
- pm_runtime_put_sync_suspend()
- pm_runtime_put_sync_autosuspend()

5. Runtime PM Initialization, Device Probing and Removal
========================================================

Initially, the runtime PM is disabled for all devices, which means that the
majority of the runtime PM helper functions described in Section 4 will return
-EAGAIN until pm_runtime_enable() is called for the device.

In addition to that, the initial runtime PM status of all devices is
'suspended', but it need not reflect the actual physical state of the device.
Thus, if the device is initially active (i.e. it is able to process I/O), its
runtime PM status must be changed to 'active', with the help of
pm_runtime_set_active(), before pm_runtime_enable() is called for the device.

However, if the device has a parent and the parent's runtime PM is enabled,
calling pm_runtime_set_active() for the device will affect the parent, unless
the parent's 'power.ignore_children' flag is set.  Namely, in that case the
parent won't be able to suspend at run time, using the PM core's helper
functions, as long as the child's status is 'active', even if the child's
runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
the child yet or pm_runtime_disable() has been called for it).  For this reason,
once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
should be called for it too as soon as reasonably possible or its runtime PM
status should be changed back to 'suspended' with the help of
pm_runtime_set_suspended().

If the default initial runtime PM status of the device (i.e. 'suspended')
reflects the actual state of the device, its bus type's or its driver's
->probe() callback will likely need to wake it up using one of the PM core's
helper functions described in Section 4.  In that case, pm_runtime_resume()
should be used.  Of course, for this purpose the device's runtime PM has to be
enabled earlier by calling pm_runtime_enable().

Note, if the device may execute pm_runtime calls during the probe (such as
if it is registered with a subsystem that may call back in) then the
pm_runtime_get_sync() call paired with a pm_runtime_put() call will be
appropriate to ensure that the device is not put back to sleep during the
probe. This can happen with systems such as the network device layer.

It may be desirable to suspend the device once ->probe() has finished.
Therefore the driver core uses the asynchronous pm_request_idle() to submit a
request to execute the subsystem-level idle callback for the device at that
time.  A driver that makes use of the runtime autosuspend feature may want to
update the last busy mark before returning from ->probe().

Moreover, the driver core prevents runtime PM callbacks from racing with the bus
notifier callback in __device_release_driver(), which is necessary because the
notifier is used by some subsystems to carry out operations affecting the
runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications.  This
resumes the device if it's in the suspended state and prevents it from
being suspended again while those routines are being executed.

To allow bus types and drivers to put devices into the suspended state by
calling pm_runtime_suspend() from their ->remove() routines, the driver core
executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
notifications in __device_release_driver().  This requires bus types and
drivers to make their ->remove() callbacks avoid races with runtime PM directly,
but it also allows more flexibility in the handling of devices during the
removal of their drivers.

Drivers in ->remove() callback should undo the runtime PM changes done
in ->probe(). Usually this means calling pm_runtime_disable(),
pm_runtime_dont_use_autosuspend() etc.

The user space can effectively disallow the driver of the device to power manage
it at run time by changing the value of its /sys/devices/.../power/control
attribute to "on", which causes pm_runtime_forbid() to be called.  In principle,
this mechanism may also be used by the driver to effectively turn off the
runtime power management of the device until the user space turns it on.
Namely, during the initialization the driver can make sure that the runtime PM
status of the device is 'active' and call pm_runtime_forbid().  It should be
noted, however, that if the user space has already intentionally changed the
value of /sys/devices/.../power/control to "auto" to allow the driver to power
manage the device at run time, the driver may confuse it by using
pm_runtime_forbid() this way.

6. Runtime PM and System Sleep
==============================

Runtime PM and system sleep (i.e., system suspend and hibernation, also known
as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
ways.  If a device is active when a system sleep starts, everything is
straightforward.  But what should happen if the device is already suspended?

The device may have different wake-up settings for runtime PM and system sleep.
For example, remote wake-up may be enabled for runtime suspend but disallowed
for system sleep (device_may_wakeup(dev) returns 'false').  When this happens,
the subsystem-level system suspend callback is responsible for changing the
device's wake-up setting (it may leave that to the device driver's system
suspend routine).  It may be necessary to resume the device and suspend it again
in order to do so.  The same is true if the driver uses different power levels
or other settings for runtime suspend and system sleep.

During system resume, the simplest approach is to bring all devices back to full
power, even if they had been suspended before the system suspend began.  There
are several reasons for this, including:

  * The device might need to switch power levels, wake-up settings, etc.

  * Remote wake-up events might have been lost by the firmware.

  * The device's children may need the device to be at full power in order
    to resume themselves.

  * The driver's idea of the device state may not agree with the device's
    physical state.  This can happen during resume from hibernation.

  * The device might need to be reset.

  * Even though the device was suspended, if its usage counter was > 0 then most
    likely it would need a runtime resume in the near future anyway.

If the device had been suspended before the system suspend began and it's
brought back to full power during resume, then its runtime PM status will have
to be updated to reflect the actual post-system sleep status.  The way to do
this is:

	 - pm_runtime_disable(dev);
	 - pm_runtime_set_active(dev);
	 - pm_runtime_enable(dev);

The PM core always increments the runtime usage counter before calling the
->suspend() callback and decrements it after calling the ->resume() callback.
Hence disabling runtime PM temporarily like this will not cause any runtime
suspend attempts to be permanently lost.  If the usage count goes to zero
following the return of the ->resume() callback, the ->runtime_idle() callback
will be invoked as usual.

On some systems, however, system sleep is not entered through a global firmware
or hardware operation.  Instead, all hardware components are put into low-power
states directly by the kernel in a coordinated way.  Then, the system sleep
state effectively follows from the states the hardware components end up in
and the system is woken up from that state by a hardware interrupt or a similar
mechanism entirely under the kernel's control.  As a result, the kernel never
gives control away and the states of all devices during resume are precisely
known to it.  If that is the case and none of the situations listed above takes
place (in particular, if the system is not waking up from hibernation), it may
be more efficient to leave the devices that had been suspended before the system
suspend began in the suspended state.

To this end, the PM core provides a mechanism allowing some coordination between
different levels of device hierarchy.  Namely, if a system suspend .prepare()
callback returns a positive number for a device, that indicates to the PM core
that the device appears to be runtime-suspended and its state is fine, so it
may be left in runtime suspend provided that all of its descendants are also
left in runtime suspend.  If that happens, the PM core will not execute any
system suspend and resume callbacks for all of those devices, except for the
.complete() callback, which is then entirely responsible for handling the device
as appropriate.  This only applies to system suspend transitions that are not
related to hibernation (see Documentation/driver-api/pm/devices.rst for more
information).

The PM core does its best to reduce the probability of race conditions between
the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
out the following operations:

  * During system suspend pm_runtime_get_noresume() is called for every device
    right before executing the subsystem-level .prepare() callback for it and
    pm_runtime_barrier() is called for every device right before executing the
    subsystem-level .suspend() callback for it.  In addition to that the PM core
    calls __pm_runtime_disable() with 'false' as the second argument for every
    device right before executing the subsystem-level .suspend_late() callback
    for it.

  * During system resume pm_runtime_enable() and pm_runtime_put() are called for
    every device right after executing the subsystem-level .resume_early()
    callback and right after executing the subsystem-level .complete() callback
    for it, respectively.

7. Generic subsystem callbacks

Subsystems may wish to conserve code space by using the set of generic power
management callbacks provided by the PM core, defined in
driver/base/power/generic_ops.c:

  `int pm_generic_runtime_suspend(struct device *dev);`
    - invoke the ->runtime_suspend() callback provided by the driver of this
      device and return its result, or return 0 if not defined

  `int pm_generic_runtime_resume(struct device *dev);`
    - invoke the ->runtime_resume() callback provided by the driver of this
      device and return its result, or return 0 if not defined

  `int pm_generic_suspend(struct device *dev);`
    - if the device has not been suspended at run time, invoke the ->suspend()
      callback provided by its driver and return its result, or return 0 if not
      defined

  `int pm_generic_suspend_noirq(struct device *dev);`
    - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
      callback provided by the device's driver and return its result, or return
      0 if not defined

  `int pm_generic_resume(struct device *dev);`
    - invoke the ->resume() callback provided by the driver of this device and,
      if successful, change the device's runtime PM status to 'active'

  `int pm_generic_resume_noirq(struct device *dev);`
    - invoke the ->resume_noirq() callback provided by the driver of this device

  `int pm_generic_freeze(struct device *dev);`
    - if the device has not been suspended at run time, invoke the ->freeze()
      callback provided by its driver and return its result, or return 0 if not
      defined

  `int pm_generic_freeze_noirq(struct device *dev);`
    - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
      callback provided by the device's driver and return its result, or return
      0 if not defined

  `int pm_generic_thaw(struct device *dev);`
    - if the device has not been suspended at run time, invoke the ->thaw()
      callback provided by its driver and return its result, or return 0 if not
      defined

  `int pm_generic_thaw_noirq(struct device *dev);`
    - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
      callback provided by the device's driver and return its result, or return
      0 if not defined

  `int pm_generic_poweroff(struct device *dev);`
    - if the device has not been suspended at run time, invoke the ->poweroff()
      callback provided by its driver and return its result, or return 0 if not
      defined

  `int pm_generic_poweroff_noirq(struct device *dev);`
    - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
      callback provided by the device's driver and return its result, or return
      0 if not defined

  `int pm_generic_restore(struct device *dev);`
    - invoke the ->restore() callback provided by the driver of this device and,
      if successful, change the device's runtime PM status to 'active'

  `int pm_generic_restore_noirq(struct device *dev);`
    - invoke the ->restore_noirq() callback provided by the device's driver

These functions are the defaults used by the PM core if a subsystem doesn't
provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
subsystem-level dev_pm_ops structure.

Device drivers that wish to use the same function as a system suspend, freeze,
poweroff and runtime suspend callback, and similarly for system resume, thaw,
restore, and runtime resume, can achieve this with the help of the
UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
last argument to NULL).

8. "No-Callback" Devices
========================

Some "devices" are only logical sub-devices of their parent and cannot be
power-managed on their own.  (The prototype example is a USB interface.  Entire
USB devices can go into low-power mode or send wake-up requests, but neither is
possible for individual interfaces.)  The drivers for these devices have no
need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
and ->runtime_resume() would always return 0 without doing anything else and
->runtime_idle() would always call pm_runtime_suspend().

Subsystems can tell the PM core about these devices by calling
pm_runtime_no_callbacks().  This should be done after the device structure is
initialized and before it is registered (although after device registration is
also okay).  The routine will set the device's power.no_callbacks flag and
prevent the non-debugging runtime PM sysfs attributes from being created.

When power.no_callbacks is set, the PM core will not invoke the
->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
Instead it will assume that suspends and resumes always succeed and that idle
devices should be suspended.

As a consequence, the PM core will never directly inform the device's subsystem
or driver about runtime power changes.  Instead, the driver for the device's
parent must take responsibility for telling the device's driver when the
parent's power state changes.

Note that, in some cases it may not be desirable for subsystems/drivers to call
pm_runtime_no_callbacks() for their devices. This could be because a subset of
the runtime PM callbacks needs to be implemented, a platform dependent PM
domain could get attached to the device or that the device is power managed
through a supplier device link. For these reasons and to avoid boilerplate code
in subsystems/drivers, the PM core allows runtime PM callbacks to be
unassigned. More precisely, if a callback pointer is NULL, the PM core will act
as though there was a callback and it returned 0.

9. Autosuspend, or automatically-delayed suspends
=================================================

Changing a device's power state isn't free; it requires both time and energy.
A device should be put in a low-power state only when there's some reason to
think it will remain in that state for a substantial time.  A common heuristic
says that a device which hasn't been used for a while is liable to remain
unused; following this advice, drivers should not allow devices to be suspended
at runtime until they have been inactive for some minimum period.  Even when
the heuristic ends up being non-optimal, it will still prevent devices from
"bouncing" too rapidly between low-power and full-power states.

The term "autosuspend" is an historical remnant.  It doesn't mean that the
device is automatically suspended (the subsystem or driver still has to call
the appropriate PM routines); rather it means that runtime suspends will
automatically be delayed until the desired period of inactivity has elapsed.

Inactivity is determined based on the power.last_busy field.  Drivers should
call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
typically just before calling pm_runtime_put_autosuspend().  The desired length
of the inactivity period is a matter of policy.  Subsystems can set this length
initially by calling pm_runtime_set_autosuspend_delay(), but after device
registration the length should be controlled by user space, using the
/sys/devices/.../power/autosuspend_delay_ms attribute.

In order to use autosuspend, subsystems or drivers must call
pm_runtime_use_autosuspend() (preferably before registering the device), and
thereafter they should use the various `*_autosuspend()` helper functions
instead of the non-autosuspend counterparts::

	Instead of: pm_runtime_suspend    use: pm_runtime_autosuspend;
	Instead of: pm_schedule_suspend   use: pm_request_autosuspend;
	Instead of: pm_runtime_put        use: pm_runtime_put_autosuspend;
	Instead of: pm_runtime_put_sync   use: pm_runtime_put_sync_autosuspend.

Drivers may also continue to use the non-autosuspend helper functions; they
will behave normally, which means sometimes taking the autosuspend delay into
account (see pm_runtime_idle).

Under some circumstances a driver or subsystem may want to prevent a device
from autosuspending immediately, even though the usage counter is zero and the
autosuspend delay time has expired.  If the ->runtime_suspend() callback
returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
in the future (as it normally would be if the callback invoked
pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
autosuspend.  The ->runtime_suspend() callback can't do this rescheduling
itself because no suspend requests of any kind are accepted while the device is
suspending (i.e., while the callback is running).

The implementation is well suited for asynchronous use in interrupt contexts.
However such use inevitably involves races, because the PM core can't
synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
This synchronization must be handled by the driver, using its private lock.
Here is a schematic pseudo-code example::

	foo_read_or_write(struct foo_priv *foo, void *data)
	{
		lock(&foo->private_lock);
		add_request_to_io_queue(foo, data);
		if (foo->num_pending_requests++ == 0)
			pm_runtime_get(&foo->dev);
		if (!foo->is_suspended)
			foo_process_next_request(foo);
		unlock(&foo->private_lock);
	}

	foo_io_completion(struct foo_priv *foo, void *req)
	{
		lock(&foo->private_lock);
		if (--foo->num_pending_requests == 0) {
			pm_runtime_mark_last_busy(&foo->dev);
			pm_runtime_put_autosuspend(&foo->dev);
		} else {
			foo_process_next_request(foo);
		}
		unlock(&foo->private_lock);
		/* Send req result back to the user ... */
	}

	int foo_runtime_suspend(struct device *dev)
	{
		struct foo_priv foo = container_of(dev, ...);
		int ret = 0;

		lock(&foo->private_lock);
		if (foo->num_pending_requests > 0) {
			ret = -EBUSY;
		} else {
			/* ... suspend the device ... */
			foo->is_suspended = 1;
		}
		unlock(&foo->private_lock);
		return ret;
	}

	int foo_runtime_resume(struct device *dev)
	{
		struct foo_priv foo = container_of(dev, ...);

		lock(&foo->private_lock);
		/* ... resume the device ... */
		foo->is_suspended = 0;
		pm_runtime_mark_last_busy(&foo->dev);
		if (foo->num_pending_requests > 0)
			foo_process_next_request(foo);
		unlock(&foo->private_lock);
		return 0;
	}

The important point is that after foo_io_completion() asks for an autosuspend,
the foo_runtime_suspend() callback may race with foo_read_or_write().
Therefore foo_runtime_suspend() has to check whether there are any pending I/O
requests (while holding the private lock) before allowing the suspend to
proceed.

In addition, the power.autosuspend_delay field can be changed by user space at
any time.  If a driver cares about this, it can call
pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
callback while holding its private lock.  If the function returns a nonzero
value then the delay has not yet expired and the callback should return
-EAGAIN.


code:
========================
How to get s2ram working
========================

2006 Linus Torvalds
2006 Pavel Machek

1) Check suspend.sf.net, program s2ram there has long whitelist of
   "known ok" machines, along with tricks to use on each one.

2) If that does not help, try reading tricks.txt and
   video.txt. Perhaps problem is as simple as broken module, and
   simple module unload can fix it.

3) You can use Linus' TRACE_RESUME infrastructure, described below.

Using TRACE_RESUME
~~~~~~~~~~~~~~~~~~

I've been working at making the machines I have able to STR, and almost
always it's a driver that is buggy. Thank God for the suspend/resume
debugging - the thing that Chuck tried to disable. That's often the _only_
way to debug these things, and it's actually pretty powerful (but
time-consuming - having to insert TRACE_RESUME() markers into the device
driver that doesn't resume and recompile and reboot).

Anyway, the way to debug this for people who are interested (have a
machine that doesn't boot) is:

 - enable PM_DEBUG, and PM_TRACE

 - use a script like this::

	#!/bin/sh
	sync
	echo 1 > /sys/power/pm_trace
	echo mem > /sys/power/state

   to suspend

 - if it doesn't come back up (which is usually the problem), reboot by
   holding the power button down, and look at the dmesg output for things
   like::

	Magic number: 4:156:725
	hash matches drivers/base/power/resume.c:28
	hash matches device 0000:01:00.0

   which means that the last trace event was just before trying to resume
   device 0000:01:00.0. Then figure out what driver is controlling that
   device (lspci and /sys/devices/pci* is your friend), and see if you can
   fix it, disable it, or trace into its resume function.

   If no device matches the hash (or any matches appear to be false positives),
   the culprit may be a device from a loadable kernel module that is not loaded
   until after the hash is checked. You can check the hash against the current
   devices again after more modules are loaded using sysfs::

	cat /sys/power/pm_trace_dev_match

For example, the above happens to be the VGA device on my EVO, which I
used to run with "radeonfb" (it's an ATI Radeon mobility). It turns out
that "radeonfb" simply cannot resume that device - it tries to set the
PLL's, and it just _hangs_. Using the regular VGA console and letting X
resume it instead works fine.

NOTE
====
pm_trace uses the system's Real Time Clock (RTC) to save the magic number.
Reason for this is that the RTC is the only reliably available piece of
hardware during resume operations where a value can be set that will
survive a reboot.

pm_trace is not compatible with asynchronous suspend, so it turns
asynchronous suspend off (which may work around timing or
ordering-sensitive bugs).

Consequence is that after a resume (even if it is successful) your system
clock will have a value corresponding to the magic number instead of the
correct date/time! It is therefore advisable to use a program like ntp-date
or rdate to reset the correct date/time from an external time source when
using this trace option.

As the clock keeps ticking it is also essential that the reboot is done
quickly after the resume failure. The trace option does not use the seconds
or the low order bits of the minutes of the RTC, but a too long delay will
corrupt the magic value.


code:
====================================================================
Interaction of Suspend code (S3) with the CPU hotplug infrastructure
====================================================================

(C) 2011 - 2014 Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>

I. Differences between CPU hotplug and Suspend-to-RAM
======================================================

How does the regular CPU hotplug code differ from how the Suspend-to-RAM
infrastructure uses it internally? And where do they share common code?

Well, a picture is worth a thousand words... So ASCII art follows :-)

[This depicts the current design in the kernel, and focusses only on the
interactions involving the freezer and CPU hotplug and also tries to explain
the locking involved. It outlines the notifications involved as well.
But please note that here, only the call paths are illustrated, with the aim
of describing where they take different paths and where they share code.
What happens when regular CPU hotplug and Suspend-to-RAM race with each other
is not depicted here.]

On a high level, the suspend-resume cycle goes like this::

  |Freeze| -> |Disable nonboot| -> |Do suspend| -> |Enable nonboot| -> |Thaw |
  |tasks |    |     cpus      |    |          |    |     cpus     |    |tasks|

More details follow::

                                Suspend call path
                                -----------------

                                  Write 'mem' to
                                /sys/power/state
                                    sysfs file
                                        |
                                        v
                               Acquire system_transition_mutex lock
                                        |
                                        v
                             Send PM_SUSPEND_PREPARE
                                   notifications
                                        |
                                        v
                                   Freeze tasks
                                        |
                                        |
                                        v
                              freeze_secondary_cpus()
                                   /* start */
                                        |
                                        v
                            Acquire cpu_add_remove_lock
                                        |
                                        v
                             Iterate over CURRENTLY
                                   online CPUs
                                        |
                                        |
                                        |                ----------
                                        v                          | L
             ======>               _cpu_down()                     |
            |              [This takes cpuhotplug.lock             |
  Common    |               before taking down the CPU             |
   code     |               and releases it when done]             | O
            |            While it is at it, notifications          |
            |            are sent when notable events occur,       |
             ======>     by running all registered callbacks.      |
                                        |                          | O
                                        |                          |
                                        |                          |
                                        v                          |
                            Note down these cpus in                | P
                                frozen_cpus mask         ----------
                                        |
                                        v
                           Disable regular cpu hotplug
                        by increasing cpu_hotplug_disabled
                                        |
                                        v
                            Release cpu_add_remove_lock
                                        |
                                        v
                       /* freeze_secondary_cpus() complete */
                                        |
                                        v
                                   Do suspend

Resuming back is likewise, with the counterparts being (in the order of
execution during resume):

* thaw_secondary_cpus() which involves::

   |  Acquire cpu_add_remove_lock
   |  Decrease cpu_hotplug_disabled, thereby enabling regular cpu hotplug
   |  Call _cpu_up() [for all those cpus in the frozen_cpus mask, in a loop]
   |  Release cpu_add_remove_lock
   v

* thaw tasks
* send PM_POST_SUSPEND notifications
* Release system_transition_mutex lock.

It is to be noted here that the system_transition_mutex lock is acquired at the
very beginning, when we are just starting out to suspend, and then released only
after the entire cycle is complete (i.e., suspend + resume).

::

                          Regular CPU hotplug call path
                          -----------------------------

                                Write 0 (or 1) to
                       /sys/devices/system/cpu/cpu*/online
                                    sysfs file
                                        |
                                        |
                                        v
                                    cpu_down()
                                        |
                                        v
                           Acquire cpu_add_remove_lock
                                        |
                                        v
                          If cpu_hotplug_disabled > 0
                                return gracefully
                                        |
                                        |
                                        v
             ======>                _cpu_down()
            |              [This takes cpuhotplug.lock
  Common    |               before taking down the CPU
   code     |               and releases it when done]
            |            While it is at it, notifications
            |           are sent when notable events occur,
             ======>    by running all registered callbacks.
                                        |
                                        |
                                        v
                          Release cpu_add_remove_lock
                               [That's it!, for
                              regular CPU hotplug]

So, as can be seen from the two diagrams (the parts marked as "Common code"),
regular CPU hotplug and the suspend code path converge at the _cpu_down() and
_cpu_up() functions. They differ in the arguments passed to these functions,
in that during regular CPU hotplug, 0 is passed for the 'tasks_frozen'
argument. But during suspend, since the tasks are already frozen by the time
the non-boot CPUs are offlined or onlined, the _cpu_*() functions are called
with the 'tasks_frozen' argument set to 1.
[See below for some known issues regarding this.]

Important files and functions/entry points:
-------------------------------------------

- kernel/power/process.c : freeze_processes(), thaw_processes()
- kernel/power/suspend.c : suspend_prepare(), suspend_enter(), suspend_finish()
- kernel/cpu.c: cpu_[up|down](), _cpu_[up|down](),
  [disable|enable]_nonboot_cpus()

II. What are the issues involved in CPU hotplug?
------------------------------------------------

There are some interesting situations involving CPU hotplug and microcode
update on the CPUs, as discussed below:

[Please bear in mind that the kernel requests the microcode images from
userspace, using the request_firmware() function defined in
drivers/base/firmware_loader/main.c]

a. When all the CPUs are identical:

   This is the most common situation and it is quite straightforward: we want
   to apply the same microcode revision to each of the CPUs.
   To give an example of x86, the collect_cpu_info() function defined in
   arch/x86/kernel/microcode_core.c helps in discovering the type of the CPU
   and thereby in applying the correct microcode revision to it.
   But note that the kernel does not maintain a common microcode image for the
   all CPUs, in order to handle case 'b' described below.

b. When some of the CPUs are different than the rest:

   In this case since we probably need to apply different microcode revisions
   to different CPUs, the kernel maintains a copy of the correct microcode
   image for each CPU (after appropriate CPU type/model discovery using
   functions such as collect_cpu_info()).

c. When a CPU is physically hot-unplugged and a new (and possibly different
   type of) CPU is hot-plugged into the system:

   In the current design of the kernel, whenever a CPU is taken offline during
   a regular CPU hotplug operation, upon receiving the CPU_DEAD notification
   (which is sent by the CPU hotplug code), the microcode update driver's
   callback for that event reacts by freeing the kernel's copy of the
   microcode image for that CPU.

   Hence, when a new CPU is brought online, since the kernel finds that it
   doesn't have the microcode image, it does the CPU type/model discovery
   afresh and then requests the userspace for the appropriate microcode image
   for that CPU, which is subsequently applied.

   For example, in x86, the mc_cpu_callback() function (which is the microcode
   update driver's callback registered for CPU hotplug events) calls
   microcode_update_cpu() which would call microcode_init_cpu() in this case,
   instead of microcode_resume_cpu() when it finds that the kernel doesn't
   have a valid microcode image. This ensures that the CPU type/model
   discovery is performed and the right microcode is applied to the CPU after
   getting it from userspace.

d. Handling microcode update during suspend/hibernate:

   Strictly speaking, during a CPU hotplug operation which does not involve
   physically removing or inserting CPUs, the CPUs are not actually powered
   off during a CPU offline. They are just put to the lowest C-states possible.
   Hence, in such a case, it is not really necessary to re-apply microcode
   when the CPUs are brought back online, since they wouldn't have lost the
   image during the CPU offline operation.

   This is the usual scenario encountered during a resume after a suspend.
   However, in the case of hibernation, since all the CPUs are completely
   powered off, during restore it becomes necessary to apply the microcode
   images to all the CPUs.

   [Note that we don't expect someone to physically pull out nodes and insert
   nodes with a different type of CPUs in-between a suspend-resume or a
   hibernate/restore cycle.]

   In the current design of the kernel however, during a CPU offline operation
   as part of the suspend/hibernate cycle (cpuhp_tasks_frozen is set),
   the existing copy of microcode image in the kernel is not freed up.
   And during the CPU online operations (during resume/restore), since the
   kernel finds that it already has copies of the microcode images for all the
   CPUs, it just applies them to the CPUs, avoiding any re-discovery of CPU
   type/model and the need for validating whether the microcode revisions are
   right for the CPUs or not (due to the above assumption that physical CPU
   hotplug will not be done in-between suspend/resume or hibernate/restore
   cycles).

III. Known problems
===================

Are there any known problems when regular CPU hotplug and suspend race
with each other?

Yes, they are listed below:

1. When invoking regular CPU hotplug, the 'tasks_frozen' argument passed to
   the _cpu_down() and _cpu_up() functions is *always* 0.
   This might not reflect the true current state of the system, since the
   tasks could have been frozen by an out-of-band event such as a suspend
   operation in progress. Hence, the cpuhp_tasks_frozen variable will not
   reflect the frozen state and the CPU hotplug callbacks which evaluate
   that variable might execute the wrong code path.

2. If a regular CPU hotplug stress test happens to race with the freezer due
   to a suspend operation in progress at the same time, then we could hit the
   situation described below:

    * A regular cpu online operation continues its journey from userspace
      into the kernel, since the freezing has not yet begun.
    * Then freezer gets to work and freezes userspace.
    * If cpu online has not yet completed the microcode update stuff by now,
      it will now start waiting on the frozen userspace in the
      TASK_UNINTERRUPTIBLE state, in order to get the microcode image.
    * Now the freezer continues and tries to freeze the remaining tasks. But
      due to this wait mentioned above, the freezer won't be able to freeze
      the cpu online hotplug task and hence freezing of tasks fails.

   As a result of this task freezing failure, the suspend operation gets
   aborted.


code:
====================================
System Suspend and Device Interrupts
====================================

Copyright (C) 2014 Intel Corp.
Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Suspending and Resuming Device IRQs
-----------------------------------

Device interrupt request lines (IRQs) are generally disabled during system
suspend after the "late" phase of suspending devices (that is, after all of the
->prepare, ->suspend and ->suspend_late callbacks have been executed for all
devices).  That is done by suspend_device_irqs().

The rationale for doing so is that after the "late" phase of device suspend
there is no legitimate reason why any interrupts from suspended devices should
trigger and if any devices have not been suspended properly yet, it is better to
block interrupts from them anyway.  Also, in the past we had problems with
interrupt handlers for shared IRQs that device drivers implementing them were
not prepared for interrupts triggering after their devices had been suspended.
In some cases they would attempt to access, for example, memory address spaces
of suspended devices and cause unpredictable behavior to ensue as a result.
Unfortunately, such problems are very difficult to debug and the introduction
of suspend_device_irqs(), along with the "noirq" phase of device suspend and
resume, was the only practical way to mitigate them.

Device IRQs are re-enabled during system resume, right before the "early" phase
of resuming devices (that is, before starting to execute ->resume_early
callbacks for devices).  The function doing that is resume_device_irqs().

The IRQF_NO_SUSPEND Flag
------------------------

There are interrupts that can legitimately trigger during the entire system
suspend-resume cycle, including the "noirq" phases of suspending and resuming
devices as well as during the time when nonboot CPUs are taken offline and
brought back online.  That applies to timer interrupts in the first place,
but also to IPIs and to some other special-purpose interrupts.

The IRQF_NO_SUSPEND flag is used to indicate that to the IRQ subsystem when
requesting a special-purpose interrupt.  It causes suspend_device_irqs() to
leave the corresponding IRQ enabled so as to allow the interrupt to work as
expected during the suspend-resume cycle, but does not guarantee that the
interrupt will wake the system from a suspended state -- for such cases it is
necessary to use enable_irq_wake().

Note that the IRQF_NO_SUSPEND flag affects the entire IRQ and not just one
user of it.  Thus, if the IRQ is shared, all of the interrupt handlers installed
for it will be executed as usual after suspend_device_irqs(), even if the
IRQF_NO_SUSPEND flag was not passed to request_irq() (or equivalent) by some of
the IRQ's users.  For this reason, using IRQF_NO_SUSPEND and IRQF_SHARED at the
same time should be avoided.

System Wakeup Interrupts, enable_irq_wake() and disable_irq_wake()
------------------------------------------------------------------

System wakeup interrupts generally need to be configured to wake up the system
from sleep states, especially if they are used for different purposes (e.g. as
I/O interrupts) in the working state.

That may involve turning on a special signal handling logic within the platform
(such as an SoC) so that signals from a given line are routed in a different way
during system sleep so as to trigger a system wakeup when needed.  For example,
the platform may include a dedicated interrupt controller used specifically for
handling system wakeup events.  Then, if a given interrupt line is supposed to
wake up the system from sleep sates, the corresponding input of that interrupt
controller needs to be enabled to receive signals from the line in question.
After wakeup, it generally is better to disable that input to prevent the
dedicated controller from triggering interrupts unnecessarily.

The IRQ subsystem provides two helper functions to be used by device drivers for
those purposes.  Namely, enable_irq_wake() turns on the platform's logic for
handling the given IRQ as a system wakeup interrupt line and disable_irq_wake()
turns that logic off.

Calling enable_irq_wake() causes suspend_device_irqs() to treat the given IRQ
in a special way.  Namely, the IRQ remains enabled, by on the first interrupt
it will be disabled, marked as pending and "suspended" so that it will be
re-enabled by resume_device_irqs() during the subsequent system resume.  Also
the PM core is notified about the event which causes the system suspend in
progress to be aborted (that doesn't have to happen immediately, but at one
of the points where the suspend thread looks for pending wakeup events).

This way every interrupt from a wakeup interrupt source will either cause the
system suspend currently in progress to be aborted or wake up the system if
already suspended.  However, after suspend_device_irqs() interrupt handlers are
not executed for system wakeup IRQs.  They are only executed for IRQF_NO_SUSPEND
IRQs at that time, but those IRQs should not be configured for system wakeup
using enable_irq_wake().

Interrupts and Suspend-to-Idle
------------------------------

Suspend-to-idle (also known as the "freeze" sleep state) is a relatively new
system sleep state that works by idling all of the processors and waiting for
interrupts right after the "noirq" phase of suspending devices.

Of course, this means that all of the interrupts with the IRQF_NO_SUSPEND flag
set will bring CPUs out of idle while in that state, but they will not cause the
IRQ subsystem to trigger a system wakeup.

System wakeup interrupts, in turn, will trigger wakeup from suspend-to-idle in
analogy with what they do in the full system suspend case.  The only difference
is that the wakeup from suspend-to-idle is signaled using the usual working
state interrupt delivery mechanisms and doesn't require the platform to use
any special interrupt handling logic for it to work.

IRQF_NO_SUSPEND and enable_irq_wake()
-------------------------------------

There are very few valid reasons to use both enable_irq_wake() and the
IRQF_NO_SUSPEND flag on the same IRQ, and it is never valid to use both for the
same device.

First of all, if the IRQ is not shared, the rules for handling IRQF_NO_SUSPEND
interrupts (interrupt handlers are invoked after suspend_device_irqs()) are
directly at odds with the rules for handling system wakeup interrupts (interrupt
handlers are not invoked after suspend_device_irqs()).

Second, both enable_irq_wake() and IRQF_NO_SUSPEND apply to entire IRQs and not
to individual interrupt handlers, so sharing an IRQ between a system wakeup
interrupt source and an IRQF_NO_SUSPEND interrupt source does not generally
make sense.

In rare cases an IRQ can be shared between a wakeup device driver and an
IRQF_NO_SUSPEND user. In order for this to be safe, the wakeup device driver
must be able to discern spurious IRQs from genuine wakeup events (signalling
the latter to the core with pm_system_wakeup()), must use enable_irq_wake() to
ensure that the IRQ will function as a wakeup source, and must request the IRQ
with IRQF_COND_SUSPEND to tell the core that it meets these requirements. If
these requirements are not met, it is not valid to use IRQF_COND_SUSPEND.


code:
===============================================
Using swap files with software suspend (swsusp)
===============================================

	(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>

The Linux kernel handles swap files almost in the same way as it handles swap
partitions and there are only two differences between these two types of swap
areas:
(1) swap files need not be contiguous,
(2) the header of a swap file is not in the first block of the partition that
holds it.  From the swsusp's point of view (1) is not a problem, because it is
already taken care of by the swap-handling code, but (2) has to be taken into
consideration.

In principle the location of a swap file's header may be determined with the
help of appropriate filesystem driver.  Unfortunately, however, it requires the
filesystem holding the swap file to be mounted, and if this filesystem is
journaled, it cannot be mounted during resume from disk.  For this reason to
identify a swap file swsusp uses the name of the partition that holds the file
and the offset from the beginning of the partition at which the swap file's
header is located.  For convenience, this offset is expressed in <PAGE_SIZE>
units.

In order to use a swap file with swsusp, you need to:

1) Create the swap file and make it active, eg.::

    # dd if=/dev/zero of=<swap_file_path> bs=1024 count=<swap_file_size_in_k>
    # mkswap <swap_file_path>
    # swapon <swap_file_path>

2) Use an application that will bmap the swap file with the help of the
FIBMAP ioctl and determine the location of the file's swap header, as the
offset, in <PAGE_SIZE> units, from the beginning of the partition which
holds the swap file.

3) Add the following parameters to the kernel command line::

    resume=<swap_file_partition> resume_offset=<swap_file_offset>

where <swap_file_partition> is the partition on which the swap file is located
and <swap_file_offset> is the offset of the swap header determined by the
application in 2) (of course, this step may be carried out automatically
by the same application that determines the swap file's header offset using the
FIBMAP ioctl)

OR

Use a userland suspend application that will set the partition and offset
with the help of the SNAPSHOT_SET_SWAP_AREA ioctl described in
Documentation/power/userland-swsusp.rst (this is the only method to suspend
to a swap file allowing the resume to be initiated from an initrd or initramfs
image).

Now, swsusp will use the swap file in the same way in which it would use a swap
partition.  In particular, the swap file has to be active (ie. be present in
/proc/swaps) so that it can be used for suspending.

Note that if the swap file used for suspending is deleted and recreated,
the location of its header need not be the same as before.  Thus every time
this happens the value of the "resume_offset=" kernel command line parameter
has to be updated.