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Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +01001Device Power Management
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Rafael J. Wysocki7538e3d2011-02-16 21:53:17 +01003Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
Alan Sternd6f9cda2010-03-26 23:53:55 +01004Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
5
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +01006
David Brownell4fc08402006-08-10 16:38:28 -07007Most of the code in Linux is device drivers, so most of the Linux power
Alan Sternd6f9cda2010-03-26 23:53:55 +01008management (PM) code is also driver-specific. Most drivers will do very
9little; others, especially for platforms with small batteries (like cell
10phones), will do a lot.
Linus Torvalds1da177e2005-04-16 15:20:36 -070011
David Brownell4fc08402006-08-10 16:38:28 -070012This writeup gives an overview of how drivers interact with system-wide
13power management goals, emphasizing the models and interfaces that are
14shared by everything that hooks up to the driver model core. Read it as
15background for the domain-specific work you'd do with any specific driver.
Linus Torvalds1da177e2005-04-16 15:20:36 -070016
17
David Brownell4fc08402006-08-10 16:38:28 -070018Two Models for Device Power Management
19======================================
20Drivers will use one or both of these models to put devices into low-power
21states:
22
23 System Sleep model:
Alan Sternd6f9cda2010-03-26 23:53:55 +010024 Drivers can enter low-power states as part of entering system-wide
25 low-power states like "suspend" (also known as "suspend-to-RAM"), or
26 (mostly for systems with disks) "hibernation" (also known as
27 "suspend-to-disk").
David Brownell4fc08402006-08-10 16:38:28 -070028
29 This is something that device, bus, and class drivers collaborate on
30 by implementing various role-specific suspend and resume methods to
31 cleanly power down hardware and software subsystems, then reactivate
32 them without loss of data.
33
34 Some drivers can manage hardware wakeup events, which make the system
Alan Sternd6f9cda2010-03-26 23:53:55 +010035 leave the low-power state. This feature may be enabled or disabled
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +010036 using the relevant /sys/devices/.../power/wakeup file (for Ethernet
37 drivers the ioctl interface used by ethtool may also be used for this
38 purpose); enabling it may cost some power usage, but let the whole
Alan Sternd6f9cda2010-03-26 23:53:55 +010039 system enter low-power states more often.
David Brownell4fc08402006-08-10 16:38:28 -070040
41 Runtime Power Management model:
Alan Sternd6f9cda2010-03-26 23:53:55 +010042 Devices may also be put into low-power states while the system is
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +010043 running, independently of other power management activity in principle.
44 However, devices are not generally independent of each other (for
Alan Sternd6f9cda2010-03-26 23:53:55 +010045 example, a parent device cannot be suspended unless all of its child
46 devices have been suspended). Moreover, depending on the bus type the
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +010047 device is on, it may be necessary to carry out some bus-specific
Alan Sternd6f9cda2010-03-26 23:53:55 +010048 operations on the device for this purpose. Devices put into low power
49 states at run time may require special handling during system-wide power
50 transitions (suspend or hibernation).
David Brownell4fc08402006-08-10 16:38:28 -070051
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +010052 For these reasons not only the device driver itself, but also the
Alan Sternd6f9cda2010-03-26 23:53:55 +010053 appropriate subsystem (bus type, device type or device class) driver and
54 the PM core are involved in runtime power management. As in the system
55 sleep power management case, they need to collaborate by implementing
56 various role-specific suspend and resume methods, so that the hardware
57 is cleanly powered down and reactivated without data or service loss.
David Brownell4fc08402006-08-10 16:38:28 -070058
Alan Sternd6f9cda2010-03-26 23:53:55 +010059There's not a lot to be said about those low-power states except that they are
60very system-specific, and often device-specific. Also, that if enough devices
61have been put into low-power states (at runtime), the effect may be very similar
62to entering some system-wide low-power state (system sleep) ... and that
63synergies exist, so that several drivers using runtime PM might put the system
64into a state where even deeper power saving options are available.
David Brownell4fc08402006-08-10 16:38:28 -070065
Alan Sternd6f9cda2010-03-26 23:53:55 +010066Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
67for wakeup events), no more data read or written, and requests from upstream
68drivers are no longer accepted. A given bus or platform may have different
69requirements though.
David Brownell4fc08402006-08-10 16:38:28 -070070
71Examples of hardware wakeup events include an alarm from a real time clock,
72network wake-on-LAN packets, keyboard or mouse activity, and media insertion
73or removal (for PCMCIA, MMC/SD, USB, and so on).
Linus Torvalds1da177e2005-04-16 15:20:36 -070074
75
David Brownell4fc08402006-08-10 16:38:28 -070076Interfaces for Entering System Sleep States
77===========================================
Alan Sternd6f9cda2010-03-26 23:53:55 +010078There are programming interfaces provided for subsystems (bus type, device type,
79device class) and device drivers to allow them to participate in the power
80management of devices they are concerned with. These interfaces cover both
81system sleep and runtime power management.
Linus Torvalds1da177e2005-04-16 15:20:36 -070082
David Brownell4fc08402006-08-10 16:38:28 -070083
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +010084Device Power Management Operations
85----------------------------------
86Device power management operations, at the subsystem level as well as at the
87device driver level, are implemented by defining and populating objects of type
88struct dev_pm_ops:
Linus Torvalds1da177e2005-04-16 15:20:36 -070089
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +010090struct dev_pm_ops {
91 int (*prepare)(struct device *dev);
92 void (*complete)(struct device *dev);
93 int (*suspend)(struct device *dev);
94 int (*resume)(struct device *dev);
95 int (*freeze)(struct device *dev);
96 int (*thaw)(struct device *dev);
97 int (*poweroff)(struct device *dev);
98 int (*restore)(struct device *dev);
Rafael J. Wysockicf579df2012-01-29 20:38:29 +010099 int (*suspend_late)(struct device *dev);
100 int (*resume_early)(struct device *dev);
101 int (*freeze_late)(struct device *dev);
102 int (*thaw_early)(struct device *dev);
103 int (*poweroff_late)(struct device *dev);
104 int (*restore_early)(struct device *dev);
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100105 int (*suspend_noirq)(struct device *dev);
106 int (*resume_noirq)(struct device *dev);
107 int (*freeze_noirq)(struct device *dev);
108 int (*thaw_noirq)(struct device *dev);
109 int (*poweroff_noirq)(struct device *dev);
110 int (*restore_noirq)(struct device *dev);
111 int (*runtime_suspend)(struct device *dev);
112 int (*runtime_resume)(struct device *dev);
113 int (*runtime_idle)(struct device *dev);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700114};
115
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100116This structure is defined in include/linux/pm.h and the methods included in it
117are also described in that file. Their roles will be explained in what follows.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100118For now, it should be sufficient to remember that the last three methods are
119specific to runtime power management while the remaining ones are used during
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100120system-wide power transitions.
121
Alan Sternd6f9cda2010-03-26 23:53:55 +0100122There also is a deprecated "old" or "legacy" interface for power management
123operations available at least for some subsystems. This approach does not use
124struct dev_pm_ops objects and it is suitable only for implementing system sleep
125power management methods. Therefore it is not described in this document, so
126please refer directly to the source code for more information about it.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100127
128
129Subsystem-Level Methods
130-----------------------
131The core methods to suspend and resume devices reside in struct dev_pm_ops
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +0100132pointed to by the ops member of struct dev_pm_domain, or by the pm member of
133struct bus_type, struct device_type and struct class. They are mostly of
134interest to the people writing infrastructure for platforms and buses, like PCI
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100135or USB, or device type and device class drivers. They also are relevant to the
136writers of device drivers whose subsystems (PM domains, device types, device
137classes and bus types) don't provide all power management methods.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100138
Alan Sternd6f9cda2010-03-26 23:53:55 +0100139Bus drivers implement these methods as appropriate for the hardware and the
140drivers using it; PCI works differently from USB, and so on. Not many people
141write subsystem-level drivers; most driver code is a "device driver" that builds
142on top of bus-specific framework code.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700143
David Brownell4fc08402006-08-10 16:38:28 -0700144For more information on these driver calls, see the description later;
145they are called in phases for every device, respecting the parent-child
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100146sequencing in the driver model tree.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700147
148
David Brownell4fc08402006-08-10 16:38:28 -0700149/sys/devices/.../power/wakeup files
150-----------------------------------
Rafael J. Wysockifafba482011-11-23 21:20:15 +0100151All device objects in the driver model contain fields that control the handling
152of system wakeup events (hardware signals that can force the system out of a
153sleep state). These fields are initialized by bus or device driver code using
Alan Sternd6f9cda2010-03-26 23:53:55 +0100154device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
155include/linux/pm_wakeup.h.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700156
Rafael J. Wysockifafba482011-11-23 21:20:15 +0100157The "power.can_wakeup" flag just records whether the device (and its driver) can
Alan Sternd6f9cda2010-03-26 23:53:55 +0100158physically support wakeup events. The device_set_wakeup_capable() routine
Rafael J. Wysockifafba482011-11-23 21:20:15 +0100159affects this flag. The "power.wakeup" field is a pointer to an object of type
160struct wakeup_source used for controlling whether or not the device should use
161its system wakeup mechanism and for notifying the PM core of system wakeup
162events signaled by the device. This object is only present for wakeup-capable
163devices (i.e. devices whose "can_wakeup" flags are set) and is created (or
164removed) by device_set_wakeup_capable().
Linus Torvalds1da177e2005-04-16 15:20:36 -0700165
Alan Sternd6f9cda2010-03-26 23:53:55 +0100166Whether or not a device is capable of issuing wakeup events is a hardware
167matter, and the kernel is responsible for keeping track of it. By contrast,
168whether or not a wakeup-capable device should issue wakeup events is a policy
169decision, and it is managed by user space through a sysfs attribute: the
Rafael J. Wysockifafba482011-11-23 21:20:15 +0100170"power/wakeup" file. User space can write the strings "enabled" or "disabled"
171to it to indicate whether or not, respectively, the device is supposed to signal
172system wakeup. This file is only present if the "power.wakeup" object exists
173for the given device and is created (or removed) along with that object, by
174device_set_wakeup_capable(). Reads from the file will return the corresponding
175string.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100176
Rafael J. Wysockifafba482011-11-23 21:20:15 +0100177The "power/wakeup" file is supposed to contain the "disabled" string initially
178for the majority of devices; the major exceptions are power buttons, keyboards,
179and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with
180ethtool. It should also default to "enabled" for devices that don't generate
181wakeup requests on their own but merely forward wakeup requests from one bus to
182another (like PCI Express ports).
183
184The device_may_wakeup() routine returns true only if the "power.wakeup" object
185exists and the corresponding "power/wakeup" file contains the string "enabled".
Rafael J. Wysockicb8f51b2011-02-08 23:26:02 +0100186This information is used by subsystems, like the PCI bus type code, to see
187whether or not to enable the devices' wakeup mechanisms. If device wakeup
188mechanisms are enabled or disabled directly by drivers, they also should use
189device_may_wakeup() to decide what to do during a system sleep transition.
Rafael J. Wysockifafba482011-11-23 21:20:15 +0100190Device drivers, however, are not supposed to call device_set_wakeup_enable()
191directly in any case.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700192
Rafael J. Wysockifafba482011-11-23 21:20:15 +0100193It ought to be noted that system wakeup is conceptually different from "remote
194wakeup" used by runtime power management, although it may be supported by the
195same physical mechanism. Remote wakeup is a feature allowing devices in
196low-power states to trigger specific interrupts to signal conditions in which
197they should be put into the full-power state. Those interrupts may or may not
198be used to signal system wakeup events, depending on the hardware design. On
199some systems it is impossible to trigger them from system sleep states. In any
200case, remote wakeup should always be enabled for runtime power management for
201all devices and drivers that support it.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700202
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100203/sys/devices/.../power/control files
204------------------------------------
Alan Sternd6f9cda2010-03-26 23:53:55 +0100205Each device in the driver model has a flag to control whether it is subject to
206runtime power management. This flag, called runtime_auto, is initialized by the
207bus type (or generally subsystem) code using pm_runtime_allow() or
208pm_runtime_forbid(); the default is to allow runtime power management.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700209
Alan Sternd6f9cda2010-03-26 23:53:55 +0100210The setting can be adjusted by user space by writing either "on" or "auto" to
211the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(),
212setting the flag and allowing the device to be runtime power-managed by its
213driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
214the device to full power if it was in a low-power state, and preventing the
215device from being runtime power-managed. User space can check the current value
216of the runtime_auto flag by reading the file.
David Brownell4fc08402006-08-10 16:38:28 -0700217
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100218The device's runtime_auto flag has no effect on the handling of system-wide
Alan Sternd6f9cda2010-03-26 23:53:55 +0100219power transitions. In particular, the device can (and in the majority of cases
220should and will) be put into a low-power state during a system-wide transition
221to a sleep state even though its runtime_auto flag is clear.
David Brownell4fc08402006-08-10 16:38:28 -0700222
Alan Sternd6f9cda2010-03-26 23:53:55 +0100223For more information about the runtime power management framework, refer to
224Documentation/power/runtime_pm.txt.
David Brownell4fc08402006-08-10 16:38:28 -0700225
226
Alan Sternd6f9cda2010-03-26 23:53:55 +0100227Calling Drivers to Enter and Leave System Sleep States
228======================================================
229When the system goes into a sleep state, each device's driver is asked to
230suspend the device by putting it into a state compatible with the target
David Brownell4fc08402006-08-10 16:38:28 -0700231system state. That's usually some version of "off", but the details are
232system-specific. Also, wakeup-enabled devices will usually stay partly
233functional in order to wake the system.
234
Alan Sternd6f9cda2010-03-26 23:53:55 +0100235When the system leaves that low-power state, the device's driver is asked to
236resume it by returning it to full power. The suspend and resume operations
237always go together, and both are multi-phase operations.
David Brownell4fc08402006-08-10 16:38:28 -0700238
Alan Sternd6f9cda2010-03-26 23:53:55 +0100239For simple drivers, suspend might quiesce the device using class code
240and then turn its hardware as "off" as possible during suspend_noirq. The
David Brownell4fc08402006-08-10 16:38:28 -0700241matching resume calls would then completely reinitialize the hardware
242before reactivating its class I/O queues.
243
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100244More power-aware drivers might prepare the devices for triggering system wakeup
245events.
David Brownell4fc08402006-08-10 16:38:28 -0700246
247
248Call Sequence Guarantees
249------------------------
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100250To ensure that bridges and similar links needing to talk to a device are
David Brownell4fc08402006-08-10 16:38:28 -0700251available when the device is suspended or resumed, the device tree is
252walked in a bottom-up order to suspend devices. A top-down order is
253used to resume those devices.
254
255The ordering of the device tree is defined by the order in which devices
256get registered: a child can never be registered, probed or resumed before
257its parent; and can't be removed or suspended after that parent.
258
259The policy is that the device tree should match hardware bus topology.
260(Or at least the control bus, for devices which use multiple busses.)
Rafael J. Wysocki58aca232008-03-12 00:57:22 +0100261In particular, this means that a device registration may fail if the parent of
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100262the device is suspending (i.e. has been chosen by the PM core as the next
Rafael J. Wysocki58aca232008-03-12 00:57:22 +0100263device to suspend) or has already suspended, as well as after all of the other
264devices have been suspended. Device drivers must be prepared to cope with such
265situations.
David Brownell4fc08402006-08-10 16:38:28 -0700266
267
Alan Sternd6f9cda2010-03-26 23:53:55 +0100268System Power Management Phases
269------------------------------
270Suspending or resuming the system is done in several phases. Different phases
271are used for standby or memory sleep states ("suspend-to-RAM") and the
272hibernation state ("suspend-to-disk"). Each phase involves executing callbacks
273for every device before the next phase begins. Not all busses or classes
274support all these callbacks and not all drivers use all the callbacks. The
275various phases always run after tasks have been frozen and before they are
276unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have
Rafael J. Wysockifa8ce722011-11-23 21:19:57 +0100277been disabled (except for those marked with the IRQF_NO_SUSPEND flag).
David Brownell4fc08402006-08-10 16:38:28 -0700278
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100279All phases use PM domain, bus, type, class or driver callbacks (that is, methods
280defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or
281dev->driver->pm). These callbacks are regarded by the PM core as mutually
282exclusive. Moreover, PM domain callbacks always take precedence over all of the
283other callbacks and, for example, type callbacks take precedence over bus, class
284and driver callbacks. To be precise, the following rules are used to determine
285which callback to execute in the given phase:
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +0100286
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100287 1. If dev->pm_domain is present, the PM core will choose the callback
288 included in dev->pm_domain->ops for execution
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +0100289
290 2. Otherwise, if both dev->type and dev->type->pm are present, the callback
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100291 included in dev->type->pm will be chosen for execution.
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +0100292
293 3. Otherwise, if both dev->class and dev->class->pm are present, the
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100294 callback included in dev->class->pm will be chosen for execution.
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +0100295
296 4. Otherwise, if both dev->bus and dev->bus->pm are present, the callback
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100297 included in dev->bus->pm will be chosen for execution.
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +0100298
299This allows PM domains and device types to override callbacks provided by bus
300types or device classes if necessary.
David Brownell4fc08402006-08-10 16:38:28 -0700301
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100302The PM domain, type, class and bus callbacks may in turn invoke device- or
303driver-specific methods stored in dev->driver->pm, but they don't have to do
304that.
305
306If the subsystem callback chosen for execution is not present, the PM core will
307execute the corresponding method from dev->driver->pm instead if there is one.
David Brownell4fc08402006-08-10 16:38:28 -0700308
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100309
Alan Sternd6f9cda2010-03-26 23:53:55 +0100310Entering System Suspend
311-----------------------
312When the system goes into the standby or memory sleep state, the phases are:
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100313
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100314 prepare, suspend, suspend_late, suspend_noirq.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100315
Alan Sternd6f9cda2010-03-26 23:53:55 +0100316 1. The prepare phase is meant to prevent races by preventing new devices
317 from being registered; the PM core would never know that all the
318 children of a device had been suspended if new children could be
319 registered at will. (By contrast, devices may be unregistered at any
320 time.) Unlike the other suspend-related phases, during the prepare
321 phase the device tree is traversed top-down.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100322
Rafael J. Wysocki91e7c752011-05-17 23:26:00 +0200323 After the prepare callback method returns, no new children may be
324 registered below the device. The method may also prepare the device or
Rafael J. Wysockifa8ce722011-11-23 21:19:57 +0100325 driver in some way for the upcoming system power transition, but it
326 should not put the device into a low-power state.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100327
328 2. The suspend methods should quiesce the device to stop it from performing
329 I/O. They also may save the device registers and put it into the
330 appropriate low-power state, depending on the bus type the device is on,
331 and they may enable wakeup events.
332
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100333 3 For a number of devices it is convenient to split suspend into the
334 "quiesce device" and "save device state" phases, in which cases
335 suspend_late is meant to do the latter. It is always executed after
336 runtime power management has been disabled for all devices.
337
338 4. The suspend_noirq phase occurs after IRQ handlers have been disabled,
Alan Sternd6f9cda2010-03-26 23:53:55 +0100339 which means that the driver's interrupt handler will not be called while
340 the callback method is running. The methods should save the values of
341 the device's registers that weren't saved previously and finally put the
342 device into the appropriate low-power state.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100343
344 The majority of subsystems and device drivers need not implement this
Alan Sternd6f9cda2010-03-26 23:53:55 +0100345 callback. However, bus types allowing devices to share interrupt
346 vectors, like PCI, generally need it; otherwise a driver might encounter
347 an error during the suspend phase by fielding a shared interrupt
348 generated by some other device after its own device had been set to low
349 power.
David Brownell4fc08402006-08-10 16:38:28 -0700350
Alan Sternd6f9cda2010-03-26 23:53:55 +0100351At the end of these phases, drivers should have stopped all I/O transactions
352(DMA, IRQs), saved enough state that they can re-initialize or restore previous
353state (as needed by the hardware), and placed the device into a low-power state.
354On many platforms they will gate off one or more clock sources; sometimes they
355will also switch off power supplies or reduce voltages. (Drivers supporting
356runtime PM may already have performed some or all of these steps.)
David Brownell4fc08402006-08-10 16:38:28 -0700357
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100358If device_may_wakeup(dev) returns true, the device should be prepared for
Alan Sternd6f9cda2010-03-26 23:53:55 +0100359generating hardware wakeup signals to trigger a system wakeup event when the
360system is in the sleep state. For example, enable_irq_wake() might identify
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100361GPIO signals hooked up to a switch or other external hardware, and
362pci_enable_wake() does something similar for the PCI PME signal.
David Brownell4fc08402006-08-10 16:38:28 -0700363
Alan Sternd6f9cda2010-03-26 23:53:55 +0100364If any of these callbacks returns an error, the system won't enter the desired
365low-power state. Instead the PM core will unwind its actions by resuming all
366the devices that were suspended.
David Brownell4fc08402006-08-10 16:38:28 -0700367
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100368
Alan Sternd6f9cda2010-03-26 23:53:55 +0100369Leaving System Suspend
370----------------------
371When resuming from standby or memory sleep, the phases are:
372
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100373 resume_noirq, resume_early, resume, complete.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100374
375 1. The resume_noirq callback methods should perform any actions needed
376 before the driver's interrupt handlers are invoked. This generally
377 means undoing the actions of the suspend_noirq phase. If the bus type
378 permits devices to share interrupt vectors, like PCI, the method should
379 bring the device and its driver into a state in which the driver can
380 recognize if the device is the source of incoming interrupts, if any,
381 and handle them correctly.
382
383 For example, the PCI bus type's ->pm.resume_noirq() puts the device into
384 the full-power state (D0 in the PCI terminology) and restores the
385 standard configuration registers of the device. Then it calls the
386 device driver's ->pm.resume_noirq() method to perform device-specific
387 actions.
388
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100389 2. The resume_early methods should prepare devices for the execution of
390 the resume methods. This generally involves undoing the actions of the
391 preceding suspend_late phase.
392
393 3 The resume methods should bring the the device back to its operating
Alan Sternd6f9cda2010-03-26 23:53:55 +0100394 state, so that it can perform normal I/O. This generally involves
395 undoing the actions of the suspend phase.
396
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100397 4. The complete phase should undo the actions of the prepare phase. Note,
398 however, that new children may be registered below the device as soon as
399 the resume callbacks occur; it's not necessary to wait until the
400 complete phase.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100401
402At the end of these phases, drivers should be as functional as they were before
403suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
404gated on. Even if the device was in a low-power state before the system sleep
405because of runtime power management, afterwards it should be back in its
406full-power state. There are multiple reasons why it's best to do this; they are
407discussed in more detail in Documentation/power/runtime_pm.txt.
408
409However, the details here may again be platform-specific. For example,
410some systems support multiple "run" states, and the mode in effect at
411the end of resume might not be the one which preceded suspension.
412That means availability of certain clocks or power supplies changed,
413which could easily affect how a driver works.
414
415Drivers need to be able to handle hardware which has been reset since the
416suspend methods were called, for example by complete reinitialization.
417This may be the hardest part, and the one most protected by NDA'd documents
418and chip errata. It's simplest if the hardware state hasn't changed since
Lucas De Marchi25985ed2011-03-30 22:57:33 -0300419the suspend was carried out, but that can't be guaranteed (in fact, it usually
Alan Sternd6f9cda2010-03-26 23:53:55 +0100420is not the case).
421
422Drivers must also be prepared to notice that the device has been removed
423while the system was powered down, whenever that's physically possible.
424PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
425where common Linux platforms will see such removal. Details of how drivers
426will notice and handle such removals are currently bus-specific, and often
427involve a separate thread.
428
429These callbacks may return an error value, but the PM core will ignore such
430errors since there's nothing it can do about them other than printing them in
431the system log.
432
433
434Entering Hibernation
435--------------------
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100436Hibernating the system is more complicated than putting it into the standby or
Alan Sternd6f9cda2010-03-26 23:53:55 +0100437memory sleep state, because it involves creating and saving a system image.
438Therefore there are more phases for hibernation, with a different set of
439callbacks. These phases always run after tasks have been frozen and memory has
440been freed.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100441
Alan Sternd6f9cda2010-03-26 23:53:55 +0100442The general procedure for hibernation is to quiesce all devices (freeze), create
443an image of the system memory while everything is stable, reactivate all
444devices (thaw), write the image to permanent storage, and finally shut down the
445system (poweroff). The phases used to accomplish this are:
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100446
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100447 prepare, freeze, freeze_late, freeze_noirq, thaw_noirq, thaw_early,
448 thaw, complete, prepare, poweroff, poweroff_late, poweroff_noirq
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100449
Alan Sternd6f9cda2010-03-26 23:53:55 +0100450 1. The prepare phase is discussed in the "Entering System Suspend" section
451 above.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100452
Alan Sternd6f9cda2010-03-26 23:53:55 +0100453 2. The freeze methods should quiesce the device so that it doesn't generate
454 IRQs or DMA, and they may need to save the values of device registers.
455 However the device does not have to be put in a low-power state, and to
456 save time it's best not to do so. Also, the device should not be
457 prepared to generate wakeup events.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100458
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100459 3. The freeze_late phase is analogous to the suspend_late phase described
460 above, except that the device should not be put in a low-power state and
461 should not be allowed to generate wakeup events by it.
462
463 4. The freeze_noirq phase is analogous to the suspend_noirq phase discussed
Alan Sternd6f9cda2010-03-26 23:53:55 +0100464 above, except again that the device should not be put in a low-power
465 state and should not be allowed to generate wakeup events.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100466
Alan Sternd6f9cda2010-03-26 23:53:55 +0100467At this point the system image is created. All devices should be inactive and
468the contents of memory should remain undisturbed while this happens, so that the
469image forms an atomic snapshot of the system state.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100470
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100471 5. The thaw_noirq phase is analogous to the resume_noirq phase discussed
Alan Sternd6f9cda2010-03-26 23:53:55 +0100472 above. The main difference is that its methods can assume the device is
473 in the same state as at the end of the freeze_noirq phase.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100474
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100475 6. The thaw_early phase is analogous to the resume_early phase described
476 above. Its methods should undo the actions of the preceding
477 freeze_late, if necessary.
478
479 7. The thaw phase is analogous to the resume phase discussed above. Its
Alan Sternd6f9cda2010-03-26 23:53:55 +0100480 methods should bring the device back to an operating state, so that it
481 can be used for saving the image if necessary.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100482
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100483 8. The complete phase is discussed in the "Leaving System Suspend" section
Alan Sternd6f9cda2010-03-26 23:53:55 +0100484 above.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100485
Alan Sternd6f9cda2010-03-26 23:53:55 +0100486At this point the system image is saved, and the devices then need to be
487prepared for the upcoming system shutdown. This is much like suspending them
488before putting the system into the standby or memory sleep state, and the phases
489are similar.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100490
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100491 9. The prepare phase is discussed above.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100492
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100493 10. The poweroff phase is analogous to the suspend phase.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100494
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100495 11. The poweroff_late phase is analogous to the suspend_late phase.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100496
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100497 12. The poweroff_noirq phase is analogous to the suspend_noirq phase.
498
499The poweroff, poweroff_late and poweroff_noirq callbacks should do essentially
500the same things as the suspend, suspend_late and suspend_noirq callbacks,
501respectively. The only notable difference is that they need not store the
502device register values, because the registers should already have been stored
503during the freeze, freeze_late or freeze_noirq phases.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100504
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100505
Alan Sternd6f9cda2010-03-26 23:53:55 +0100506Leaving Hibernation
507-------------------
508Resuming from hibernation is, again, more complicated than resuming from a sleep
509state in which the contents of main memory are preserved, because it requires
510a system image to be loaded into memory and the pre-hibernation memory contents
511to be restored before control can be passed back to the image kernel.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100512
Alan Sternd6f9cda2010-03-26 23:53:55 +0100513Although in principle, the image might be loaded into memory and the
514pre-hibernation memory contents restored by the boot loader, in practice this
515can't be done because boot loaders aren't smart enough and there is no
516established protocol for passing the necessary information. So instead, the
517boot loader loads a fresh instance of the kernel, called the boot kernel, into
518memory and passes control to it in the usual way. Then the boot kernel reads
519the system image, restores the pre-hibernation memory contents, and passes
520control to the image kernel. Thus two different kernels are involved in
521resuming from hibernation. In fact, the boot kernel may be completely different
522from the image kernel: a different configuration and even a different version.
523This has important consequences for device drivers and their subsystems.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100524
Alan Sternd6f9cda2010-03-26 23:53:55 +0100525To be able to load the system image into memory, the boot kernel needs to
526include at least a subset of device drivers allowing it to access the storage
527medium containing the image, although it doesn't need to include all of the
528drivers present in the image kernel. After the image has been loaded, the
529devices managed by the boot kernel need to be prepared for passing control back
530to the image kernel. This is very similar to the initial steps involved in
531creating a system image, and it is accomplished in the same way, using prepare,
532freeze, and freeze_noirq phases. However the devices affected by these phases
533are only those having drivers in the boot kernel; other devices will still be in
534whatever state the boot loader left them.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100535
Alan Sternd6f9cda2010-03-26 23:53:55 +0100536Should the restoration of the pre-hibernation memory contents fail, the boot
537kernel would go through the "thawing" procedure described above, using the
538thaw_noirq, thaw, and complete phases, and then continue running normally. This
539happens only rarely. Most often the pre-hibernation memory contents are
540restored successfully and control is passed to the image kernel, which then
541becomes responsible for bringing the system back to the working state.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100542
Alan Sternd6f9cda2010-03-26 23:53:55 +0100543To achieve this, the image kernel must restore the devices' pre-hibernation
544functionality. The operation is much like waking up from the memory sleep
545state, although it involves different phases:
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100546
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100547 restore_noirq, restore_early, restore, complete
Alan Sternd6f9cda2010-03-26 23:53:55 +0100548
549 1. The restore_noirq phase is analogous to the resume_noirq phase.
550
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100551 2. The restore_early phase is analogous to the resume_early phase.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100552
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100553 3. The restore phase is analogous to the resume phase.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100554
Rafael J. Wysockicf579df2012-01-29 20:38:29 +0100555 4. The complete phase is discussed above.
556
557The main difference from resume[_early|_noirq] is that restore[_early|_noirq]
558must assume the device has been accessed and reconfigured by the boot loader or
559the boot kernel. Consequently the state of the device may be different from the
560state remembered from the freeze, freeze_late and freeze_noirq phases. The
561device may even need to be reset and completely re-initialized. In many cases
562this difference doesn't matter, so the resume[_early|_noirq] and
563restore[_early|_norq] method pointers can be set to the same routines.
564Nevertheless, different callback pointers are used in case there is a situation
565where it actually does matter.
Alan Sternd6f9cda2010-03-26 23:53:55 +0100566
567
Rafael J. Wysocki564b9052011-06-23 01:52:55 +0200568Device Power Management Domains
569-------------------------------
Rafael J. Wysocki7538e3d2011-02-16 21:53:17 +0100570Sometimes devices share reference clocks or other power resources. In those
571cases it generally is not possible to put devices into low-power states
572individually. Instead, a set of devices sharing a power resource can be put
573into a low-power state together at the same time by turning off the shared
574power resource. Of course, they also need to be put into the full-power state
575together, by turning the shared power resource on. A set of devices with this
576property is often referred to as a power domain.
577
Rafael J. Wysocki564b9052011-06-23 01:52:55 +0200578Support for power domains is provided through the pm_domain field of struct
579device. This field is a pointer to an object of type struct dev_pm_domain,
Rafael J. Wysocki7538e3d2011-02-16 21:53:17 +0100580defined in include/linux/pm.h, providing a set of power management callbacks
581analogous to the subsystem-level and device driver callbacks that are executed
Rafael J. Wysockica9c68902011-06-21 23:25:32 +0200582for the given device during all power transitions, instead of the respective
583subsystem-level callbacks. Specifically, if a device's pm_domain pointer is
584not NULL, the ->suspend() callback from the object pointed to by it will be
585executed instead of its subsystem's (e.g. bus type's) ->suspend() callback and
Oskar Schirmer8d2c7942012-07-03 09:27:24 +0000586analogously for all of the remaining callbacks. In other words, power
587management domain callbacks, if defined for the given device, always take
588precedence over the callbacks provided by the device's subsystem (e.g. bus
589type).
Rafael J. Wysocki7538e3d2011-02-16 21:53:17 +0100590
Rafael J. Wysockica9c68902011-06-21 23:25:32 +0200591The support for device power management domains is only relevant to platforms
592needing to use the same device driver power management callbacks in many
593different power domain configurations and wanting to avoid incorporating the
594support for power domains into subsystem-level callbacks, for example by
595modifying the platform bus type. Other platforms need not implement it or take
596it into account in any way.
Rafael J. Wysocki7538e3d2011-02-16 21:53:17 +0100597
598
David Brownell4fc08402006-08-10 16:38:28 -0700599Device Low Power (suspend) States
600---------------------------------
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100601Device low-power states aren't standard. One device might only handle
Oskar Schirmer8d2c7942012-07-03 09:27:24 +0000602"on" and "off", while another might support a dozen different versions of
David Brownell4fc08402006-08-10 16:38:28 -0700603"on" (how many engines are active?), plus a state that gets back to "on"
604faster than from a full "off".
605
606Some busses define rules about what different suspend states mean. PCI
607gives one example: after the suspend sequence completes, a non-legacy
608PCI device may not perform DMA or issue IRQs, and any wakeup events it
609issues would be issued through the PME# bus signal. Plus, there are
610several PCI-standard device states, some of which are optional.
611
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100612In contrast, integrated system-on-chip processors often use IRQs as the
David Brownell4fc08402006-08-10 16:38:28 -0700613wakeup event sources (so drivers would call enable_irq_wake) and might
614be able to treat DMA completion as a wakeup event (sometimes DMA can stay
615active too, it'd only be the CPU and some peripherals that sleep).
616
617Some details here may be platform-specific. Systems may have devices that
618can be fully active in certain sleep states, such as an LCD display that's
619refreshed using DMA while most of the system is sleeping lightly ... and
620its frame buffer might even be updated by a DSP or other non-Linux CPU while
621the Linux control processor stays idle.
622
623Moreover, the specific actions taken may depend on the target system state.
624One target system state might allow a given device to be very operational;
625another might require a hard shut down with re-initialization on resume.
626And two different target systems might use the same device in different
627ways; the aforementioned LCD might be active in one product's "standby",
628but a different product using the same SOC might work differently.
629
630
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100631Power Management Notifiers
632--------------------------
Alan Sternd6f9cda2010-03-26 23:53:55 +0100633There are some operations that cannot be carried out by the power management
634callbacks discussed above, because the callbacks occur too late or too early.
635To handle these cases, subsystems and device drivers may register power
636management notifiers that are called before tasks are frozen and after they have
637been thawed. Generally speaking, the PM notifiers are suitable for performing
638actions that either require user space to be available, or at least won't
639interfere with user space.
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100640
641For details refer to Documentation/power/notifiers.txt.
642
643
Linus Torvalds1da177e2005-04-16 15:20:36 -0700644Runtime Power Management
David Brownell4fc08402006-08-10 16:38:28 -0700645========================
646Many devices are able to dynamically power down while the system is still
647running. This feature is useful for devices that are not being used, and
648can offer significant power savings on a running system. These devices
649often support a range of runtime power states, which might use names such
650as "off", "sleep", "idle", "active", and so on. Those states will in some
Alan Sternd6f9cda2010-03-26 23:53:55 +0100651cases (like PCI) be partially constrained by the bus the device uses, and will
David Brownell4fc08402006-08-10 16:38:28 -0700652usually include hardware states that are also used in system sleep states.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700653
Alan Sternd6f9cda2010-03-26 23:53:55 +0100654A system-wide power transition can be started while some devices are in low
655power states due to runtime power management. The system sleep PM callbacks
656should recognize such situations and react to them appropriately, but the
657necessary actions are subsystem-specific.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700658
Alan Sternd6f9cda2010-03-26 23:53:55 +0100659In some cases the decision may be made at the subsystem level while in other
660cases the device driver may be left to decide. In some cases it may be
661desirable to leave a suspended device in that state during a system-wide power
662transition, but in other cases the device must be put back into the full-power
663state temporarily, for example so that its system wakeup capability can be
664disabled. This all depends on the hardware and the design of the subsystem and
665device driver in question.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700666
Rafael J. Wysocki455716e2011-07-01 22:29:05 +0200667During system-wide resume from a sleep state it's easiest to put devices into
668the full-power state, as explained in Documentation/power/runtime_pm.txt. Refer
669to that document for more information regarding this particular issue as well as
Rafael J. Wysocki624f6ec2010-03-26 23:53:42 +0100670for information on the device runtime power management framework in general.