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Based on kernel version 4.3. Page generated on 2015-11-02 12:50 EST.

1	Device Power Management
3	Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
4	Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
5	Copyright (c) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
8	Most of the code in Linux is device drivers, so most of the Linux power
9	management (PM) code is also driver-specific.  Most drivers will do very
10	little; others, especially for platforms with small batteries (like cell
11	phones), will do a lot.
13	This writeup gives an overview of how drivers interact with system-wide
14	power management goals, emphasizing the models and interfaces that are
15	shared by everything that hooks up to the driver model core.  Read it as
16	background for the domain-specific work you'd do with any specific driver.
19	Two Models for Device Power Management
20	======================================
21	Drivers will use one or both of these models to put devices into low-power
22	states:
24	    System Sleep model:
25		Drivers can enter low-power states as part of entering system-wide
26		low-power states like "suspend" (also known as "suspend-to-RAM"), or
27		(mostly for systems with disks) "hibernation" (also known as
28		"suspend-to-disk").
30		This is something that device, bus, and class drivers collaborate on
31		by implementing various role-specific suspend and resume methods to
32		cleanly power down hardware and software subsystems, then reactivate
33		them without loss of data.
35		Some drivers can manage hardware wakeup events, which make the system
36		leave the low-power state.  This feature may be enabled or disabled
37		using the relevant /sys/devices/.../power/wakeup file (for Ethernet
38		drivers the ioctl interface used by ethtool may also be used for this
39		purpose); enabling it may cost some power usage, but let the whole
40		system enter low-power states more often.
42	    Runtime Power Management model:
43		Devices may also be put into low-power states while the system is
44		running, independently of other power management activity in principle.
45		However, devices are not generally independent of each other (for
46		example, a parent device cannot be suspended unless all of its child
47		devices have been suspended).  Moreover, depending on the bus type the
48		device is on, it may be necessary to carry out some bus-specific
49		operations on the device for this purpose.  Devices put into low power
50		states at run time may require special handling during system-wide power
51		transitions (suspend or hibernation).
53		For these reasons not only the device driver itself, but also the
54		appropriate subsystem (bus type, device type or device class) driver and
55		the PM core are involved in runtime power management.  As in the system
56		sleep power management case, they need to collaborate by implementing
57		various role-specific suspend and resume methods, so that the hardware
58		is cleanly powered down and reactivated without data or service loss.
60	There's not a lot to be said about those low-power states except that they are
61	very system-specific, and often device-specific.  Also, that if enough devices
62	have been put into low-power states (at runtime), the effect may be very similar
63	to entering some system-wide low-power state (system sleep) ... and that
64	synergies exist, so that several drivers using runtime PM might put the system
65	into a state where even deeper power saving options are available.
67	Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
68	for wakeup events), no more data read or written, and requests from upstream
69	drivers are no longer accepted.  A given bus or platform may have different
70	requirements though.
72	Examples of hardware wakeup events include an alarm from a real time clock,
73	network wake-on-LAN packets, keyboard or mouse activity, and media insertion
74	or removal (for PCMCIA, MMC/SD, USB, and so on).
77	Interfaces for Entering System Sleep States
78	===========================================
79	There are programming interfaces provided for subsystems (bus type, device type,
80	device class) and device drivers to allow them to participate in the power
81	management of devices they are concerned with.  These interfaces cover both
82	system sleep and runtime power management.
85	Device Power Management Operations
86	----------------------------------
87	Device power management operations, at the subsystem level as well as at the
88	device driver level, are implemented by defining and populating objects of type
89	struct dev_pm_ops:
91	struct dev_pm_ops {
92		int (*prepare)(struct device *dev);
93		void (*complete)(struct device *dev);
94		int (*suspend)(struct device *dev);
95		int (*resume)(struct device *dev);
96		int (*freeze)(struct device *dev);
97		int (*thaw)(struct device *dev);
98		int (*poweroff)(struct device *dev);
99		int (*restore)(struct device *dev);
100		int (*suspend_late)(struct device *dev);
101		int (*resume_early)(struct device *dev);
102		int (*freeze_late)(struct device *dev);
103		int (*thaw_early)(struct device *dev);
104		int (*poweroff_late)(struct device *dev);
105		int (*restore_early)(struct device *dev);
106		int (*suspend_noirq)(struct device *dev);
107		int (*resume_noirq)(struct device *dev);
108		int (*freeze_noirq)(struct device *dev);
109		int (*thaw_noirq)(struct device *dev);
110		int (*poweroff_noirq)(struct device *dev);
111		int (*restore_noirq)(struct device *dev);
112		int (*runtime_suspend)(struct device *dev);
113		int (*runtime_resume)(struct device *dev);
114		int (*runtime_idle)(struct device *dev);
115	};
117	This structure is defined in include/linux/pm.h and the methods included in it
118	are also described in that file.  Their roles will be explained in what follows.
119	For now, it should be sufficient to remember that the last three methods are
120	specific to runtime power management while the remaining ones are used during
121	system-wide power transitions.
123	There also is a deprecated "old" or "legacy" interface for power management
124	operations available at least for some subsystems.  This approach does not use
125	struct dev_pm_ops objects and it is suitable only for implementing system sleep
126	power management methods.  Therefore it is not described in this document, so
127	please refer directly to the source code for more information about it.
130	Subsystem-Level Methods
131	-----------------------
132	The core methods to suspend and resume devices reside in struct dev_pm_ops
133	pointed to by the ops member of struct dev_pm_domain, or by the pm member of
134	struct bus_type, struct device_type and struct class.  They are mostly of
135	interest to the people writing infrastructure for platforms and buses, like PCI
136	or USB, or device type and device class drivers.  They also are relevant to the
137	writers of device drivers whose subsystems (PM domains, device types, device
138	classes and bus types) don't provide all power management methods.
140	Bus drivers implement these methods as appropriate for the hardware and the
141	drivers using it; PCI works differently from USB, and so on.  Not many people
142	write subsystem-level drivers; most driver code is a "device driver" that builds
143	on top of bus-specific framework code.
145	For more information on these driver calls, see the description later;
146	they are called in phases for every device, respecting the parent-child
147	sequencing in the driver model tree.
150	/sys/devices/.../power/wakeup files
151	-----------------------------------
152	All device objects in the driver model contain fields that control the handling
153	of system wakeup events (hardware signals that can force the system out of a
154	sleep state).  These fields are initialized by bus or device driver code using
155	device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
156	include/linux/pm_wakeup.h.
158	The "power.can_wakeup" flag just records whether the device (and its driver) can
159	physically support wakeup events.  The device_set_wakeup_capable() routine
160	affects this flag.  The "power.wakeup" field is a pointer to an object of type
161	struct wakeup_source used for controlling whether or not the device should use
162	its system wakeup mechanism and for notifying the PM core of system wakeup
163	events signaled by the device.  This object is only present for wakeup-capable
164	devices (i.e. devices whose "can_wakeup" flags are set) and is created (or
165	removed) by device_set_wakeup_capable().
167	Whether or not a device is capable of issuing wakeup events is a hardware
168	matter, and the kernel is responsible for keeping track of it.  By contrast,
169	whether or not a wakeup-capable device should issue wakeup events is a policy
170	decision, and it is managed by user space through a sysfs attribute: the
171	"power/wakeup" file.  User space can write the strings "enabled" or "disabled"
172	to it to indicate whether or not, respectively, the device is supposed to signal
173	system wakeup.  This file is only present if the "power.wakeup" object exists
174	for the given device and is created (or removed) along with that object, by
175	device_set_wakeup_capable().  Reads from the file will return the corresponding
176	string.
178	The "power/wakeup" file is supposed to contain the "disabled" string initially
179	for the majority of devices; the major exceptions are power buttons, keyboards,
180	and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with
181	ethtool.  It should also default to "enabled" for devices that don't generate
182	wakeup requests on their own but merely forward wakeup requests from one bus to
183	another (like PCI Express ports).
185	The device_may_wakeup() routine returns true only if the "power.wakeup" object
186	exists and the corresponding "power/wakeup" file contains the string "enabled".
187	This information is used by subsystems, like the PCI bus type code, to see
188	whether or not to enable the devices' wakeup mechanisms.  If device wakeup
189	mechanisms are enabled or disabled directly by drivers, they also should use
190	device_may_wakeup() to decide what to do during a system sleep transition.
191	Device drivers, however, are not supposed to call device_set_wakeup_enable()
192	directly in any case.
194	It ought to be noted that system wakeup is conceptually different from "remote
195	wakeup" used by runtime power management, although it may be supported by the
196	same physical mechanism.  Remote wakeup is a feature allowing devices in
197	low-power states to trigger specific interrupts to signal conditions in which
198	they should be put into the full-power state.  Those interrupts may or may not
199	be used to signal system wakeup events, depending on the hardware design.  On
200	some systems it is impossible to trigger them from system sleep states.  In any
201	case, remote wakeup should always be enabled for runtime power management for
202	all devices and drivers that support it.
204	/sys/devices/.../power/control files
205	------------------------------------
206	Each device in the driver model has a flag to control whether it is subject to
207	runtime power management.  This flag, called runtime_auto, is initialized by the
208	bus type (or generally subsystem) code using pm_runtime_allow() or
209	pm_runtime_forbid(); the default is to allow runtime power management.
211	The setting can be adjusted by user space by writing either "on" or "auto" to
212	the device's power/control sysfs file.  Writing "auto" calls pm_runtime_allow(),
213	setting the flag and allowing the device to be runtime power-managed by its
214	driver.  Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
215	the device to full power if it was in a low-power state, and preventing the
216	device from being runtime power-managed.  User space can check the current value
217	of the runtime_auto flag by reading the file.
219	The device's runtime_auto flag has no effect on the handling of system-wide
220	power transitions.  In particular, the device can (and in the majority of cases
221	should and will) be put into a low-power state during a system-wide transition
222	to a sleep state even though its runtime_auto flag is clear.
224	For more information about the runtime power management framework, refer to
225	Documentation/power/runtime_pm.txt.
228	Calling Drivers to Enter and Leave System Sleep States
229	======================================================
230	When the system goes into a sleep state, each device's driver is asked to
231	suspend the device by putting it into a state compatible with the target
232	system state.  That's usually some version of "off", but the details are
233	system-specific.  Also, wakeup-enabled devices will usually stay partly
234	functional in order to wake the system.
236	When the system leaves that low-power state, the device's driver is asked to
237	resume it by returning it to full power.  The suspend and resume operations
238	always go together, and both are multi-phase operations.
240	For simple drivers, suspend might quiesce the device using class code
241	and then turn its hardware as "off" as possible during suspend_noirq.  The
242	matching resume calls would then completely reinitialize the hardware
243	before reactivating its class I/O queues.
245	More power-aware drivers might prepare the devices for triggering system wakeup
246	events.
249	Call Sequence Guarantees
250	------------------------
251	To ensure that bridges and similar links needing to talk to a device are
252	available when the device is suspended or resumed, the device tree is
253	walked in a bottom-up order to suspend devices.  A top-down order is
254	used to resume those devices.
256	The ordering of the device tree is defined by the order in which devices
257	get registered:  a child can never be registered, probed or resumed before
258	its parent; and can't be removed or suspended after that parent.
260	The policy is that the device tree should match hardware bus topology.
261	(Or at least the control bus, for devices which use multiple busses.)
262	In particular, this means that a device registration may fail if the parent of
263	the device is suspending (i.e. has been chosen by the PM core as the next
264	device to suspend) or has already suspended, as well as after all of the other
265	devices have been suspended.  Device drivers must be prepared to cope with such
266	situations.
269	System Power Management Phases
270	------------------------------
271	Suspending or resuming the system is done in several phases.  Different phases
272	are used for freeze, standby, and memory sleep states ("suspend-to-RAM") and the
273	hibernation state ("suspend-to-disk").  Each phase involves executing callbacks
274	for every device before the next phase begins.  Not all busses or classes
275	support all these callbacks and not all drivers use all the callbacks.  The
276	various phases always run after tasks have been frozen and before they are
277	unfrozen.  Furthermore, the *_noirq phases run at a time when IRQ handlers have
278	been disabled (except for those marked with the IRQF_NO_SUSPEND flag).
280	All phases use PM domain, bus, type, class or driver callbacks (that is, methods
281	defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or
282	dev->driver->pm).  These callbacks are regarded by the PM core as mutually
283	exclusive.  Moreover, PM domain callbacks always take precedence over all of the
284	other callbacks and, for example, type callbacks take precedence over bus, class
285	and driver callbacks.  To be precise, the following rules are used to determine
286	which callback to execute in the given phase:
288	    1.	If dev->pm_domain is present, the PM core will choose the callback
289		included in dev->pm_domain->ops for execution
291	    2.	Otherwise, if both dev->type and dev->type->pm are present, the callback
292		included in dev->type->pm will be chosen for execution.
294	    3.	Otherwise, if both dev->class and dev->class->pm are present, the
295		callback included in dev->class->pm will be chosen for execution.
297	    4.	Otherwise, if both dev->bus and dev->bus->pm are present, the callback
298		included in dev->bus->pm will be chosen for execution.
300	This allows PM domains and device types to override callbacks provided by bus
301	types or device classes if necessary.
303	The PM domain, type, class and bus callbacks may in turn invoke device- or
304	driver-specific methods stored in dev->driver->pm, but they don't have to do
305	that.
307	If the subsystem callback chosen for execution is not present, the PM core will
308	execute the corresponding method from dev->driver->pm instead if there is one.
311	Entering System Suspend
312	-----------------------
313	When the system goes into the freeze, standby or memory sleep state,
314	the phases are:
316			prepare, suspend, suspend_late, suspend_noirq.
318	    1.	The prepare phase is meant to prevent races by preventing new devices
319		from being registered; the PM core would never know that all the
320		children of a device had been suspended if new children could be
321		registered at will.  (By contrast, devices may be unregistered at any
322		time.)  Unlike the other suspend-related phases, during the prepare
323		phase the device tree is traversed top-down.
325		After the prepare callback method returns, no new children may be
326		registered below the device.  The method may also prepare the device or
327		driver in some way for the upcoming system power transition, but it
328		should not put the device into a low-power state.
330		For devices supporting runtime power management, the return value of the
331		prepare callback can be used to indicate to the PM core that it may
332		safely leave the device in runtime suspend (if runtime-suspended
333		already), provided that all of the device's descendants are also left in
334		runtime suspend.  Namely, if the prepare callback returns a positive
335		number and that happens for all of the descendants of the device too,
336		and all of them (including the device itself) are runtime-suspended, the
337		PM core will skip the suspend, suspend_late and	suspend_noirq suspend
338		phases as well as the resume_noirq, resume_early and resume phases of
339		the following system resume for all of these devices.	In that case,
340		the complete callback will be called directly after the prepare callback
341		and is entirely responsible for bringing the device back to the
342		functional state as appropriate.
344		Note that this direct-complete procedure applies even if the device is
345		disabled for runtime PM; only the runtime-PM status matters.  It follows
346		that if a device has system-sleep callbacks but does not support runtime
347		PM, then its prepare callback must never return a positive value.  This
348		is because all devices are initially set to runtime-suspended with
349		runtime PM disabled.
351	    2.	The suspend methods should quiesce the device to stop it from performing
352		I/O.  They also may save the device registers and put it into the
353		appropriate low-power state, depending on the bus type the device is on,
354		and they may enable wakeup events.
356	    3	For a number of devices it is convenient to split suspend into the
357		"quiesce device" and "save device state" phases, in which cases
358		suspend_late is meant to do the latter.  It is always executed after
359		runtime power management has been disabled for all devices.
361	    4.	The suspend_noirq phase occurs after IRQ handlers have been disabled,
362		which means that the driver's interrupt handler will not be called while
363		the callback method is running.  The methods should save the values of
364		the device's registers that weren't saved previously and finally put the
365		device into the appropriate low-power state.
367		The majority of subsystems and device drivers need not implement this
368		callback.  However, bus types allowing devices to share interrupt
369		vectors, like PCI, generally need it; otherwise a driver might encounter
370		an error during the suspend phase by fielding a shared interrupt
371		generated by some other device after its own device had been set to low
372		power.
374	At the end of these phases, drivers should have stopped all I/O transactions
375	(DMA, IRQs), saved enough state that they can re-initialize or restore previous
376	state (as needed by the hardware), and placed the device into a low-power state.
377	On many platforms they will gate off one or more clock sources; sometimes they
378	will also switch off power supplies or reduce voltages.  (Drivers supporting
379	runtime PM may already have performed some or all of these steps.)
381	If device_may_wakeup(dev) returns true, the device should be prepared for
382	generating hardware wakeup signals to trigger a system wakeup event when the
383	system is in the sleep state.  For example, enable_irq_wake() might identify
384	GPIO signals hooked up to a switch or other external hardware, and
385	pci_enable_wake() does something similar for the PCI PME signal.
387	If any of these callbacks returns an error, the system won't enter the desired
388	low-power state.  Instead the PM core will unwind its actions by resuming all
389	the devices that were suspended.
392	Leaving System Suspend
393	----------------------
394	When resuming from freeze, standby or memory sleep, the phases are:
396			resume_noirq, resume_early, resume, complete.
398	    1.	The resume_noirq callback methods should perform any actions needed
399		before the driver's interrupt handlers are invoked.  This generally
400		means undoing the actions of the suspend_noirq phase.  If the bus type
401		permits devices to share interrupt vectors, like PCI, the method should
402		bring the device and its driver into a state in which the driver can
403		recognize if the device is the source of incoming interrupts, if any,
404		and handle them correctly.
406		For example, the PCI bus type's ->pm.resume_noirq() puts the device into
407		the full-power state (D0 in the PCI terminology) and restores the
408		standard configuration registers of the device.  Then it calls the
409		device driver's ->pm.resume_noirq() method to perform device-specific
410		actions.
412	    2.	The resume_early methods should prepare devices for the execution of
413		the resume methods.  This generally involves undoing the actions of the
414		preceding suspend_late phase.
416	    3	The resume methods should bring the device back to its operating
417		state, so that it can perform normal I/O.  This generally involves
418		undoing the actions of the suspend phase.
420	    4.	The complete phase should undo the actions of the prepare phase.  Note,
421		however, that new children may be registered below the device as soon as
422		the resume callbacks occur; it's not necessary to wait until the
423		complete phase.
425		Moreover, if the preceding prepare callback returned a positive number,
426		the device may have been left in runtime suspend throughout the whole
427		system suspend and resume (the suspend, suspend_late, suspend_noirq
428		phases of system suspend and the resume_noirq, resume_early, resume
429		phases of system resume may have been skipped for it).  In that case,
430		the complete callback is entirely responsible for bringing the device
431		back to the functional state after system suspend if necessary.  [For
432		example, it may need to queue up a runtime resume request for the device
433		for this purpose.]  To check if that is the case, the complete callback
434		can consult the device's power.direct_complete flag.  Namely, if that
435		flag is set when the complete callback is being run, it has been called
436		directly after the preceding prepare and special action may be required
437		to make the device work correctly afterward.
439	At the end of these phases, drivers should be as functional as they were before
440	suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
441	gated on.
443	However, the details here may again be platform-specific.  For example,
444	some systems support multiple "run" states, and the mode in effect at
445	the end of resume might not be the one which preceded suspension.
446	That means availability of certain clocks or power supplies changed,
447	which could easily affect how a driver works.
449	Drivers need to be able to handle hardware which has been reset since the
450	suspend methods were called, for example by complete reinitialization.
451	This may be the hardest part, and the one most protected by NDA'd documents
452	and chip errata.  It's simplest if the hardware state hasn't changed since
453	the suspend was carried out, but that can't be guaranteed (in fact, it usually
454	is not the case).
456	Drivers must also be prepared to notice that the device has been removed
457	while the system was powered down, whenever that's physically possible.
458	PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
459	where common Linux platforms will see such removal.  Details of how drivers
460	will notice and handle such removals are currently bus-specific, and often
461	involve a separate thread.
463	These callbacks may return an error value, but the PM core will ignore such
464	errors since there's nothing it can do about them other than printing them in
465	the system log.
468	Entering Hibernation
469	--------------------
470	Hibernating the system is more complicated than putting it into the other
471	sleep states, because it involves creating and saving a system image.
472	Therefore there are more phases for hibernation, with a different set of
473	callbacks.  These phases always run after tasks have been frozen and memory has
474	been freed.
476	The general procedure for hibernation is to quiesce all devices (freeze), create
477	an image of the system memory while everything is stable, reactivate all
478	devices (thaw), write the image to permanent storage, and finally shut down the
479	system (poweroff).  The phases used to accomplish this are:
481		prepare, freeze, freeze_late, freeze_noirq, thaw_noirq, thaw_early,
482		thaw, complete, prepare, poweroff, poweroff_late, poweroff_noirq
484	    1.	The prepare phase is discussed in the "Entering System Suspend" section
485		above.
487	    2.	The freeze methods should quiesce the device so that it doesn't generate
488		IRQs or DMA, and they may need to save the values of device registers.
489		However the device does not have to be put in a low-power state, and to
490		save time it's best not to do so.  Also, the device should not be
491		prepared to generate wakeup events.
493	    3.	The freeze_late phase is analogous to the suspend_late phase described
494		above, except that the device should not be put in a low-power state and
495		should not be allowed to generate wakeup events by it.
497	    4.	The freeze_noirq phase is analogous to the suspend_noirq phase discussed
498		above, except again that the device should not be put in a low-power
499		state and should not be allowed to generate wakeup events.
501	At this point the system image is created.  All devices should be inactive and
502	the contents of memory should remain undisturbed while this happens, so that the
503	image forms an atomic snapshot of the system state.
505	    5.	The thaw_noirq phase is analogous to the resume_noirq phase discussed
506		above.  The main difference is that its methods can assume the device is
507		in the same state as at the end of the freeze_noirq phase.
509	    6.	The thaw_early phase is analogous to the resume_early phase described
510		above.  Its methods should undo the actions of the preceding
511		freeze_late, if necessary.
513	    7.	The thaw phase is analogous to the resume phase discussed above.  Its
514		methods should bring the device back to an operating state, so that it
515		can be used for saving the image if necessary.
517	    8.	The complete phase is discussed in the "Leaving System Suspend" section
518		above.
520	At this point the system image is saved, and the devices then need to be
521	prepared for the upcoming system shutdown.  This is much like suspending them
522	before putting the system into the freeze, standby or memory sleep state,
523	and the phases are similar.
525	    9.	The prepare phase is discussed above.
527	    10.	The poweroff phase is analogous to the suspend phase.
529	    11.	The poweroff_late phase is analogous to the suspend_late phase.
531	    12.	The poweroff_noirq phase is analogous to the suspend_noirq phase.
533	The poweroff, poweroff_late and poweroff_noirq callbacks should do essentially
534	the same things as the suspend, suspend_late and suspend_noirq callbacks,
535	respectively.  The only notable difference is that they need not store the
536	device register values, because the registers should already have been stored
537	during the freeze, freeze_late or freeze_noirq phases.
540	Leaving Hibernation
541	-------------------
542	Resuming from hibernation is, again, more complicated than resuming from a sleep
543	state in which the contents of main memory are preserved, because it requires
544	a system image to be loaded into memory and the pre-hibernation memory contents
545	to be restored before control can be passed back to the image kernel.
547	Although in principle, the image might be loaded into memory and the
548	pre-hibernation memory contents restored by the boot loader, in practice this
549	can't be done because boot loaders aren't smart enough and there is no
550	established protocol for passing the necessary information.  So instead, the
551	boot loader loads a fresh instance of the kernel, called the boot kernel, into
552	memory and passes control to it in the usual way.  Then the boot kernel reads
553	the system image, restores the pre-hibernation memory contents, and passes
554	control to the image kernel.  Thus two different kernels are involved in
555	resuming from hibernation.  In fact, the boot kernel may be completely different
556	from the image kernel: a different configuration and even a different version.
557	This has important consequences for device drivers and their subsystems.
559	To be able to load the system image into memory, the boot kernel needs to
560	include at least a subset of device drivers allowing it to access the storage
561	medium containing the image, although it doesn't need to include all of the
562	drivers present in the image kernel.  After the image has been loaded, the
563	devices managed by the boot kernel need to be prepared for passing control back
564	to the image kernel.  This is very similar to the initial steps involved in
565	creating a system image, and it is accomplished in the same way, using prepare,
566	freeze, and freeze_noirq phases.  However the devices affected by these phases
567	are only those having drivers in the boot kernel; other devices will still be in
568	whatever state the boot loader left them.
570	Should the restoration of the pre-hibernation memory contents fail, the boot
571	kernel would go through the "thawing" procedure described above, using the
572	thaw_noirq, thaw, and complete phases, and then continue running normally.  This
573	happens only rarely.  Most often the pre-hibernation memory contents are
574	restored successfully and control is passed to the image kernel, which then
575	becomes responsible for bringing the system back to the working state.
577	To achieve this, the image kernel must restore the devices' pre-hibernation
578	functionality.  The operation is much like waking up from the memory sleep
579	state, although it involves different phases:
581		restore_noirq, restore_early, restore, complete
583	    1.	The restore_noirq phase is analogous to the resume_noirq phase.
585	    2.	The restore_early phase is analogous to the resume_early phase.
587	    3.	The restore phase is analogous to the resume phase.
589	    4.	The complete phase is discussed above.
591	The main difference from resume[_early|_noirq] is that restore[_early|_noirq]
592	must assume the device has been accessed and reconfigured by the boot loader or
593	the boot kernel.  Consequently the state of the device may be different from the
594	state remembered from the freeze, freeze_late and freeze_noirq phases.  The
595	device may even need to be reset and completely re-initialized.  In many cases
596	this difference doesn't matter, so the resume[_early|_noirq] and
597	restore[_early|_norq] method pointers can be set to the same routines.
598	Nevertheless, different callback pointers are used in case there is a situation
599	where it actually does matter.
602	Device Power Management Domains
603	-------------------------------
604	Sometimes devices share reference clocks or other power resources.  In those
605	cases it generally is not possible to put devices into low-power states
606	individually.  Instead, a set of devices sharing a power resource can be put
607	into a low-power state together at the same time by turning off the shared
608	power resource.  Of course, they also need to be put into the full-power state
609	together, by turning the shared power resource on.  A set of devices with this
610	property is often referred to as a power domain.
612	Support for power domains is provided through the pm_domain field of struct
613	device.  This field is a pointer to an object of type struct dev_pm_domain,
614	defined in include/linux/pm.h, providing a set of power management callbacks
615	analogous to the subsystem-level and device driver callbacks that are executed
616	for the given device during all power transitions, instead of the respective
617	subsystem-level callbacks.  Specifically, if a device's pm_domain pointer is
618	not NULL, the ->suspend() callback from the object pointed to by it will be
619	executed instead of its subsystem's (e.g. bus type's) ->suspend() callback and
620	analogously for all of the remaining callbacks.  In other words, power
621	management domain callbacks, if defined for the given device, always take
622	precedence over the callbacks provided by the device's subsystem (e.g. bus
623	type).
625	The support for device power management domains is only relevant to platforms
626	needing to use the same device driver power management callbacks in many
627	different power domain configurations and wanting to avoid incorporating the
628	support for power domains into subsystem-level callbacks, for example by
629	modifying the platform bus type.  Other platforms need not implement it or take
630	it into account in any way.
633	Device Low Power (suspend) States
634	---------------------------------
635	Device low-power states aren't standard.  One device might only handle
636	"on" and "off", while another might support a dozen different versions of
637	"on" (how many engines are active?), plus a state that gets back to "on"
638	faster than from a full "off".
640	Some busses define rules about what different suspend states mean.  PCI
641	gives one example:  after the suspend sequence completes, a non-legacy
642	PCI device may not perform DMA or issue IRQs, and any wakeup events it
643	issues would be issued through the PME# bus signal.  Plus, there are
644	several PCI-standard device states, some of which are optional.
646	In contrast, integrated system-on-chip processors often use IRQs as the
647	wakeup event sources (so drivers would call enable_irq_wake) and might
648	be able to treat DMA completion as a wakeup event (sometimes DMA can stay
649	active too, it'd only be the CPU and some peripherals that sleep).
651	Some details here may be platform-specific.  Systems may have devices that
652	can be fully active in certain sleep states, such as an LCD display that's
653	refreshed using DMA while most of the system is sleeping lightly ... and
654	its frame buffer might even be updated by a DSP or other non-Linux CPU while
655	the Linux control processor stays idle.
657	Moreover, the specific actions taken may depend on the target system state.
658	One target system state might allow a given device to be very operational;
659	another might require a hard shut down with re-initialization on resume.
660	And two different target systems might use the same device in different
661	ways; the aforementioned LCD might be active in one product's "standby",
662	but a different product using the same SOC might work differently.
665	Power Management Notifiers
666	--------------------------
667	There are some operations that cannot be carried out by the power management
668	callbacks discussed above, because the callbacks occur too late or too early.
669	To handle these cases, subsystems and device drivers may register power
670	management notifiers that are called before tasks are frozen and after they have
671	been thawed.  Generally speaking, the PM notifiers are suitable for performing
672	actions that either require user space to be available, or at least won't
673	interfere with user space.
675	For details refer to Documentation/power/notifiers.txt.
678	Runtime Power Management
679	========================
680	Many devices are able to dynamically power down while the system is still
681	running. This feature is useful for devices that are not being used, and
682	can offer significant power savings on a running system.  These devices
683	often support a range of runtime power states, which might use names such
684	as "off", "sleep", "idle", "active", and so on.  Those states will in some
685	cases (like PCI) be partially constrained by the bus the device uses, and will
686	usually include hardware states that are also used in system sleep states.
688	A system-wide power transition can be started while some devices are in low
689	power states due to runtime power management.  The system sleep PM callbacks
690	should recognize such situations and react to them appropriately, but the
691	necessary actions are subsystem-specific.
693	In some cases the decision may be made at the subsystem level while in other
694	cases the device driver may be left to decide.  In some cases it may be
695	desirable to leave a suspended device in that state during a system-wide power
696	transition, but in other cases the device must be put back into the full-power
697	state temporarily, for example so that its system wakeup capability can be
698	disabled.  This all depends on the hardware and the design of the subsystem and
699	device driver in question.
701	During system-wide resume from a sleep state it's easiest to put devices into
702	the full-power state, as explained in Documentation/power/runtime_pm.txt.  Refer
703	to that document for more information regarding this particular issue as well as
704	for information on the device runtime power management framework in general.
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