Based on kernel version 3.9. Page generated on 2013-05-02 23:12 EST.
1 Device Power Management 2 3 Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. 4 Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu> 5 6 7 Most of the code in Linux is device drivers, so most of the Linux power 8 management (PM) code is also driver-specific. Most drivers will do very 9 little; others, especially for platforms with small batteries (like cell 10 phones), will do a lot. 11 12 This writeup gives an overview of how drivers interact with system-wide 13 power management goals, emphasizing the models and interfaces that are 14 shared by everything that hooks up to the driver model core. Read it as 15 background for the domain-specific work you'd do with any specific driver. 16 17 18 Two Models for Device Power Management 19 ====================================== 20 Drivers will use one or both of these models to put devices into low-power 21 states: 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 59 There's not a lot to be said about those low-power states except that they are 60 very system-specific, and often device-specific. Also, that if enough devices 61 have been put into low-power states (at runtime), the effect may be very similar 62 to entering some system-wide low-power state (system sleep) ... and that 63 synergies exist, so that several drivers using runtime PM might put the system 64 into a state where even deeper power saving options are available. 65 66 Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except 67 for wakeup events), no more data read or written, and requests from upstream 68 drivers are no longer accepted. A given bus or platform may have different 69 requirements though. 70 71 Examples of hardware wakeup events include an alarm from a real time clock, 72 network wake-on-LAN packets, keyboard or mouse activity, and media insertion 73 or removal (for PCMCIA, MMC/SD, USB, and so on). 74 75 76 Interfaces for Entering System Sleep States 77 =========================================== 78 There are programming interfaces provided for subsystems (bus type, device type, 79 device class) and device drivers to allow them to participate in the power 80 management of devices they are concerned with. These interfaces cover both 81 system sleep and runtime power management. 82 83 84 Device Power Management Operations 85 ---------------------------------- 86 Device power management operations, at the subsystem level as well as at the 87 device driver level, are implemented by defining and populating objects of type 88 struct dev_pm_ops: 89 90 struct 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_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); 105 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); 114 }; 115 116 This structure is defined in include/linux/pm.h and the methods included in it 117 are also described in that file. Their roles will be explained in what follows. 118 For now, it should be sufficient to remember that the last three methods are 119 specific to runtime power management while the remaining ones are used during 120 system-wide power transitions. 121 122 There also is a deprecated "old" or "legacy" interface for power management 123 operations available at least for some subsystems. This approach does not use 124 struct dev_pm_ops objects and it is suitable only for implementing system sleep 125 power management methods. Therefore it is not described in this document, so 126 please refer directly to the source code for more information about it. 127 128 129 Subsystem-Level Methods 130 ----------------------- 131 The core methods to suspend and resume devices reside in struct dev_pm_ops 132 pointed to by the ops member of struct dev_pm_domain, or by the pm member of 133 struct bus_type, struct device_type and struct class. They are mostly of 134 interest to the people writing infrastructure for platforms and buses, like PCI 135 or USB, or device type and device class drivers. They also are relevant to the 136 writers of device drivers whose subsystems (PM domains, device types, device 137 classes and bus types) don't provide all power management methods. 138 139 Bus drivers implement these methods as appropriate for the hardware and the 140 drivers using it; PCI works differently from USB, and so on. Not many people 141 write subsystem-level drivers; most driver code is a "device driver" that builds 142 on top of bus-specific framework code. 143 144 For more information on these driver calls, see the description later; 145 they are called in phases for every device, respecting the parent-child 146 sequencing in the driver model tree. 147 148 149 /sys/devices/.../power/wakeup files 150 ----------------------------------- 151 All device objects in the driver model contain fields that control the handling 152 of system wakeup events (hardware signals that can force the system out of a 153 sleep state). These fields are initialized by bus or device driver code using 154 device_set_wakeup_capable() and device_set_wakeup_enable(), defined in 155 include/linux/pm_wakeup.h. 156 157 The "power.can_wakeup" flag just records whether the device (and its driver) can 158 physically support wakeup events. The device_set_wakeup_capable() routine 159 affects this flag. The "power.wakeup" field is a pointer to an object of type 160 struct wakeup_source used for controlling whether or not the device should use 161 its system wakeup mechanism and for notifying the PM core of system wakeup 162 events signaled by the device. This object is only present for wakeup-capable 163 devices (i.e. devices whose "can_wakeup" flags are set) and is created (or 164 removed) by device_set_wakeup_capable(). 165 166 Whether or not a device is capable of issuing wakeup events is a hardware 167 matter, and the kernel is responsible for keeping track of it. By contrast, 168 whether or not a wakeup-capable device should issue wakeup events is a policy 169 decision, and it is managed by user space through a sysfs attribute: the 170 "power/wakeup" file. User space can write the strings "enabled" or "disabled" 171 to it to indicate whether or not, respectively, the device is supposed to signal 172 system wakeup. This file is only present if the "power.wakeup" object exists 173 for the given device and is created (or removed) along with that object, by 174 device_set_wakeup_capable(). Reads from the file will return the corresponding 175 string. 176 177 The "power/wakeup" file is supposed to contain the "disabled" string initially 178 for the majority of devices; the major exceptions are power buttons, keyboards, 179 and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with 180 ethtool. It should also default to "enabled" for devices that don't generate 181 wakeup requests on their own but merely forward wakeup requests from one bus to 182 another (like PCI Express ports). 183 184 The device_may_wakeup() routine returns true only if the "power.wakeup" object 185 exists and the corresponding "power/wakeup" file contains the string "enabled". 186 This information is used by subsystems, like the PCI bus type code, to see 187 whether or not to enable the devices' wakeup mechanisms. If device wakeup 188 mechanisms are enabled or disabled directly by drivers, they also should use 189 device_may_wakeup() to decide what to do during a system sleep transition. 190 Device drivers, however, are not supposed to call device_set_wakeup_enable() 191 directly in any case. 192 193 It ought to be noted that system wakeup is conceptually different from "remote 194 wakeup" used by runtime power management, although it may be supported by the 195 same physical mechanism. Remote wakeup is a feature allowing devices in 196 low-power states to trigger specific interrupts to signal conditions in which 197 they should be put into the full-power state. Those interrupts may or may not 198 be used to signal system wakeup events, depending on the hardware design. On 199 some systems it is impossible to trigger them from system sleep states. In any 200 case, remote wakeup should always be enabled for runtime power management for 201 all devices and drivers that support it. 202 203 /sys/devices/.../power/control files 204 ------------------------------------ 205 Each device in the driver model has a flag to control whether it is subject to 206 runtime power management. This flag, called runtime_auto, is initialized by the 207 bus type (or generally subsystem) code using pm_runtime_allow() or 208 pm_runtime_forbid(); the default is to allow runtime power management. 209 210 The setting can be adjusted by user space by writing either "on" or "auto" to 211 the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(), 212 setting the flag and allowing the device to be runtime power-managed by its 213 driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning 214 the device to full power if it was in a low-power state, and preventing the 215 device from being runtime power-managed. User space can check the current value 216 of the runtime_auto flag by reading the file. 217 218 The device's runtime_auto flag has no effect on the handling of system-wide 219 power transitions. In particular, the device can (and in the majority of cases 220 should and will) be put into a low-power state during a system-wide transition 221 to a sleep state even though its runtime_auto flag is clear. 222 223 For more information about the runtime power management framework, refer to 224 Documentation/power/runtime_pm.txt. 225 226 227 Calling Drivers to Enter and Leave System Sleep States 228 ====================================================== 229 When the system goes into a sleep state, each device's driver is asked to 230 suspend the device by putting it into a state compatible with the target 231 system state. That's usually some version of "off", but the details are 232 system-specific. Also, wakeup-enabled devices will usually stay partly 233 functional in order to wake the system. 234 235 When the system leaves that low-power state, the device's driver is asked to 236 resume it by returning it to full power. The suspend and resume operations 237 always go together, and both are multi-phase operations. 238 239 For simple drivers, suspend might quiesce the device using class code 240 and then turn its hardware as "off" as possible during suspend_noirq. The 241 matching resume calls would then completely reinitialize the hardware 242 before reactivating its class I/O queues. 243 244 More power-aware drivers might prepare the devices for triggering system wakeup 245 events. 246 247 248 Call Sequence Guarantees 249 ------------------------ 250 To ensure that bridges and similar links needing to talk to a device are 251 available when the device is suspended or resumed, the device tree is 252 walked in a bottom-up order to suspend devices. A top-down order is 253 used to resume those devices. 254 255 The ordering of the device tree is defined by the order in which devices 256 get registered: a child can never be registered, probed or resumed before 257 its parent; and can't be removed or suspended after that parent. 258 259 The policy is that the device tree should match hardware bus topology. 260 (Or at least the control bus, for devices which use multiple busses.) 261 In particular, this means that a device registration may fail if the parent of 262 the device is suspending (i.e. has been chosen by the PM core as the next 263 device to suspend) or has already suspended, as well as after all of the other 264 devices have been suspended. Device drivers must be prepared to cope with such 265 situations. 266 267 268 System Power Management Phases 269 ------------------------------ 270 Suspending or resuming the system is done in several phases. Different phases 271 are used for standby or memory sleep states ("suspend-to-RAM") and the 272 hibernation state ("suspend-to-disk"). Each phase involves executing callbacks 273 for every device before the next phase begins. Not all busses or classes 274 support all these callbacks and not all drivers use all the callbacks. The 275 various phases always run after tasks have been frozen and before they are 276 unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have 277 been disabled (except for those marked with the IRQF_NO_SUSPEND flag). 278 279 All phases use PM domain, bus, type, class or driver callbacks (that is, methods 280 defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or 281 dev->driver->pm). These callbacks are regarded by the PM core as mutually 282 exclusive. Moreover, PM domain callbacks always take precedence over all of the 283 other callbacks and, for example, type callbacks take precedence over bus, class 284 and driver callbacks. To be precise, the following rules are used to determine 285 which callback to execute in the given phase: 286 287 1. If dev->pm_domain is present, the PM core will choose the callback 288 included in dev->pm_domain->ops for execution 289 290 2. Otherwise, if both dev->type and dev->type->pm are present, the callback 291 included in dev->type->pm will be chosen for execution. 292 293 3. Otherwise, if both dev->class and dev->class->pm are present, the 294 callback included in dev->class->pm will be chosen for execution. 295 296 4. Otherwise, if both dev->bus and dev->bus->pm are present, the callback 297 included in dev->bus->pm will be chosen for execution. 298 299 This allows PM domains and device types to override callbacks provided by bus 300 types or device classes if necessary. 301 302 The PM domain, type, class and bus callbacks may in turn invoke device- or 303 driver-specific methods stored in dev->driver->pm, but they don't have to do 304 that. 305 306 If the subsystem callback chosen for execution is not present, the PM core will 307 execute the corresponding method from dev->driver->pm instead if there is one. 308 309 310 Entering System Suspend 311 ----------------------- 312 When the system goes into the standby or memory sleep state, the phases are: 313 314 prepare, suspend, suspend_late, suspend_noirq. 315 316 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. 322 323 After the prepare callback method returns, no new children may be 324 registered below the device. The method may also prepare the device or 325 driver in some way for the upcoming system power transition, but it 326 should not put the device into a low-power state. 327 328 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. 332 333 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, 339 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. 343 344 The majority of subsystems and device drivers need not implement this 345 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. 350 351 At 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 353 state (as needed by the hardware), and placed the device into a low-power state. 354 On many platforms they will gate off one or more clock sources; sometimes they 355 will also switch off power supplies or reduce voltages. (Drivers supporting 356 runtime PM may already have performed some or all of these steps.) 357 358 If device_may_wakeup(dev) returns true, the device should be prepared for 359 generating hardware wakeup signals to trigger a system wakeup event when the 360 system is in the sleep state. For example, enable_irq_wake() might identify 361 GPIO signals hooked up to a switch or other external hardware, and 362 pci_enable_wake() does something similar for the PCI PME signal. 363 364 If any of these callbacks returns an error, the system won't enter the desired 365 low-power state. Instead the PM core will unwind its actions by resuming all 366 the devices that were suspended. 367 368 369 Leaving System Suspend 370 ---------------------- 371 When resuming from standby or memory sleep, the phases are: 372 373 resume_noirq, resume_early, resume, complete. 374 375 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. 388 389 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 394 state, so that it can perform normal I/O. This generally involves 395 undoing the actions of the suspend phase. 396 397 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. 401 402 At the end of these phases, drivers should be as functional as they were before 403 suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are 404 gated on. Even if the device was in a low-power state before the system sleep 405 because of runtime power management, afterwards it should be back in its 406 full-power state. There are multiple reasons why it's best to do this; they are 407 discussed in more detail in Documentation/power/runtime_pm.txt. 408 409 However, the details here may again be platform-specific. For example, 410 some systems support multiple "run" states, and the mode in effect at 411 the end of resume might not be the one which preceded suspension. 412 That means availability of certain clocks or power supplies changed, 413 which could easily affect how a driver works. 414 415 Drivers need to be able to handle hardware which has been reset since the 416 suspend methods were called, for example by complete reinitialization. 417 This may be the hardest part, and the one most protected by NDA'd documents 418 and chip errata. It's simplest if the hardware state hasn't changed since 419 the suspend was carried out, but that can't be guaranteed (in fact, it usually 420 is not the case). 421 422 Drivers must also be prepared to notice that the device has been removed 423 while the system was powered down, whenever that's physically possible. 424 PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses 425 where common Linux platforms will see such removal. Details of how drivers 426 will notice and handle such removals are currently bus-specific, and often 427 involve a separate thread. 428 429 These callbacks may return an error value, but the PM core will ignore such 430 errors since there's nothing it can do about them other than printing them in 431 the system log. 432 433 434 Entering Hibernation 435 -------------------- 436 Hibernating the system is more complicated than putting it into the standby or 437 memory sleep state, because it involves creating and saving a system image. 438 Therefore there are more phases for hibernation, with a different set of 439 callbacks. These phases always run after tasks have been frozen and memory has 440 been freed. 441 442 The general procedure for hibernation is to quiesce all devices (freeze), create 443 an image of the system memory while everything is stable, reactivate all 444 devices (thaw), write the image to permanent storage, and finally shut down the 445 system (poweroff). The phases used to accomplish this are: 446 447 prepare, freeze, freeze_late, freeze_noirq, thaw_noirq, thaw_early, 448 thaw, complete, prepare, poweroff, poweroff_late, poweroff_noirq 449 450 1. The prepare phase is discussed in the "Entering System Suspend" section 451 above. 452 453 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. 458 459 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 464 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. 466 467 At this point the system image is created. All devices should be inactive and 468 the contents of memory should remain undisturbed while this happens, so that the 469 image forms an atomic snapshot of the system state. 470 471 5. The thaw_noirq phase is analogous to the resume_noirq phase discussed 472 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. 474 475 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 480 methods should bring the device back to an operating state, so that it 481 can be used for saving the image if necessary. 482 483 8. The complete phase is discussed in the "Leaving System Suspend" section 484 above. 485 486 At this point the system image is saved, and the devices then need to be 487 prepared for the upcoming system shutdown. This is much like suspending them 488 before putting the system into the standby or memory sleep state, and the phases 489 are similar. 490 491 9. The prepare phase is discussed above. 492 493 10. The poweroff phase is analogous to the suspend phase. 494 495 11. The poweroff_late phase is analogous to the suspend_late phase. 496 497 12. The poweroff_noirq phase is analogous to the suspend_noirq phase. 498 499 The poweroff, poweroff_late and poweroff_noirq callbacks should do essentially 500 the same things as the suspend, suspend_late and suspend_noirq callbacks, 501 respectively. The only notable difference is that they need not store the 502 device register values, because the registers should already have been stored 503 during the freeze, freeze_late or freeze_noirq phases. 504 505 506 Leaving Hibernation 507 ------------------- 508 Resuming from hibernation is, again, more complicated than resuming from a sleep 509 state in which the contents of main memory are preserved, because it requires 510 a system image to be loaded into memory and the pre-hibernation memory contents 511 to be restored before control can be passed back to the image kernel. 512 513 Although in principle, the image might be loaded into memory and the 514 pre-hibernation memory contents restored by the boot loader, in practice this 515 can't be done because boot loaders aren't smart enough and there is no 516 established protocol for passing the necessary information. So instead, the 517 boot loader loads a fresh instance of the kernel, called the boot kernel, into 518 memory and passes control to it in the usual way. Then the boot kernel reads 519 the system image, restores the pre-hibernation memory contents, and passes 520 control to the image kernel. Thus two different kernels are involved in 521 resuming from hibernation. In fact, the boot kernel may be completely different 522 from the image kernel: a different configuration and even a different version. 523 This has important consequences for device drivers and their subsystems. 524 525 To be able to load the system image into memory, the boot kernel needs to 526 include at least a subset of device drivers allowing it to access the storage 527 medium containing the image, although it doesn't need to include all of the 528 drivers present in the image kernel. After the image has been loaded, the 529 devices managed by the boot kernel need to be prepared for passing control back 530 to the image kernel. This is very similar to the initial steps involved in 531 creating a system image, and it is accomplished in the same way, using prepare, 532 freeze, and freeze_noirq phases. However the devices affected by these phases 533 are only those having drivers in the boot kernel; other devices will still be in 534 whatever state the boot loader left them. 535 536 Should the restoration of the pre-hibernation memory contents fail, the boot 537 kernel would go through the "thawing" procedure described above, using the 538 thaw_noirq, thaw, and complete phases, and then continue running normally. This 539 happens only rarely. Most often the pre-hibernation memory contents are 540 restored successfully and control is passed to the image kernel, which then 541 becomes responsible for bringing the system back to the working state. 542 543 To achieve this, the image kernel must restore the devices' pre-hibernation 544 functionality. The operation is much like waking up from the memory sleep 545 state, although it involves different phases: 546 547 restore_noirq, restore_early, restore, complete 548 549 1. The restore_noirq phase is analogous to the resume_noirq phase. 550 551 2. The restore_early phase is analogous to the resume_early phase. 552 553 3. The restore phase is analogous to the resume phase. 554 555 4. The complete phase is discussed above. 556 557 The main difference from resume[_early|_noirq] is that restore[_early|_noirq] 558 must assume the device has been accessed and reconfigured by the boot loader or 559 the boot kernel. Consequently the state of the device may be different from the 560 state remembered from the freeze, freeze_late and freeze_noirq phases. The 561 device may even need to be reset and completely re-initialized. In many cases 562 this difference doesn't matter, so the resume[_early|_noirq] and 563 restore[_early|_norq] method pointers can be set to the same routines. 564 Nevertheless, different callback pointers are used in case there is a situation 565 where it actually does matter. 566 567 568 Device Power Management Domains 569 ------------------------------- 570 Sometimes devices share reference clocks or other power resources. In those 571 cases it generally is not possible to put devices into low-power states 572 individually. Instead, a set of devices sharing a power resource can be put 573 into a low-power state together at the same time by turning off the shared 574 power resource. Of course, they also need to be put into the full-power state 575 together, by turning the shared power resource on. A set of devices with this 576 property is often referred to as a power domain. 577 578 Support for power domains is provided through the pm_domain field of struct 579 device. This field is a pointer to an object of type struct dev_pm_domain, 580 defined in include/linux/pm.h, providing a set of power management callbacks 581 analogous to the subsystem-level and device driver callbacks that are executed 582 for the given device during all power transitions, instead of the respective 583 subsystem-level callbacks. Specifically, if a device's pm_domain pointer is 584 not NULL, the ->suspend() callback from the object pointed to by it will be 585 executed instead of its subsystem's (e.g. bus type's) ->suspend() callback and 586 analogously for all of the remaining callbacks. In other words, power 587 management domain callbacks, if defined for the given device, always take 588 precedence over the callbacks provided by the device's subsystem (e.g. bus 589 type). 590 591 The support for device power management domains is only relevant to platforms 592 needing to use the same device driver power management callbacks in many 593 different power domain configurations and wanting to avoid incorporating the 594 support for power domains into subsystem-level callbacks, for example by 595 modifying the platform bus type. Other platforms need not implement it or take 596 it into account in any way. 597 598 599 Device Low Power (suspend) States 600 --------------------------------- 601 Device low-power states aren't standard. One device might only handle 602 "on" and "off", while another might support a dozen different versions of 603 "on" (how many engines are active?), plus a state that gets back to "on" 604 faster than from a full "off". 605 606 Some busses define rules about what different suspend states mean. PCI 607 gives one example: after the suspend sequence completes, a non-legacy 608 PCI device may not perform DMA or issue IRQs, and any wakeup events it 609 issues would be issued through the PME# bus signal. Plus, there are 610 several PCI-standard device states, some of which are optional. 611 612 In contrast, integrated system-on-chip processors often use IRQs as the 613 wakeup event sources (so drivers would call enable_irq_wake) and might 614 be able to treat DMA completion as a wakeup event (sometimes DMA can stay 615 active too, it'd only be the CPU and some peripherals that sleep). 616 617 Some details here may be platform-specific. Systems may have devices that 618 can be fully active in certain sleep states, such as an LCD display that's 619 refreshed using DMA while most of the system is sleeping lightly ... and 620 its frame buffer might even be updated by a DSP or other non-Linux CPU while 621 the Linux control processor stays idle. 622 623 Moreover, the specific actions taken may depend on the target system state. 624 One target system state might allow a given device to be very operational; 625 another might require a hard shut down with re-initialization on resume. 626 And two different target systems might use the same device in different 627 ways; the aforementioned LCD might be active in one product's "standby", 628 but a different product using the same SOC might work differently. 629 630 631 Power Management Notifiers 632 -------------------------- 633 There are some operations that cannot be carried out by the power management 634 callbacks discussed above, because the callbacks occur too late or too early. 635 To handle these cases, subsystems and device drivers may register power 636 management notifiers that are called before tasks are frozen and after they have 637 been thawed. Generally speaking, the PM notifiers are suitable for performing 638 actions that either require user space to be available, or at least won't 639 interfere with user space. 640 641 For details refer to Documentation/power/notifiers.txt. 642 643 644 Runtime Power Management 645 ======================== 646 Many devices are able to dynamically power down while the system is still 647 running. This feature is useful for devices that are not being used, and 648 can offer significant power savings on a running system. These devices 649 often support a range of runtime power states, which might use names such 650 as "off", "sleep", "idle", "active", and so on. Those states will in some 651 cases (like PCI) be partially constrained by the bus the device uses, and will 652 usually include hardware states that are also used in system sleep states. 653 654 A system-wide power transition can be started while some devices are in low 655 power states due to runtime power management. The system sleep PM callbacks 656 should recognize such situations and react to them appropriately, but the 657 necessary actions are subsystem-specific. 658 659 In some cases the decision may be made at the subsystem level while in other 660 cases the device driver may be left to decide. In some cases it may be 661 desirable to leave a suspended device in that state during a system-wide power 662 transition, but in other cases the device must be put back into the full-power 663 state temporarily, for example so that its system wakeup capability can be 664 disabled. This all depends on the hardware and the design of the subsystem and 665 device driver in question. 666 667 During system-wide resume from a sleep state it's easiest to put devices into 668 the full-power state, as explained in Documentation/power/runtime_pm.txt. Refer 669 to that document for more information regarding this particular issue as well as 670 for information on the device runtime power management framework in general.