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+++ b/Documentation/power/devices.txt
@@ -1,7 +1,13 @@
1Device Power Management
2
3Copyright (c) 2010 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
4Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
5
6
1Most of the code in Linux is device drivers, so most of the Linux power 7Most of the code in Linux is device drivers, so most of the Linux power
2management code is also driver-specific. Most drivers will do very little; 8management (PM) code is also driver-specific. Most drivers will do very
3others, especially for platforms with small batteries (like cell phones), 9little; others, especially for platforms with small batteries (like cell
4will do a lot. 10phones), will do a lot.
5 11
6This writeup gives an overview of how drivers interact with system-wide 12This writeup gives an overview of how drivers interact with system-wide
7power management goals, emphasizing the models and interfaces that are 13power management goals, emphasizing the models and interfaces that are
@@ -15,9 +21,10 @@ Drivers will use one or both of these models to put devices into low-power
15states: 21states:
16 22
17 System Sleep model: 23 System Sleep model:
18 Drivers can enter low power states as part of entering system-wide 24 Drivers can enter low-power states as part of entering system-wide
19 low-power states like "suspend-to-ram", or (mostly for systems with 25 low-power states like "suspend" (also known as "suspend-to-RAM"), or
20 disks) "hibernate" (suspend-to-disk). 26 (mostly for systems with disks) "hibernation" (also known as
27 "suspend-to-disk").
21 28
22 This is something that device, bus, and class drivers collaborate on 29 This is something that device, bus, and class drivers collaborate on
23 by implementing various role-specific suspend and resume methods to 30 by implementing various role-specific suspend and resume methods to
@@ -25,33 +32,41 @@ states:
25 them without loss of data. 32 them without loss of data.
26 33
27 Some drivers can manage hardware wakeup events, which make the system 34 Some drivers can manage hardware wakeup events, which make the system
28 leave that low-power state. This feature may be disabled using the 35 leave the low-power state. This feature may be enabled or disabled
29 relevant /sys/devices/.../power/wakeup file; enabling it may cost some 36 using the relevant /sys/devices/.../power/wakeup file (for Ethernet
30 power usage, but let the whole system enter low power states more often. 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
39 system enter low-power states more often.
31 40
32 Runtime Power Management model: 41 Runtime Power Management model:
33 Drivers may also enter low power states while the system is running, 42 Devices may also be put into low-power states while the system is
34 independently of other power management activity. Upstream drivers 43 running, independently of other power management activity in principle.
35 will normally not know (or care) if the device is in some low power 44 However, devices are not generally independent of each other (for
36 state when issuing requests; the driver will auto-resume anything 45 example, a parent device cannot be suspended unless all of its child
37 that's needed when it gets a request. 46 devices have been suspended). Moreover, depending on the bus type the
38 47 device is on, it may be necessary to carry out some bus-specific
39 This doesn't have, or need much infrastructure; it's just something you 48 operations on the device for this purpose. Devices put into low power
40 should do when writing your drivers. For example, clk_disable() unused 49 states at run time may require special handling during system-wide power
41 clocks as part of minimizing power drain for currently-unused hardware. 50 transitions (suspend or hibernation).
42 Of course, sometimes clusters of drivers will collaborate with each 51
43 other, which could involve task-specific power management. 52 For these reasons not only the device driver itself, but also the
44 53 appropriate subsystem (bus type, device type or device class) driver and
45There's not a lot to be said about those low power states except that they 54 the PM core are involved in runtime power management. As in the system
46are very system-specific, and often device-specific. Also, that if enough 55 sleep power management case, they need to collaborate by implementing
47drivers put themselves into low power states (at "runtime"), the effect may be 56 various role-specific suspend and resume methods, so that the hardware
48the same as entering some system-wide low-power state (system sleep) ... and 57 is cleanly powered down and reactivated without data or service loss.
49that synergies exist, so that several drivers using runtime pm might put the 58
50system into a state where even deeper power saving options are available. 59There's not a lot to be said about those low-power states except that they are
51 60very system-specific, and often device-specific. Also, that if enough devices
52Most suspended devices will have quiesced all I/O: no more DMA or irqs, no 61have been put into low-power states (at runtime), the effect may be very similar
53more data read or written, and requests from upstream drivers are no longer 62to entering some system-wide low-power state (system sleep) ... and that
54accepted. A given bus or platform may have different requirements though. 63synergies exist, so that several drivers using runtime PM might put the system
64into a state where even deeper power saving options are available.
65
66Most 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.
55 70
56Examples of hardware wakeup events include an alarm from a real time clock, 71Examples of hardware wakeup events include an alarm from a real time clock,
57network wake-on-LAN packets, keyboard or mouse activity, and media insertion 72network wake-on-LAN packets, keyboard or mouse activity, and media insertion
@@ -60,129 +75,152 @@ or removal (for PCMCIA, MMC/SD, USB, and so on).
60 75
61Interfaces for Entering System Sleep States 76Interfaces for Entering System Sleep States
62=========================================== 77===========================================
63Most of the programming interfaces a device driver needs to know about 78There are programming interfaces provided for subsystems (bus type, device type,
64relate to that first model: entering a system-wide low power state, 79device class) and device drivers to allow them to participate in the power
65rather than just minimizing power consumption by one device. 80management of devices they are concerned with. These interfaces cover both
66 81system sleep and runtime power management.
67 82
68Bus Driver Methods 83
69------------------ 84Device Power Management Operations
70The core methods to suspend and resume devices reside in struct bus_type. 85----------------------------------
71These are mostly of interest to people writing infrastructure for busses 86Device power management operations, at the subsystem level as well as at the
72like PCI or USB, or because they define the primitives that device drivers 87device driver level, are implemented by defining and populating objects of type
73may need to apply in domain-specific ways to their devices: 88struct dev_pm_ops:
74 89
75struct bus_type { 90struct dev_pm_ops {
76 ... 91 int (*prepare)(struct device *dev);
77 int (*suspend)(struct device *dev, pm_message_t state); 92 void (*complete)(struct device *dev);
78 int (*resume)(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);
99 int (*suspend_noirq)(struct device *dev);
100 int (*resume_noirq)(struct device *dev);
101 int (*freeze_noirq)(struct device *dev);
102 int (*thaw_noirq)(struct device *dev);
103 int (*poweroff_noirq)(struct device *dev);
104 int (*restore_noirq)(struct device *dev);
105 int (*runtime_suspend)(struct device *dev);
106 int (*runtime_resume)(struct device *dev);
107 int (*runtime_idle)(struct device *dev);
79}; 108};
80 109
81Bus drivers implement those methods as appropriate for the hardware and 110This structure is defined in include/linux/pm.h and the methods included in it
82the drivers using it; PCI works differently from USB, and so on. Not many 111are also described in that file. Their roles will be explained in what follows.
83people write bus drivers; most driver code is a "device driver" that 112For now, it should be sufficient to remember that the last three methods are
84builds on top of bus-specific framework code. 113specific to runtime power management while the remaining ones are used during
114system-wide power transitions.
85 115
86For more information on these driver calls, see the description later; 116There also is a deprecated "old" or "legacy" interface for power management
87they are called in phases for every device, respecting the parent-child 117operations available at least for some subsystems. This approach does not use
88sequencing in the driver model tree. Note that as this is being written, 118struct dev_pm_ops objects and it is suitable only for implementing system sleep
89only the suspend() and resume() are widely available; not many bus drivers 119power management methods. Therefore it is not described in this document, so
90leverage all of those phases, or pass them down to lower driver levels. 120please refer directly to the source code for more information about it.
91 121
92 122
93/sys/devices/.../power/wakeup files 123Subsystem-Level Methods
94----------------------------------- 124-----------------------
95All devices in the driver model have two flags to control handling of 125The core methods to suspend and resume devices reside in struct dev_pm_ops
96wakeup events, which are hardware signals that can force the device and/or 126pointed to by the pm member of struct bus_type, struct device_type and
97system out of a low power state. These are initialized by bus or device 127struct class. They are mostly of interest to the people writing infrastructure
98driver code using device_init_wakeup(dev,can_wakeup). 128for buses, like PCI or USB, or device type and device class drivers.
99 129
100The "can_wakeup" flag just records whether the device (and its driver) can 130Bus drivers implement these methods as appropriate for the hardware and the
101physically support wakeup events. When that flag is clear, the sysfs 131drivers using it; PCI works differently from USB, and so on. Not many people
102"wakeup" file is empty, and device_may_wakeup() returns false. 132write subsystem-level drivers; most driver code is a "device driver" that builds
133on top of bus-specific framework code.
103 134
104For devices that can issue wakeup events, a separate flag controls whether 135For more information on these driver calls, see the description later;
105that device should try to use its wakeup mechanism. The initial value of 136they are called in phases for every device, respecting the parent-child
106device_may_wakeup() will be true, so that the device's "wakeup" file holds 137sequencing in the driver model tree.
107the value "enabled". Userspace can change that to "disabled" so that
108device_may_wakeup() returns false; or change it back to "enabled" (so that
109it returns true again).
110 138
111 139
112EXAMPLE: PCI Device Driver Methods 140/sys/devices/.../power/wakeup files
113----------------------------------- 141-----------------------------------
114PCI framework software calls these methods when the PCI device driver bound 142All devices in the driver model have two flags to control handling of wakeup
115to a device device has provided them: 143events (hardware signals that can force the device and/or system out of a low
116 144power state). These flags are initialized by bus or device driver code using
117struct pci_driver { 145device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
118 ... 146include/linux/pm_wakeup.h.
119 int (*suspend)(struct pci_device *pdev, pm_message_t state);
120 int (*suspend_late)(struct pci_device *pdev, pm_message_t state);
121 147
122 int (*resume_early)(struct pci_device *pdev); 148The "can_wakeup" flag just records whether the device (and its driver) can
123 int (*resume)(struct pci_device *pdev); 149physically support wakeup events. The device_set_wakeup_capable() routine
124}; 150affects this flag. The "should_wakeup" flag controls whether the device should
125 151try to use its wakeup mechanism. device_set_wakeup_enable() affects this flag;
126Drivers will implement those methods, and call PCI-specific procedures 152for the most part drivers should not change its value. The initial value of
127like pci_set_power_state(), pci_enable_wake(), pci_save_state(), and 153should_wakeup is supposed to be false for the majority of devices; the major
128pci_restore_state() to manage PCI-specific mechanisms. (PCI config space 154exceptions are power buttons, keyboards, and Ethernet adapters whose WoL
129could be saved during driver probe, if it weren't for the fact that some 155(wake-on-LAN) feature has been set up with ethtool.
130systems rely on userspace tweaking using setpci.) Devices are suspended 156
131before their bridges enter low power states, and likewise bridges resume 157Whether or not a device is capable of issuing wakeup events is a hardware
132before their devices. 158matter, and the kernel is responsible for keeping track of it. By contrast,
133 159whether or not a wakeup-capable device should issue wakeup events is a policy
134 160decision, and it is managed by user space through a sysfs attribute: the
135Upper Layers of Driver Stacks 161power/wakeup file. User space can write the strings "enabled" or "disabled" to
136----------------------------- 162set or clear the should_wakeup flag, respectively. Reads from the file will
137Device drivers generally have at least two interfaces, and the methods 163return the corresponding string if can_wakeup is true, but if can_wakeup is
138sketched above are the ones which apply to the lower level (nearer PCI, USB, 164false then reads will return an empty string, to indicate that the device
139or other bus hardware). The network and block layers are examples of upper 165doesn't support wakeup events. (But even though the file appears empty, writes
140level interfaces, as is a character device talking to userspace. 166will still affect the should_wakeup flag.)
141 167
142Power management requests normally need to flow through those upper levels, 168The device_may_wakeup() routine returns true only if both flags are set.
143which often use domain-oriented requests like "blank that screen". In 169Drivers should check this routine when putting devices in a low-power state
144some cases those upper levels will have power management intelligence that 170during a system sleep transition, to see whether or not to enable the devices'
145relates to end-user activity, or other devices that work in cooperation. 171wakeup mechanisms. However for runtime power management, wakeup events should
146 172be enabled whenever the device and driver both support them, regardless of the
147When those interfaces are structured using class interfaces, there is a 173should_wakeup flag.
148standard way to have the upper layer stop issuing requests to a given 174
149class device (and restart later): 175
150 176/sys/devices/.../power/control files
151struct class { 177------------------------------------
152 ... 178Each device in the driver model has a flag to control whether it is subject to
153 int (*suspend)(struct device *dev, pm_message_t state); 179runtime power management. This flag, called runtime_auto, is initialized by the
154 int (*resume)(struct device *dev); 180bus type (or generally subsystem) code using pm_runtime_allow() or
155}; 181pm_runtime_forbid(); the default is to allow runtime power management.
156 182
157Those calls are issued in specific phases of the process by which the 183The setting can be adjusted by user space by writing either "on" or "auto" to
158system enters a low power "suspend" state, or resumes from it. 184the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(),
159 185setting the flag and allowing the device to be runtime power-managed by its
160 186driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
161Calling Drivers to Enter System Sleep States 187the device to full power if it was in a low-power state, and preventing the
162============================================ 188device from being runtime power-managed. User space can check the current value
163When the system enters a low power state, each device's driver is asked 189of the runtime_auto flag by reading the file.
164to suspend the device by putting it into state compatible with the target 190
191The device's runtime_auto flag has no effect on the handling of system-wide
192power transitions. In particular, the device can (and in the majority of cases
193should and will) be put into a low-power state during a system-wide transition
194to a sleep state even though its runtime_auto flag is clear.
195
196For more information about the runtime power management framework, refer to
197Documentation/power/runtime_pm.txt.
198
199
200Calling Drivers to Enter and Leave System Sleep States
201======================================================
202When the system goes into a sleep state, each device's driver is asked to
203suspend the device by putting it into a state compatible with the target
165system state. That's usually some version of "off", but the details are 204system state. That's usually some version of "off", but the details are
166system-specific. Also, wakeup-enabled devices will usually stay partly 205system-specific. Also, wakeup-enabled devices will usually stay partly
167functional in order to wake the system. 206functional in order to wake the system.
168 207
169When the system leaves that low power state, the device's driver is asked 208When the system leaves that low-power state, the device's driver is asked to
170to resume it. The suspend and resume operations always go together, and 209resume it by returning it to full power. The suspend and resume operations
171both are multi-phase operations. 210always go together, and both are multi-phase operations.
172 211
173For simple drivers, suspend might quiesce the device using the class code 212For simple drivers, suspend might quiesce the device using class code
174and then turn its hardware as "off" as possible with late_suspend. The 213and then turn its hardware as "off" as possible during suspend_noirq. The
175matching resume calls would then completely reinitialize the hardware 214matching resume calls would then completely reinitialize the hardware
176before reactivating its class I/O queues. 215before reactivating its class I/O queues.
177 216
178More power-aware drivers drivers will use more than one device low power 217More power-aware drivers might prepare the devices for triggering system wakeup
179state, either at runtime or during system sleep states, and might trigger 218events.
180system wakeup events.
181 219
182 220
183Call Sequence Guarantees 221Call Sequence Guarantees
184------------------------ 222------------------------
185To ensure that bridges and similar links needed to talk to a device are 223To ensure that bridges and similar links needing to talk to a device are
186available when the device is suspended or resumed, the device tree is 224available when the device is suspended or resumed, the device tree is
187walked in a bottom-up order to suspend devices. A top-down order is 225walked in a bottom-up order to suspend devices. A top-down order is
188used to resume those devices. 226used to resume those devices.
@@ -194,67 +232,310 @@ its parent; and can't be removed or suspended after that parent.
194The policy is that the device tree should match hardware bus topology. 232The policy is that the device tree should match hardware bus topology.
195(Or at least the control bus, for devices which use multiple busses.) 233(Or at least the control bus, for devices which use multiple busses.)
196In particular, this means that a device registration may fail if the parent of 234In particular, this means that a device registration may fail if the parent of
197the device is suspending (ie. has been chosen by the PM core as the next 235the device is suspending (i.e. has been chosen by the PM core as the next
198device to suspend) or has already suspended, as well as after all of the other 236device to suspend) or has already suspended, as well as after all of the other
199devices have been suspended. Device drivers must be prepared to cope with such 237devices have been suspended. Device drivers must be prepared to cope with such
200situations. 238situations.
201 239
202 240
203Suspending Devices 241System Power Management Phases
204------------------ 242------------------------------
205Suspending a given device is done in several phases. Suspending the 243Suspending or resuming the system is done in several phases. Different phases
206system always includes every phase, executing calls for every device 244are used for standby or memory sleep states ("suspend-to-RAM") and the
207before the next phase begins. Not all busses or classes support all 245hibernation state ("suspend-to-disk"). Each phase involves executing callbacks
208these callbacks; and not all drivers use all the callbacks. 246for every device before the next phase begins. Not all busses or classes
247support all these callbacks and not all drivers use all the callbacks. The
248various phases always run after tasks have been frozen and before they are
249unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have
250been disabled (except for those marked with the IRQ_WAKEUP flag).
209 251
210The phases are seen by driver notifications issued in this order: 252Most phases use bus, type, and class callbacks (that is, methods defined in
253dev->bus->pm, dev->type->pm, and dev->class->pm). The prepare and complete
254phases are exceptions; they use only bus callbacks. When multiple callbacks
255are used in a phase, they are invoked in the order: <class, type, bus> during
256power-down transitions and in the opposite order during power-up transitions.
257For example, during the suspend phase the PM core invokes
211 258
212 1 class.suspend(dev, message) is called after tasks are frozen, for 259 dev->class->pm.suspend(dev);
213 devices associated with a class that has such a method. This 260 dev->type->pm.suspend(dev);
214 method may sleep. 261 dev->bus->pm.suspend(dev);
215 262
216 Since I/O activity usually comes from such higher layers, this is 263before moving on to the next device, whereas during the resume phase the core
217 a good place to quiesce all drivers of a given type (and keep such 264invokes
218 code out of those drivers).
219 265
220 2 bus.suspend(dev, message) is called next. This method may sleep, 266 dev->bus->pm.resume(dev);
221 and is often morphed into a device driver call with bus-specific 267 dev->type->pm.resume(dev);
222 parameters and/or rules. 268 dev->class->pm.resume(dev);
223 269
224 This call should handle parts of device suspend logic that require 270These callbacks may in turn invoke device- or driver-specific methods stored in
225 sleeping. It probably does work to quiesce the device which hasn't 271dev->driver->pm, but they don't have to.
226 been abstracted into class.suspend().
227 272
228The pm_message_t parameter is currently used to refine those semantics
229(described later).
230 273
231At the end of those phases, drivers should normally have stopped all I/O 274Entering System Suspend
232transactions (DMA, IRQs), saved enough state that they can re-initialize 275-----------------------
233or restore previous state (as needed by the hardware), and placed the 276When the system goes into the standby or memory sleep state, the phases are:
234device into a low-power state. On many platforms they will also use 277
235clk_disable() to gate off one or more clock sources; sometimes they will 278 prepare, suspend, suspend_noirq.
236also switch off power supplies, or reduce voltages. Drivers which have 279
237runtime PM support may already have performed some or all of the steps 280 1. The prepare phase is meant to prevent races by preventing new devices
238needed to prepare for the upcoming system sleep state. 281 from being registered; the PM core would never know that all the
282 children of a device had been suspended if new children could be
283 registered at will. (By contrast, devices may be unregistered at any
284 time.) Unlike the other suspend-related phases, during the prepare
285 phase the device tree is traversed top-down.
286
287 The prepare phase uses only a bus callback. After the callback method
288 returns, no new children may be registered below the device. The method
289 may also prepare the device or driver in some way for the upcoming
290 system power transition, but it should not put the device into a
291 low-power state.
292
293 2. The suspend methods should quiesce the device to stop it from performing
294 I/O. They also may save the device registers and put it into the
295 appropriate low-power state, depending on the bus type the device is on,
296 and they may enable wakeup events.
297
298 3. The suspend_noirq phase occurs after IRQ handlers have been disabled,
299 which means that the driver's interrupt handler will not be called while
300 the callback method is running. The methods should save the values of
301 the device's registers that weren't saved previously and finally put the
302 device into the appropriate low-power state.
303
304 The majority of subsystems and device drivers need not implement this
305 callback. However, bus types allowing devices to share interrupt
306 vectors, like PCI, generally need it; otherwise a driver might encounter
307 an error during the suspend phase by fielding a shared interrupt
308 generated by some other device after its own device had been set to low
309 power.
310
311At the end of these phases, drivers should have stopped all I/O transactions
312(DMA, IRQs), saved enough state that they can re-initialize or restore previous
313state (as needed by the hardware), and placed the device into a low-power state.
314On many platforms they will gate off one or more clock sources; sometimes they
315will also switch off power supplies or reduce voltages. (Drivers supporting
316runtime PM may already have performed some or all of these steps.)
317
318If device_may_wakeup(dev) returns true, the device should be prepared for
319generating hardware wakeup signals to trigger a system wakeup event when the
320system is in the sleep state. For example, enable_irq_wake() might identify
321GPIO signals hooked up to a switch or other external hardware, and
322pci_enable_wake() does something similar for the PCI PME signal.
323
324If any of these callbacks returns an error, the system won't enter the desired
325low-power state. Instead the PM core will unwind its actions by resuming all
326the devices that were suspended.
327
328
329Leaving System Suspend
330----------------------
331When resuming from standby or memory sleep, the phases are:
332
333 resume_noirq, resume, complete.
334
335 1. The resume_noirq callback methods should perform any actions needed
336 before the driver's interrupt handlers are invoked. This generally
337 means undoing the actions of the suspend_noirq phase. If the bus type
338 permits devices to share interrupt vectors, like PCI, the method should
339 bring the device and its driver into a state in which the driver can
340 recognize if the device is the source of incoming interrupts, if any,
341 and handle them correctly.
342
343 For example, the PCI bus type's ->pm.resume_noirq() puts the device into
344 the full-power state (D0 in the PCI terminology) and restores the
345 standard configuration registers of the device. Then it calls the
346 device driver's ->pm.resume_noirq() method to perform device-specific
347 actions.
348
349 2. The resume methods should bring the the device back to its operating
350 state, so that it can perform normal I/O. This generally involves
351 undoing the actions of the suspend phase.
352
353 3. The complete phase uses only a bus callback. The method should undo the
354 actions of the prepare phase. Note, however, that new children may be
355 registered below the device as soon as the resume callbacks occur; it's
356 not necessary to wait until the complete phase.
357
358At the end of these phases, drivers should be as functional as they were before
359suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
360gated on. Even if the device was in a low-power state before the system sleep
361because of runtime power management, afterwards it should be back in its
362full-power state. There are multiple reasons why it's best to do this; they are
363discussed in more detail in Documentation/power/runtime_pm.txt.
239 364
240When any driver sees that its device_can_wakeup(dev), it should make sure 365However, the details here may again be platform-specific. For example,
241to use the relevant hardware signals to trigger a system wakeup event. 366some systems support multiple "run" states, and the mode in effect at
242For example, enable_irq_wake() might identify GPIO signals hooked up to 367the end of resume might not be the one which preceded suspension.
243a switch or other external hardware, and pci_enable_wake() does something 368That means availability of certain clocks or power supplies changed,
244similar for PCI's PME# signal. 369which could easily affect how a driver works.
370
371Drivers need to be able to handle hardware which has been reset since the
372suspend methods were called, for example by complete reinitialization.
373This may be the hardest part, and the one most protected by NDA'd documents
374and chip errata. It's simplest if the hardware state hasn't changed since
375the suspend was carried out, but that can't be guaranteed (in fact, it ususally
376is not the case).
377
378Drivers must also be prepared to notice that the device has been removed
379while the system was powered down, whenever that's physically possible.
380PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
381where common Linux platforms will see such removal. Details of how drivers
382will notice and handle such removals are currently bus-specific, and often
383involve a separate thread.
384
385These callbacks may return an error value, but the PM core will ignore such
386errors since there's nothing it can do about them other than printing them in
387the system log.
388
389
390Entering Hibernation
391--------------------
392Hibernating the system is more complicated than putting it into the standby or
393memory sleep state, because it involves creating and saving a system image.
394Therefore there are more phases for hibernation, with a different set of
395callbacks. These phases always run after tasks have been frozen and memory has
396been freed.
397
398The general procedure for hibernation is to quiesce all devices (freeze), create
399an image of the system memory while everything is stable, reactivate all
400devices (thaw), write the image to permanent storage, and finally shut down the
401system (poweroff). The phases used to accomplish this are:
402
403 prepare, freeze, freeze_noirq, thaw_noirq, thaw, complete,
404 prepare, poweroff, poweroff_noirq
405
406 1. The prepare phase is discussed in the "Entering System Suspend" section
407 above.
408
409 2. The freeze methods should quiesce the device so that it doesn't generate
410 IRQs or DMA, and they may need to save the values of device registers.
411 However the device does not have to be put in a low-power state, and to
412 save time it's best not to do so. Also, the device should not be
413 prepared to generate wakeup events.
414
415 3. The freeze_noirq phase is analogous to the suspend_noirq phase discussed
416 above, except again that the device should not be put in a low-power
417 state and should not be allowed to generate wakeup events.
418
419At this point the system image is created. All devices should be inactive and
420the contents of memory should remain undisturbed while this happens, so that the
421image forms an atomic snapshot of the system state.
422
423 4. The thaw_noirq phase is analogous to the resume_noirq phase discussed
424 above. The main difference is that its methods can assume the device is
425 in the same state as at the end of the freeze_noirq phase.
426
427 5. The thaw phase is analogous to the resume phase discussed above. Its
428 methods should bring the device back to an operating state, so that it
429 can be used for saving the image if necessary.
430
431 6. The complete phase is discussed in the "Leaving System Suspend" section
432 above.
433
434At this point the system image is saved, and the devices then need to be
435prepared for the upcoming system shutdown. This is much like suspending them
436before putting the system into the standby or memory sleep state, and the phases
437are similar.
438
439 7. The prepare phase is discussed above.
440
441 8. The poweroff phase is analogous to the suspend phase.
442
443 9. The poweroff_noirq phase is analogous to the suspend_noirq phase.
444
445The poweroff and poweroff_noirq callbacks should do essentially the same things
446as the suspend and suspend_noirq callbacks. The only notable difference is that
447they need not store the device register values, because the registers should
448already have been stored during the freeze or freeze_noirq phases.
449
450
451Leaving Hibernation
452-------------------
453Resuming from hibernation is, again, more complicated than resuming from a sleep
454state in which the contents of main memory are preserved, because it requires
455a system image to be loaded into memory and the pre-hibernation memory contents
456to be restored before control can be passed back to the image kernel.
457
458Although in principle, the image might be loaded into memory and the
459pre-hibernation memory contents restored by the boot loader, in practice this
460can't be done because boot loaders aren't smart enough and there is no
461established protocol for passing the necessary information. So instead, the
462boot loader loads a fresh instance of the kernel, called the boot kernel, into
463memory and passes control to it in the usual way. Then the boot kernel reads
464the system image, restores the pre-hibernation memory contents, and passes
465control to the image kernel. Thus two different kernels are involved in
466resuming from hibernation. In fact, the boot kernel may be completely different
467from the image kernel: a different configuration and even a different version.
468This has important consequences for device drivers and their subsystems.
469
470To be able to load the system image into memory, the boot kernel needs to
471include at least a subset of device drivers allowing it to access the storage
472medium containing the image, although it doesn't need to include all of the
473drivers present in the image kernel. After the image has been loaded, the
474devices managed by the boot kernel need to be prepared for passing control back
475to the image kernel. This is very similar to the initial steps involved in
476creating a system image, and it is accomplished in the same way, using prepare,
477freeze, and freeze_noirq phases. However the devices affected by these phases
478are only those having drivers in the boot kernel; other devices will still be in
479whatever state the boot loader left them.
480
481Should the restoration of the pre-hibernation memory contents fail, the boot
482kernel would go through the "thawing" procedure described above, using the
483thaw_noirq, thaw, and complete phases, and then continue running normally. This
484happens only rarely. Most often the pre-hibernation memory contents are
485restored successfully and control is passed to the image kernel, which then
486becomes responsible for bringing the system back to the working state.
487
488To achieve this, the image kernel must restore the devices' pre-hibernation
489functionality. The operation is much like waking up from the memory sleep
490state, although it involves different phases:
491
492 restore_noirq, restore, complete
493
494 1. The restore_noirq phase is analogous to the resume_noirq phase.
495
496 2. The restore phase is analogous to the resume phase.
497
498 3. The complete phase is discussed above.
499
500The main difference from resume[_noirq] is that restore[_noirq] must assume the
501device has been accessed and reconfigured by the boot loader or the boot kernel.
502Consequently the state of the device may be different from the state remembered
503from the freeze and freeze_noirq phases. The device may even need to be reset
504and completely re-initialized. In many cases this difference doesn't matter, so
505the resume[_noirq] and restore[_norq] method pointers can be set to the same
506routines. Nevertheless, different callback pointers are used in case there is a
507situation where it actually matters.
245 508
246If a driver (or bus, or class) fails it suspend method, the system won't
247enter the desired low power state; it will resume all the devices it's
248suspended so far.
249 509
250Note that drivers may need to perform different actions based on the target 510System Devices
251system lowpower/sleep state. At this writing, there are only platform 511--------------
252specific APIs through which drivers could determine those target states. 512System devices (sysdevs) follow a slightly different API, which can be found in
513
514 include/linux/sysdev.h
515 drivers/base/sys.c
516
517System devices will be suspended with interrupts disabled, and after all other
518devices have been suspended. On resume, they will be resumed before any other
519devices, and also with interrupts disabled. These things occur in special
520"sysdev_driver" phases, which affect only system devices.
521
522Thus, after the suspend_noirq (or freeze_noirq or poweroff_noirq) phase, when
523the non-boot CPUs are all offline and IRQs are disabled on the remaining online
524CPU, then a sysdev_driver.suspend phase is carried out, and the system enters a
525sleep state (or a system image is created). During resume (or after the image
526has been created or loaded) a sysdev_driver.resume phase is carried out, IRQs
527are enabled on the only online CPU, the non-boot CPUs are enabled, and the
528resume_noirq (or thaw_noirq or restore_noirq) phase begins.
529
530Code to actually enter and exit the system-wide low power state sometimes
531involves hardware details that are only known to the boot firmware, and
532may leave a CPU running software (from SRAM or flash memory) that monitors
533the system and manages its wakeup sequence.
253 534
254 535
255Device Low Power (suspend) States 536Device Low Power (suspend) States
256--------------------------------- 537---------------------------------
257Device low-power states aren't very standard. One device might only handle 538Device low-power states aren't standard. One device might only handle
258"on" and "off, while another might support a dozen different versions of 539"on" and "off, while another might support a dozen different versions of
259"on" (how many engines are active?), plus a state that gets back to "on" 540"on" (how many engines are active?), plus a state that gets back to "on"
260faster than from a full "off". 541faster than from a full "off".
@@ -265,7 +546,7 @@ PCI device may not perform DMA or issue IRQs, and any wakeup events it
265issues would be issued through the PME# bus signal. Plus, there are 546issues would be issued through the PME# bus signal. Plus, there are
266several PCI-standard device states, some of which are optional. 547several PCI-standard device states, some of which are optional.
267 548
268In contrast, integrated system-on-chip processors often use irqs as the 549In contrast, integrated system-on-chip processors often use IRQs as the
269wakeup event sources (so drivers would call enable_irq_wake) and might 550wakeup event sources (so drivers would call enable_irq_wake) and might
270be able to treat DMA completion as a wakeup event (sometimes DMA can stay 551be able to treat DMA completion as a wakeup event (sometimes DMA can stay
271active too, it'd only be the CPU and some peripherals that sleep). 552active too, it'd only be the CPU and some peripherals that sleep).
@@ -284,120 +565,17 @@ ways; the aforementioned LCD might be active in one product's "standby",
284but a different product using the same SOC might work differently. 565but a different product using the same SOC might work differently.
285 566
286 567
287Meaning of pm_message_t.event 568Power Management Notifiers
288----------------------------- 569--------------------------
289Parameters to suspend calls include the device affected and a message of 570There are some operations that cannot be carried out by the power management
290type pm_message_t, which has one field: the event. If driver does not 571callbacks discussed above, because the callbacks occur too late or too early.
291recognize the event code, suspend calls may abort the request and return 572To handle these cases, subsystems and device drivers may register power
292a negative errno. However, most drivers will be fine if they implement 573management notifiers that are called before tasks are frozen and after they have
293PM_EVENT_SUSPEND semantics for all messages. 574been thawed. Generally speaking, the PM notifiers are suitable for performing
575actions that either require user space to be available, or at least won't
576interfere with user space.
294 577
295The event codes are used to refine the goal of suspending the device, and 578For details refer to Documentation/power/notifiers.txt.
296mostly matter when creating or resuming system memory image snapshots, as
297used with suspend-to-disk:
298
299 PM_EVENT_SUSPEND -- quiesce the driver and put hardware into a low-power
300 state. When used with system sleep states like "suspend-to-RAM" or
301 "standby", the upcoming resume() call will often be able to rely on
302 state kept in hardware, or issue system wakeup events.
303
304 PM_EVENT_HIBERNATE -- Put hardware into a low-power state and enable wakeup
305 events as appropriate. It is only used with hibernation
306 (suspend-to-disk) and few devices are able to wake up the system from
307 this state; most are completely powered off.
308
309 PM_EVENT_FREEZE -- quiesce the driver, but don't necessarily change into
310 any low power mode. A system snapshot is about to be taken, often
311 followed by a call to the driver's resume() method. Neither wakeup
312 events nor DMA are allowed.
313
314 PM_EVENT_PRETHAW -- quiesce the driver, knowing that the upcoming resume()
315 will restore a suspend-to-disk snapshot from a different kernel image.
316 Drivers that are smart enough to look at their hardware state during
317 resume() processing need that state to be correct ... a PRETHAW could
318 be used to invalidate that state (by resetting the device), like a
319 shutdown() invocation would before a kexec() or system halt. Other
320 drivers might handle this the same way as PM_EVENT_FREEZE. Neither
321 wakeup events nor DMA are allowed.
322
323To enter "standby" (ACPI S1) or "Suspend to RAM" (STR, ACPI S3) states, or
324the similarly named APM states, only PM_EVENT_SUSPEND is used; the other event
325codes are used for hibernation ("Suspend to Disk", STD, ACPI S4).
326
327There's also PM_EVENT_ON, a value which never appears as a suspend event
328but is sometimes used to record the "not suspended" device state.
329
330
331Resuming Devices
332----------------
333Resuming is done in multiple phases, much like suspending, with all
334devices processing each phase's calls before the next phase begins.
335
336The phases are seen by driver notifications issued in this order:
337
338 1 bus.resume(dev) reverses the effects of bus.suspend(). This may
339 be morphed into a device driver call with bus-specific parameters;
340 implementations may sleep.
341
342 2 class.resume(dev) is called for devices associated with a class
343 that has such a method. Implementations may sleep.
344
345 This reverses the effects of class.suspend(), and would usually
346 reactivate the device's I/O queue.
347
348At the end of those phases, drivers should normally be as functional as
349they were before suspending: I/O can be performed using DMA and IRQs, and
350the relevant clocks are gated on. The device need not be "fully on"; it
351might be in a runtime lowpower/suspend state that acts as if it were.
352
353However, the details here may again be platform-specific. For example,
354some systems support multiple "run" states, and the mode in effect at
355the end of resume() might not be the one which preceded suspension.
356That means availability of certain clocks or power supplies changed,
357which could easily affect how a driver works.
358
359
360Drivers need to be able to handle hardware which has been reset since the
361suspend methods were called, for example by complete reinitialization.
362This may be the hardest part, and the one most protected by NDA'd documents
363and chip errata. It's simplest if the hardware state hasn't changed since
364the suspend() was called, but that can't always be guaranteed.
365
366Drivers must also be prepared to notice that the device has been removed
367while the system was powered off, whenever that's physically possible.
368PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
369where common Linux platforms will see such removal. Details of how drivers
370will notice and handle such removals are currently bus-specific, and often
371involve a separate thread.
372
373
374Note that the bus-specific runtime PM wakeup mechanism can exist, and might
375be defined to share some of the same driver code as for system wakeup. For
376example, a bus-specific device driver's resume() method might be used there,
377so it wouldn't only be called from bus.resume() during system-wide wakeup.
378See bus-specific information about how runtime wakeup events are handled.
379
380
381System Devices
382--------------
383System devices follow a slightly different API, which can be found in
384
385 include/linux/sysdev.h
386 drivers/base/sys.c
387
388System devices will only be suspended with interrupts disabled, and after
389all other devices have been suspended. On resume, they will be resumed
390before any other devices, and also with interrupts disabled.
391
392That is, IRQs are disabled, the suspend_late() phase begins, then the
393sysdev_driver.suspend() phase, and the system enters a sleep state. Then
394the sysdev_driver.resume() phase begins, followed by the resume_early()
395phase, after which IRQs are enabled.
396
397Code to actually enter and exit the system-wide low power state sometimes
398involves hardware details that are only known to the boot firmware, and
399may leave a CPU running software (from SRAM or flash memory) that monitors
400the system and manages its wakeup sequence.
401 579
402 580
403Runtime Power Management 581Runtime Power Management
@@ -407,82 +585,23 @@ running. This feature is useful for devices that are not being used, and
407can offer significant power savings on a running system. These devices 585can offer significant power savings on a running system. These devices
408often support a range of runtime power states, which might use names such 586often support a range of runtime power states, which might use names such
409as "off", "sleep", "idle", "active", and so on. Those states will in some 587as "off", "sleep", "idle", "active", and so on. Those states will in some
410cases (like PCI) be partially constrained by a bus the device uses, and will 588cases (like PCI) be partially constrained by the bus the device uses, and will
411usually include hardware states that are also used in system sleep states. 589usually include hardware states that are also used in system sleep states.
412 590
413However, note that if a driver puts a device into a runtime low power state 591A system-wide power transition can be started while some devices are in low
414and the system then goes into a system-wide sleep state, it normally ought 592power states due to runtime power management. The system sleep PM callbacks
415to resume into that runtime low power state rather than "full on". Such 593should recognize such situations and react to them appropriately, but the
416distinctions would be part of the driver-internal state machine for that 594necessary actions are subsystem-specific.
417hardware; the whole point of runtime power management is to be sure that 595
418drivers are decoupled in that way from the state machine governing phases 596In some cases the decision may be made at the subsystem level while in other
419of the system-wide power/sleep state transitions. 597cases the device driver may be left to decide. In some cases it may be
420 598desirable to leave a suspended device in that state during a system-wide power
421 599transition, but in other cases the device must be put back into the full-power
422Power Saving Techniques 600state temporarily, for example so that its system wakeup capability can be
423----------------------- 601disabled. This all depends on the hardware and the design of the subsystem and
424Normally runtime power management is handled by the drivers without specific 602device driver in question.
425userspace or kernel intervention, by device-aware use of techniques like: 603
426 604During system-wide resume from a sleep state it's best to put devices into the
427 Using information provided by other system layers 605full-power state, as explained in Documentation/power/runtime_pm.txt. Refer to
428 - stay deeply "off" except between open() and close() 606that document for more information regarding this particular issue as well as
429 - if transceiver/PHY indicates "nobody connected", stay "off" 607for information on the device runtime power management framework in general.
430 - application protocols may include power commands or hints
431
432 Using fewer CPU cycles
433 - using DMA instead of PIO
434 - removing timers, or making them lower frequency
435 - shortening "hot" code paths
436 - eliminating cache misses
437 - (sometimes) offloading work to device firmware
438
439 Reducing other resource costs
440 - gating off unused clocks in software (or hardware)
441 - switching off unused power supplies
442 - eliminating (or delaying/merging) IRQs
443 - tuning DMA to use word and/or burst modes
444
445 Using device-specific low power states
446 - using lower voltages
447 - avoiding needless DMA transfers
448
449Read your hardware documentation carefully to see the opportunities that
450may be available. If you can, measure the actual power usage and check
451it against the budget established for your project.
452
453
454Examples: USB hosts, system timer, system CPU
455----------------------------------------------
456USB host controllers make interesting, if complex, examples. In many cases
457these have no work to do: no USB devices are connected, or all of them are
458in the USB "suspend" state. Linux host controller drivers can then disable
459periodic DMA transfers that would otherwise be a constant power drain on the
460memory subsystem, and enter a suspend state. In power-aware controllers,
461entering that suspend state may disable the clock used with USB signaling,
462saving a certain amount of power.
463
464The controller will be woken from that state (with an IRQ) by changes to the
465signal state on the data lines of a given port, for example by an existing
466peripheral requesting "remote wakeup" or by plugging a new peripheral. The
467same wakeup mechanism usually works from "standby" sleep states, and on some
468systems also from "suspend to RAM" (or even "suspend to disk") states.
469(Except that ACPI may be involved instead of normal IRQs, on some hardware.)
470
471System devices like timers and CPUs may have special roles in the platform
472power management scheme. For example, system timers using a "dynamic tick"
473approach don't just save CPU cycles (by eliminating needless timer IRQs),
474but they may also open the door to using lower power CPU "idle" states that
475cost more than a jiffie to enter and exit. On x86 systems these are states
476like "C3"; note that periodic DMA transfers from a USB host controller will
477also prevent entry to a C3 state, much like a periodic timer IRQ.
478
479That kind of runtime mechanism interaction is common. "System On Chip" (SOC)
480processors often have low power idle modes that can't be entered unless
481certain medium-speed clocks (often 12 or 48 MHz) are gated off. When the
482drivers gate those clocks effectively, then the system idle task may be able
483to use the lower power idle modes and thereby increase battery life.
484
485If the CPU can have a "cpufreq" driver, there also may be opportunities
486to shift to lower voltage settings and reduce the power cost of executing
487a given number of instructions. (Without voltage adjustment, it's rare
488for cpufreq to save much power; the cost-per-instruction must go down.)