1Device Power Management 2 3Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. 4Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu> 5 6 7Most of the code in Linux is device drivers, so most of the Linux power 8management (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. 11 12This 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. 16 17 18Two 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: 24 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"). 28 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 35 leave the low-power state. This feature may be enabled or disabled 36 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 39 system enter low-power states more often. 40 41 Runtime Power Management model: 42 Devices may also be put into low-power states while the system is 43 running, independently of other power management activity in principle. 44 However, devices are not generally independent of each other (for 45 example, a parent device cannot be suspended unless all of its child 46 devices have been suspended). Moreover, depending on the bus type the 47 device is on, it may be necessary to carry out some bus-specific 48 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). 51 52 For these reasons not only the device driver itself, but also the 53 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. 58 59There'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. 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. 70 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). 74 75 76Interfaces for Entering System Sleep States 77=========================================== 78There 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. 82 83 84Device 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: 89 90struct 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); 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); 108}; 109 110This structure is defined in include/linux/pm.h and the methods included in it 111are also described in that file. Their roles will be explained in what follows. 112For now, it should be sufficient to remember that the last three methods are 113specific to runtime power management while the remaining ones are used during 114system-wide power transitions. 115 116There also is a deprecated "old" or "legacy" interface for power management 117operations available at least for some subsystems. This approach does not use 118struct dev_pm_ops objects and it is suitable only for implementing system sleep 119power management methods. Therefore it is not described in this document, so 120please refer directly to the source code for more information about it. 121 122 123Subsystem-Level Methods 124----------------------- 125The core methods to suspend and resume devices reside in struct dev_pm_ops 126pointed to by the ops member of struct dev_pm_domain, or by the pm member of 127struct bus_type, struct device_type and struct class. They are mostly of 128interest to the people writing infrastructure for platforms and buses, like PCI 129or USB, or device type and device class drivers. They also are relevant to the 130writers of device drivers whose subsystems (PM domains, device types, device 131classes and bus types) don't provide all power management methods. 132 133Bus drivers implement these methods as appropriate for the hardware and the 134drivers using it; PCI works differently from USB, and so on. Not many people 135write subsystem-level drivers; most driver code is a "device driver" that builds 136on top of bus-specific framework code. 137 138For more information on these driver calls, see the description later; 139they are called in phases for every device, respecting the parent-child 140sequencing in the driver model tree. 141 142 143/sys/devices/.../power/wakeup files 144----------------------------------- 145All device objects in the driver model contain fields that control the handling 146of system wakeup events (hardware signals that can force the system out of a 147sleep state). These fields are initialized by bus or device driver code using 148device_set_wakeup_capable() and device_set_wakeup_enable(), defined in 149include/linux/pm_wakeup.h. 150 151The "power.can_wakeup" flag just records whether the device (and its driver) can 152physically support wakeup events. The device_set_wakeup_capable() routine 153affects this flag. The "power.wakeup" field is a pointer to an object of type 154struct wakeup_source used for controlling whether or not the device should use 155its system wakeup mechanism and for notifying the PM core of system wakeup 156events signaled by the device. This object is only present for wakeup-capable 157devices (i.e. devices whose "can_wakeup" flags are set) and is created (or 158removed) by device_set_wakeup_capable(). 159 160Whether or not a device is capable of issuing wakeup events is a hardware 161matter, and the kernel is responsible for keeping track of it. By contrast, 162whether or not a wakeup-capable device should issue wakeup events is a policy 163decision, and it is managed by user space through a sysfs attribute: the 164"power/wakeup" file. User space can write the strings "enabled" or "disabled" 165to it to indicate whether or not, respectively, the device is supposed to signal 166system wakeup. This file is only present if the "power.wakeup" object exists 167for the given device and is created (or removed) along with that object, by 168device_set_wakeup_capable(). Reads from the file will return the corresponding 169string. 170 171The "power/wakeup" file is supposed to contain the "disabled" string initially 172for the majority of devices; the major exceptions are power buttons, keyboards, 173and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with 174ethtool. It should also default to "enabled" for devices that don't generate 175wakeup requests on their own but merely forward wakeup requests from one bus to 176another (like PCI Express ports). 177 178The device_may_wakeup() routine returns true only if the "power.wakeup" object 179exists and the corresponding "power/wakeup" file contains the string "enabled". 180This information is used by subsystems, like the PCI bus type code, to see 181whether or not to enable the devices' wakeup mechanisms. If device wakeup 182mechanisms are enabled or disabled directly by drivers, they also should use 183device_may_wakeup() to decide what to do during a system sleep transition. 184Device drivers, however, are not supposed to call device_set_wakeup_enable() 185directly in any case. 186 187It ought to be noted that system wakeup is conceptually different from "remote 188wakeup" used by runtime power management, although it may be supported by the 189same physical mechanism. Remote wakeup is a feature allowing devices in 190low-power states to trigger specific interrupts to signal conditions in which 191they should be put into the full-power state. Those interrupts may or may not 192be used to signal system wakeup events, depending on the hardware design. On 193some systems it is impossible to trigger them from system sleep states. In any 194case, remote wakeup should always be enabled for runtime power management for 195all devices and drivers that support it. 196 197/sys/devices/.../power/control files 198------------------------------------ 199Each device in the driver model has a flag to control whether it is subject to 200runtime power management. This flag, called runtime_auto, is initialized by the 201bus type (or generally subsystem) code using pm_runtime_allow() or 202pm_runtime_forbid(); the default is to allow runtime power management. 203 204The setting can be adjusted by user space by writing either "on" or "auto" to 205the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(), 206setting the flag and allowing the device to be runtime power-managed by its 207driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning 208the device to full power if it was in a low-power state, and preventing the 209device from being runtime power-managed. User space can check the current value 210of the runtime_auto flag by reading the file. 211 212The device's runtime_auto flag has no effect on the handling of system-wide 213power transitions. In particular, the device can (and in the majority of cases 214should and will) be put into a low-power state during a system-wide transition 215to a sleep state even though its runtime_auto flag is clear. 216 217For more information about the runtime power management framework, refer to 218Documentation/power/runtime_pm.txt. 219 220 221Calling Drivers to Enter and Leave System Sleep States 222====================================================== 223When the system goes into a sleep state, each device's driver is asked to 224suspend the device by putting it into a state compatible with the target 225system state. That's usually some version of "off", but the details are 226system-specific. Also, wakeup-enabled devices will usually stay partly 227functional in order to wake the system. 228 229When the system leaves that low-power state, the device's driver is asked to 230resume it by returning it to full power. The suspend and resume operations 231always go together, and both are multi-phase operations. 232 233For simple drivers, suspend might quiesce the device using class code 234and then turn its hardware as "off" as possible during suspend_noirq. The 235matching resume calls would then completely reinitialize the hardware 236before reactivating its class I/O queues. 237 238More power-aware drivers might prepare the devices for triggering system wakeup 239events. 240 241 242Call Sequence Guarantees 243------------------------ 244To ensure that bridges and similar links needing to talk to a device are 245available when the device is suspended or resumed, the device tree is 246walked in a bottom-up order to suspend devices. A top-down order is 247used to resume those devices. 248 249The ordering of the device tree is defined by the order in which devices 250get registered: a child can never be registered, probed or resumed before 251its parent; and can't be removed or suspended after that parent. 252 253The policy is that the device tree should match hardware bus topology. 254(Or at least the control bus, for devices which use multiple busses.) 255In particular, this means that a device registration may fail if the parent of 256the device is suspending (i.e. has been chosen by the PM core as the next 257device to suspend) or has already suspended, as well as after all of the other 258devices have been suspended. Device drivers must be prepared to cope with such 259situations. 260 261 262System Power Management Phases 263------------------------------ 264Suspending or resuming the system is done in several phases. Different phases 265are used for standby or memory sleep states ("suspend-to-RAM") and the 266hibernation state ("suspend-to-disk"). Each phase involves executing callbacks 267for every device before the next phase begins. Not all busses or classes 268support all these callbacks and not all drivers use all the callbacks. The 269various phases always run after tasks have been frozen and before they are 270unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have 271been disabled (except for those marked with the IRQF_NO_SUSPEND flag). 272 273All phases use PM domain, bus, type, class or driver callbacks (that is, methods 274defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or 275dev->driver->pm). These callbacks are regarded by the PM core as mutually 276exclusive. Moreover, PM domain callbacks always take precedence over all of the 277other callbacks and, for example, type callbacks take precedence over bus, class 278and driver callbacks. To be precise, the following rules are used to determine 279which callback to execute in the given phase: 280 281 1. If dev->pm_domain is present, the PM core will choose the callback 282 included in dev->pm_domain->ops for execution 283 284 2. Otherwise, if both dev->type and dev->type->pm are present, the callback 285 included in dev->type->pm will be chosen for execution. 286 287 3. Otherwise, if both dev->class and dev->class->pm are present, the 288 callback included in dev->class->pm will be chosen for execution. 289 290 4. Otherwise, if both dev->bus and dev->bus->pm are present, the callback 291 included in dev->bus->pm will be chosen for execution. 292 293This allows PM domains and device types to override callbacks provided by bus 294types or device classes if necessary. 295 296The PM domain, type, class and bus callbacks may in turn invoke device- or 297driver-specific methods stored in dev->driver->pm, but they don't have to do 298that. 299 300If the subsystem callback chosen for execution is not present, the PM core will 301execute the corresponding method from dev->driver->pm instead if there is one. 302 303 304Entering System Suspend 305----------------------- 306When the system goes into the standby or memory sleep state, the phases are: 307 308 prepare, suspend, suspend_noirq. 309 310 1. The prepare phase is meant to prevent races by preventing new devices 311 from being registered; the PM core would never know that all the 312 children of a device had been suspended if new children could be 313 registered at will. (By contrast, devices may be unregistered at any 314 time.) Unlike the other suspend-related phases, during the prepare 315 phase the device tree is traversed top-down. 316 317 After the prepare callback method returns, no new children may be 318 registered below the device. The method may also prepare the device or 319 driver in some way for the upcoming system power transition, but it 320 should not put the device into a low-power state. 321 322 2. The suspend methods should quiesce the device to stop it from performing 323 I/O. They also may save the device registers and put it into the 324 appropriate low-power state, depending on the bus type the device is on, 325 and they may enable wakeup events. 326 327 3. The suspend_noirq phase occurs after IRQ handlers have been disabled, 328 which means that the driver's interrupt handler will not be called while 329 the callback method is running. The methods should save the values of 330 the device's registers that weren't saved previously and finally put the 331 device into the appropriate low-power state. 332 333 The majority of subsystems and device drivers need not implement this 334 callback. However, bus types allowing devices to share interrupt 335 vectors, like PCI, generally need it; otherwise a driver might encounter 336 an error during the suspend phase by fielding a shared interrupt 337 generated by some other device after its own device had been set to low 338 power. 339 340At the end of these phases, drivers should have stopped all I/O transactions 341(DMA, IRQs), saved enough state that they can re-initialize or restore previous 342state (as needed by the hardware), and placed the device into a low-power state. 343On many platforms they will gate off one or more clock sources; sometimes they 344will also switch off power supplies or reduce voltages. (Drivers supporting 345runtime PM may already have performed some or all of these steps.) 346 347If device_may_wakeup(dev) returns true, the device should be prepared for 348generating hardware wakeup signals to trigger a system wakeup event when the 349system is in the sleep state. For example, enable_irq_wake() might identify 350GPIO signals hooked up to a switch or other external hardware, and 351pci_enable_wake() does something similar for the PCI PME signal. 352 353If any of these callbacks returns an error, the system won't enter the desired 354low-power state. Instead the PM core will unwind its actions by resuming all 355the devices that were suspended. 356 357 358Leaving System Suspend 359---------------------- 360When resuming from standby or memory sleep, the phases are: 361 362 resume_noirq, resume, complete. 363 364 1. The resume_noirq callback methods should perform any actions needed 365 before the driver's interrupt handlers are invoked. This generally 366 means undoing the actions of the suspend_noirq phase. If the bus type 367 permits devices to share interrupt vectors, like PCI, the method should 368 bring the device and its driver into a state in which the driver can 369 recognize if the device is the source of incoming interrupts, if any, 370 and handle them correctly. 371 372 For example, the PCI bus type's ->pm.resume_noirq() puts the device into 373 the full-power state (D0 in the PCI terminology) and restores the 374 standard configuration registers of the device. Then it calls the 375 device driver's ->pm.resume_noirq() method to perform device-specific 376 actions. 377 378 2. The resume methods should bring the the device back to its operating 379 state, so that it can perform normal I/O. This generally involves 380 undoing the actions of the suspend phase. 381 382 3. The complete phase uses only a bus callback. The method should undo the 383 actions of the prepare phase. Note, however, that new children may be 384 registered below the device as soon as the resume callbacks occur; it's 385 not necessary to wait until the complete phase. 386 387At the end of these phases, drivers should be as functional as they were before 388suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are 389gated on. Even if the device was in a low-power state before the system sleep 390because of runtime power management, afterwards it should be back in its 391full-power state. There are multiple reasons why it's best to do this; they are 392discussed in more detail in Documentation/power/runtime_pm.txt. 393 394However, the details here may again be platform-specific. For example, 395some systems support multiple "run" states, and the mode in effect at 396the end of resume might not be the one which preceded suspension. 397That means availability of certain clocks or power supplies changed, 398which could easily affect how a driver works. 399 400Drivers need to be able to handle hardware which has been reset since the 401suspend methods were called, for example by complete reinitialization. 402This may be the hardest part, and the one most protected by NDA'd documents 403and chip errata. It's simplest if the hardware state hasn't changed since 404the suspend was carried out, but that can't be guaranteed (in fact, it usually 405is not the case). 406 407Drivers must also be prepared to notice that the device has been removed 408while the system was powered down, whenever that's physically possible. 409PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses 410where common Linux platforms will see such removal. Details of how drivers 411will notice and handle such removals are currently bus-specific, and often 412involve a separate thread. 413 414These callbacks may return an error value, but the PM core will ignore such 415errors since there's nothing it can do about them other than printing them in 416the system log. 417 418 419Entering Hibernation 420-------------------- 421Hibernating the system is more complicated than putting it into the standby or 422memory sleep state, because it involves creating and saving a system image. 423Therefore there are more phases for hibernation, with a different set of 424callbacks. These phases always run after tasks have been frozen and memory has 425been freed. 426 427The general procedure for hibernation is to quiesce all devices (freeze), create 428an image of the system memory while everything is stable, reactivate all 429devices (thaw), write the image to permanent storage, and finally shut down the 430system (poweroff). The phases used to accomplish this are: 431 432 prepare, freeze, freeze_noirq, thaw_noirq, thaw, complete, 433 prepare, poweroff, poweroff_noirq 434 435 1. The prepare phase is discussed in the "Entering System Suspend" section 436 above. 437 438 2. The freeze methods should quiesce the device so that it doesn't generate 439 IRQs or DMA, and they may need to save the values of device registers. 440 However the device does not have to be put in a low-power state, and to 441 save time it's best not to do so. Also, the device should not be 442 prepared to generate wakeup events. 443 444 3. The freeze_noirq phase is analogous to the suspend_noirq phase discussed 445 above, except again that the device should not be put in a low-power 446 state and should not be allowed to generate wakeup events. 447 448At this point the system image is created. All devices should be inactive and 449the contents of memory should remain undisturbed while this happens, so that the 450image forms an atomic snapshot of the system state. 451 452 4. The thaw_noirq phase is analogous to the resume_noirq phase discussed 453 above. The main difference is that its methods can assume the device is 454 in the same state as at the end of the freeze_noirq phase. 455 456 5. The thaw phase is analogous to the resume phase discussed above. Its 457 methods should bring the device back to an operating state, so that it 458 can be used for saving the image if necessary. 459 460 6. The complete phase is discussed in the "Leaving System Suspend" section 461 above. 462 463At this point the system image is saved, and the devices then need to be 464prepared for the upcoming system shutdown. This is much like suspending them 465before putting the system into the standby or memory sleep state, and the phases 466are similar. 467 468 7. The prepare phase is discussed above. 469 470 8. The poweroff phase is analogous to the suspend phase. 471 472 9. The poweroff_noirq phase is analogous to the suspend_noirq phase. 473 474The poweroff and poweroff_noirq callbacks should do essentially the same things 475as the suspend and suspend_noirq callbacks. The only notable difference is that 476they need not store the device register values, because the registers should 477already have been stored during the freeze or freeze_noirq phases. 478 479 480Leaving Hibernation 481------------------- 482Resuming from hibernation is, again, more complicated than resuming from a sleep 483state in which the contents of main memory are preserved, because it requires 484a system image to be loaded into memory and the pre-hibernation memory contents 485to be restored before control can be passed back to the image kernel. 486 487Although in principle, the image might be loaded into memory and the 488pre-hibernation memory contents restored by the boot loader, in practice this 489can't be done because boot loaders aren't smart enough and there is no 490established protocol for passing the necessary information. So instead, the 491boot loader loads a fresh instance of the kernel, called the boot kernel, into 492memory and passes control to it in the usual way. Then the boot kernel reads 493the system image, restores the pre-hibernation memory contents, and passes 494control to the image kernel. Thus two different kernels are involved in 495resuming from hibernation. In fact, the boot kernel may be completely different 496from the image kernel: a different configuration and even a different version. 497This has important consequences for device drivers and their subsystems. 498 499To be able to load the system image into memory, the boot kernel needs to 500include at least a subset of device drivers allowing it to access the storage 501medium containing the image, although it doesn't need to include all of the 502drivers present in the image kernel. After the image has been loaded, the 503devices managed by the boot kernel need to be prepared for passing control back 504to the image kernel. This is very similar to the initial steps involved in 505creating a system image, and it is accomplished in the same way, using prepare, 506freeze, and freeze_noirq phases. However the devices affected by these phases 507are only those having drivers in the boot kernel; other devices will still be in 508whatever state the boot loader left them. 509 510Should the restoration of the pre-hibernation memory contents fail, the boot 511kernel would go through the "thawing" procedure described above, using the 512thaw_noirq, thaw, and complete phases, and then continue running normally. This 513happens only rarely. Most often the pre-hibernation memory contents are 514restored successfully and control is passed to the image kernel, which then 515becomes responsible for bringing the system back to the working state. 516 517To achieve this, the image kernel must restore the devices' pre-hibernation 518functionality. The operation is much like waking up from the memory sleep 519state, although it involves different phases: 520 521 restore_noirq, restore, complete 522 523 1. The restore_noirq phase is analogous to the resume_noirq phase. 524 525 2. The restore phase is analogous to the resume phase. 526 527 3. The complete phase is discussed above. 528 529The main difference from resume[_noirq] is that restore[_noirq] must assume the 530device has been accessed and reconfigured by the boot loader or the boot kernel. 531Consequently the state of the device may be different from the state remembered 532from the freeze and freeze_noirq phases. The device may even need to be reset 533and completely re-initialized. In many cases this difference doesn't matter, so 534the resume[_noirq] and restore[_norq] method pointers can be set to the same 535routines. Nevertheless, different callback pointers are used in case there is a 536situation where it actually matters. 537 538 539Device Power Management Domains 540------------------------------- 541Sometimes devices share reference clocks or other power resources. In those 542cases it generally is not possible to put devices into low-power states 543individually. Instead, a set of devices sharing a power resource can be put 544into a low-power state together at the same time by turning off the shared 545power resource. Of course, they also need to be put into the full-power state 546together, by turning the shared power resource on. A set of devices with this 547property is often referred to as a power domain. 548 549Support for power domains is provided through the pm_domain field of struct 550device. This field is a pointer to an object of type struct dev_pm_domain, 551defined in include/linux/pm.h, providing a set of power management callbacks 552analogous to the subsystem-level and device driver callbacks that are executed 553for the given device during all power transitions, instead of the respective 554subsystem-level callbacks. Specifically, if a device's pm_domain pointer is 555not NULL, the ->suspend() callback from the object pointed to by it will be 556executed instead of its subsystem's (e.g. bus type's) ->suspend() callback and 557anlogously for all of the remaining callbacks. In other words, power management 558domain callbacks, if defined for the given device, always take precedence over 559the callbacks provided by the device's subsystem (e.g. bus type). 560 561The support for device power management domains is only relevant to platforms 562needing to use the same device driver power management callbacks in many 563different power domain configurations and wanting to avoid incorporating the 564support for power domains into subsystem-level callbacks, for example by 565modifying the platform bus type. Other platforms need not implement it or take 566it into account in any way. 567 568 569Device Low Power (suspend) States 570--------------------------------- 571Device low-power states aren't standard. One device might only handle 572"on" and "off, while another might support a dozen different versions of 573"on" (how many engines are active?), plus a state that gets back to "on" 574faster than from a full "off". 575 576Some busses define rules about what different suspend states mean. PCI 577gives one example: after the suspend sequence completes, a non-legacy 578PCI device may not perform DMA or issue IRQs, and any wakeup events it 579issues would be issued through the PME# bus signal. Plus, there are 580several PCI-standard device states, some of which are optional. 581 582In contrast, integrated system-on-chip processors often use IRQs as the 583wakeup event sources (so drivers would call enable_irq_wake) and might 584be able to treat DMA completion as a wakeup event (sometimes DMA can stay 585active too, it'd only be the CPU and some peripherals that sleep). 586 587Some details here may be platform-specific. Systems may have devices that 588can be fully active in certain sleep states, such as an LCD display that's 589refreshed using DMA while most of the system is sleeping lightly ... and 590its frame buffer might even be updated by a DSP or other non-Linux CPU while 591the Linux control processor stays idle. 592 593Moreover, the specific actions taken may depend on the target system state. 594One target system state might allow a given device to be very operational; 595another might require a hard shut down with re-initialization on resume. 596And two different target systems might use the same device in different 597ways; the aforementioned LCD might be active in one product's "standby", 598but a different product using the same SOC might work differently. 599 600 601Power Management Notifiers 602-------------------------- 603There are some operations that cannot be carried out by the power management 604callbacks discussed above, because the callbacks occur too late or too early. 605To handle these cases, subsystems and device drivers may register power 606management notifiers that are called before tasks are frozen and after they have 607been thawed. Generally speaking, the PM notifiers are suitable for performing 608actions that either require user space to be available, or at least won't 609interfere with user space. 610 611For details refer to Documentation/power/notifiers.txt. 612 613 614Runtime Power Management 615======================== 616Many devices are able to dynamically power down while the system is still 617running. This feature is useful for devices that are not being used, and 618can offer significant power savings on a running system. These devices 619often support a range of runtime power states, which might use names such 620as "off", "sleep", "idle", "active", and so on. Those states will in some 621cases (like PCI) be partially constrained by the bus the device uses, and will 622usually include hardware states that are also used in system sleep states. 623 624A system-wide power transition can be started while some devices are in low 625power states due to runtime power management. The system sleep PM callbacks 626should recognize such situations and react to them appropriately, but the 627necessary actions are subsystem-specific. 628 629In some cases the decision may be made at the subsystem level while in other 630cases the device driver may be left to decide. In some cases it may be 631desirable to leave a suspended device in that state during a system-wide power 632transition, but in other cases the device must be put back into the full-power 633state temporarily, for example so that its system wakeup capability can be 634disabled. This all depends on the hardware and the design of the subsystem and 635device driver in question. 636 637During system-wide resume from a sleep state it's easiest to put devices into 638the full-power state, as explained in Documentation/power/runtime_pm.txt. Refer 639to that document for more information regarding this particular issue as well as 640for information on the device runtime power management framework in general. 641