1Runtime Power Management Framework for I/O Devices 2 3(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. 4(C) 2010 Alan Stern <stern@rowland.harvard.edu> 5 61. Introduction 7 8Support for runtime power management (runtime PM) of I/O devices is provided 9at the power management core (PM core) level by means of: 10 11* The power management workqueue pm_wq in which bus types and device drivers can 12 put their PM-related work items. It is strongly recommended that pm_wq be 13 used for queuing all work items related to runtime PM, because this allows 14 them to be synchronized with system-wide power transitions (suspend to RAM, 15 hibernation and resume from system sleep states). pm_wq is declared in 16 include/linux/pm_runtime.h and defined in kernel/power/main.c. 17 18* A number of runtime PM fields in the 'power' member of 'struct device' (which 19 is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can 20 be used for synchronizing runtime PM operations with one another. 21 22* Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in 23 include/linux/pm.h). 24 25* A set of helper functions defined in drivers/base/power/runtime.c that can be 26 used for carrying out runtime PM operations in such a way that the 27 synchronization between them is taken care of by the PM core. Bus types and 28 device drivers are encouraged to use these functions. 29 30The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM 31fields of 'struct dev_pm_info' and the core helper functions provided for 32runtime PM are described below. 33 342. Device Runtime PM Callbacks 35 36There are three device runtime PM callbacks defined in 'struct dev_pm_ops': 37 38struct dev_pm_ops { 39 ... 40 int (*runtime_suspend)(struct device *dev); 41 int (*runtime_resume)(struct device *dev); 42 int (*runtime_idle)(struct device *dev); 43 ... 44}; 45 46The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks 47are executed by the PM core for the device's subsystem that may be either of 48the following: 49 50 1. PM domain of the device, if the device's PM domain object, dev->pm_domain, 51 is present. 52 53 2. Device type of the device, if both dev->type and dev->type->pm are present. 54 55 3. Device class of the device, if both dev->class and dev->class->pm are 56 present. 57 58 4. Bus type of the device, if both dev->bus and dev->bus->pm are present. 59 60If the subsystem chosen by applying the above rules doesn't provide the relevant 61callback, the PM core will invoke the corresponding driver callback stored in 62dev->driver->pm directly (if present). 63 64The PM core always checks which callback to use in the order given above, so the 65priority order of callbacks from high to low is: PM domain, device type, class 66and bus type. Moreover, the high-priority one will always take precedence over 67a low-priority one. The PM domain, bus type, device type and class callbacks 68are referred to as subsystem-level callbacks in what follows. 69 70By default, the callbacks are always invoked in process context with interrupts 71enabled. However, the pm_runtime_irq_safe() helper function can be used to tell 72the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume() 73and ->runtime_idle() callbacks for the given device in atomic context with 74interrupts disabled. This implies that the callback routines in question must 75not block or sleep, but it also means that the synchronous helper functions 76listed at the end of Section 4 may be used for that device within an interrupt 77handler or generally in an atomic context. 78 79The subsystem-level suspend callback, if present, is _entirely_ _responsible_ 80for handling the suspend of the device as appropriate, which may, but need not 81include executing the device driver's own ->runtime_suspend() callback (from the 82PM core's point of view it is not necessary to implement a ->runtime_suspend() 83callback in a device driver as long as the subsystem-level suspend callback 84knows what to do to handle the device). 85 86 * Once the subsystem-level suspend callback (or the driver suspend callback, 87 if invoked directly) has completed successfully for the given device, the PM 88 core regards the device as suspended, which need not mean that it has been 89 put into a low power state. It is supposed to mean, however, that the 90 device will not process data and will not communicate with the CPU(s) and 91 RAM until the appropriate resume callback is executed for it. The runtime 92 PM status of a device after successful execution of the suspend callback is 93 'suspended'. 94 95 * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM 96 status remains 'active', which means that the device _must_ be fully 97 operational afterwards. 98 99 * If the suspend callback returns an error code different from -EBUSY and 100 -EAGAIN, the PM core regards this as a fatal error and will refuse to run 101 the helper functions described in Section 4 for the device until its status 102 is directly set to either'active', or 'suspended' (the PM core provides 103 special helper functions for this purpose). 104 105In particular, if the driver requires remote wakeup capability (i.e. hardware 106mechanism allowing the device to request a change of its power state, such as 107PCI PME) for proper functioning and device_run_wake() returns 'false' for the 108device, then ->runtime_suspend() should return -EBUSY. On the other hand, if 109device_run_wake() returns 'true' for the device and the device is put into a 110low-power state during the execution of the suspend callback, it is expected 111that remote wakeup will be enabled for the device. Generally, remote wakeup 112should be enabled for all input devices put into low-power states at run time. 113 114The subsystem-level resume callback, if present, is _entirely_ _responsible_ for 115handling the resume of the device as appropriate, which may, but need not 116include executing the device driver's own ->runtime_resume() callback (from the 117PM core's point of view it is not necessary to implement a ->runtime_resume() 118callback in a device driver as long as the subsystem-level resume callback knows 119what to do to handle the device). 120 121 * Once the subsystem-level resume callback (or the driver resume callback, if 122 invoked directly) has completed successfully, the PM core regards the device 123 as fully operational, which means that the device _must_ be able to complete 124 I/O operations as needed. The runtime PM status of the device is then 125 'active'. 126 127 * If the resume callback returns an error code, the PM core regards this as a 128 fatal error and will refuse to run the helper functions described in Section 129 4 for the device, until its status is directly set to either 'active', or 130 'suspended' (by means of special helper functions provided by the PM core 131 for this purpose). 132 133The idle callback (a subsystem-level one, if present, or the driver one) is 134executed by the PM core whenever the device appears to be idle, which is 135indicated to the PM core by two counters, the device's usage counter and the 136counter of 'active' children of the device. 137 138 * If any of these counters is decreased using a helper function provided by 139 the PM core and it turns out to be equal to zero, the other counter is 140 checked. If that counter also is equal to zero, the PM core executes the 141 idle callback with the device as its argument. 142 143The action performed by the idle callback is totally dependent on the subsystem 144(or driver) in question, but the expected and recommended action is to check 145if the device can be suspended (i.e. if all of the conditions necessary for 146suspending the device are satisfied) and to queue up a suspend request for the 147device in that case. The value returned by this callback is ignored by the PM 148core. 149 150The helper functions provided by the PM core, described in Section 4, guarantee 151that the following constraints are met with respect to runtime PM callbacks for 152one device: 153 154(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute 155 ->runtime_suspend() in parallel with ->runtime_resume() or with another 156 instance of ->runtime_suspend() for the same device) with the exception that 157 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with 158 ->runtime_idle() (although ->runtime_idle() will not be started while any 159 of the other callbacks is being executed for the same device). 160 161(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active' 162 devices (i.e. the PM core will only execute ->runtime_idle() or 163 ->runtime_suspend() for the devices the runtime PM status of which is 164 'active'). 165 166(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device 167 the usage counter of which is equal to zero _and_ either the counter of 168 'active' children of which is equal to zero, or the 'power.ignore_children' 169 flag of which is set. 170 171(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the 172 PM core will only execute ->runtime_resume() for the devices the runtime 173 PM status of which is 'suspended'). 174 175Additionally, the helper functions provided by the PM core obey the following 176rules: 177 178 * If ->runtime_suspend() is about to be executed or there's a pending request 179 to execute it, ->runtime_idle() will not be executed for the same device. 180 181 * A request to execute or to schedule the execution of ->runtime_suspend() 182 will cancel any pending requests to execute ->runtime_idle() for the same 183 device. 184 185 * If ->runtime_resume() is about to be executed or there's a pending request 186 to execute it, the other callbacks will not be executed for the same device. 187 188 * A request to execute ->runtime_resume() will cancel any pending or 189 scheduled requests to execute the other callbacks for the same device, 190 except for scheduled autosuspends. 191 1923. Runtime PM Device Fields 193 194The following device runtime PM fields are present in 'struct dev_pm_info', as 195defined in include/linux/pm.h: 196 197 struct timer_list suspend_timer; 198 - timer used for scheduling (delayed) suspend and autosuspend requests 199 200 unsigned long timer_expires; 201 - timer expiration time, in jiffies (if this is different from zero, the 202 timer is running and will expire at that time, otherwise the timer is not 203 running) 204 205 struct work_struct work; 206 - work structure used for queuing up requests (i.e. work items in pm_wq) 207 208 wait_queue_head_t wait_queue; 209 - wait queue used if any of the helper functions needs to wait for another 210 one to complete 211 212 spinlock_t lock; 213 - lock used for synchronisation 214 215 atomic_t usage_count; 216 - the usage counter of the device 217 218 atomic_t child_count; 219 - the count of 'active' children of the device 220 221 unsigned int ignore_children; 222 - if set, the value of child_count is ignored (but still updated) 223 224 unsigned int disable_depth; 225 - used for disabling the helper funcions (they work normally if this is 226 equal to zero); the initial value of it is 1 (i.e. runtime PM is 227 initially disabled for all devices) 228 229 unsigned int runtime_error; 230 - if set, there was a fatal error (one of the callbacks returned error code 231 as described in Section 2), so the helper funtions will not work until 232 this flag is cleared; this is the error code returned by the failing 233 callback 234 235 unsigned int idle_notification; 236 - if set, ->runtime_idle() is being executed 237 238 unsigned int request_pending; 239 - if set, there's a pending request (i.e. a work item queued up into pm_wq) 240 241 enum rpm_request request; 242 - type of request that's pending (valid if request_pending is set) 243 244 unsigned int deferred_resume; 245 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is 246 being executed for that device and it is not practical to wait for the 247 suspend to complete; means "start a resume as soon as you've suspended" 248 249 unsigned int run_wake; 250 - set if the device is capable of generating runtime wake-up events 251 252 enum rpm_status runtime_status; 253 - the runtime PM status of the device; this field's initial value is 254 RPM_SUSPENDED, which means that each device is initially regarded by the 255 PM core as 'suspended', regardless of its real hardware status 256 257 unsigned int runtime_auto; 258 - if set, indicates that the user space has allowed the device driver to 259 power manage the device at run time via the /sys/devices/.../power/control 260 interface; it may only be modified with the help of the pm_runtime_allow() 261 and pm_runtime_forbid() helper functions 262 263 unsigned int no_callbacks; 264 - indicates that the device does not use the runtime PM callbacks (see 265 Section 8); it may be modified only by the pm_runtime_no_callbacks() 266 helper function 267 268 unsigned int irq_safe; 269 - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks 270 will be invoked with the spinlock held and interrupts disabled 271 272 unsigned int use_autosuspend; 273 - indicates that the device's driver supports delayed autosuspend (see 274 Section 9); it may be modified only by the 275 pm_runtime{_dont}_use_autosuspend() helper functions 276 277 unsigned int timer_autosuspends; 278 - indicates that the PM core should attempt to carry out an autosuspend 279 when the timer expires rather than a normal suspend 280 281 int autosuspend_delay; 282 - the delay time (in milliseconds) to be used for autosuspend 283 284 unsigned long last_busy; 285 - the time (in jiffies) when the pm_runtime_mark_last_busy() helper 286 function was last called for this device; used in calculating inactivity 287 periods for autosuspend 288 289All of the above fields are members of the 'power' member of 'struct device'. 290 2914. Runtime PM Device Helper Functions 292 293The following runtime PM helper functions are defined in 294drivers/base/power/runtime.c and include/linux/pm_runtime.h: 295 296 void pm_runtime_init(struct device *dev); 297 - initialize the device runtime PM fields in 'struct dev_pm_info' 298 299 void pm_runtime_remove(struct device *dev); 300 - make sure that the runtime PM of the device will be disabled after 301 removing the device from device hierarchy 302 303 int pm_runtime_idle(struct device *dev); 304 - execute the subsystem-level idle callback for the device; returns 0 on 305 success or error code on failure, where -EINPROGRESS means that 306 ->runtime_idle() is already being executed 307 308 int pm_runtime_suspend(struct device *dev); 309 - execute the subsystem-level suspend callback for the device; returns 0 on 310 success, 1 if the device's runtime PM status was already 'suspended', or 311 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt 312 to suspend the device again in future and -EACCES means that 313 'power.disable_depth' is different from 0 314 315 int pm_runtime_autosuspend(struct device *dev); 316 - same as pm_runtime_suspend() except that the autosuspend delay is taken 317 into account; if pm_runtime_autosuspend_expiration() says the delay has 318 not yet expired then an autosuspend is scheduled for the appropriate time 319 and 0 is returned 320 321 int pm_runtime_resume(struct device *dev); 322 - execute the subsystem-level resume callback for the device; returns 0 on 323 success, 1 if the device's runtime PM status was already 'active' or 324 error code on failure, where -EAGAIN means it may be safe to attempt to 325 resume the device again in future, but 'power.runtime_error' should be 326 checked additionally, and -EACCES means that 'power.disable_depth' is 327 different from 0 328 329 int pm_request_idle(struct device *dev); 330 - submit a request to execute the subsystem-level idle callback for the 331 device (the request is represented by a work item in pm_wq); returns 0 on 332 success or error code if the request has not been queued up 333 334 int pm_request_autosuspend(struct device *dev); 335 - schedule the execution of the subsystem-level suspend callback for the 336 device when the autosuspend delay has expired; if the delay has already 337 expired then the work item is queued up immediately 338 339 int pm_schedule_suspend(struct device *dev, unsigned int delay); 340 - schedule the execution of the subsystem-level suspend callback for the 341 device in future, where 'delay' is the time to wait before queuing up a 342 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work 343 item is queued up immediately); returns 0 on success, 1 if the device's PM 344 runtime status was already 'suspended', or error code if the request 345 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of 346 ->runtime_suspend() is already scheduled and not yet expired, the new 347 value of 'delay' will be used as the time to wait 348 349 int pm_request_resume(struct device *dev); 350 - submit a request to execute the subsystem-level resume callback for the 351 device (the request is represented by a work item in pm_wq); returns 0 on 352 success, 1 if the device's runtime PM status was already 'active', or 353 error code if the request hasn't been queued up 354 355 void pm_runtime_get_noresume(struct device *dev); 356 - increment the device's usage counter 357 358 int pm_runtime_get(struct device *dev); 359 - increment the device's usage counter, run pm_request_resume(dev) and 360 return its result 361 362 int pm_runtime_get_sync(struct device *dev); 363 - increment the device's usage counter, run pm_runtime_resume(dev) and 364 return its result 365 366 void pm_runtime_put_noidle(struct device *dev); 367 - decrement the device's usage counter 368 369 int pm_runtime_put(struct device *dev); 370 - decrement the device's usage counter; if the result is 0 then run 371 pm_request_idle(dev) and return its result 372 373 int pm_runtime_put_autosuspend(struct device *dev); 374 - decrement the device's usage counter; if the result is 0 then run 375 pm_request_autosuspend(dev) and return its result 376 377 int pm_runtime_put_sync(struct device *dev); 378 - decrement the device's usage counter; if the result is 0 then run 379 pm_runtime_idle(dev) and return its result 380 381 int pm_runtime_put_sync_suspend(struct device *dev); 382 - decrement the device's usage counter; if the result is 0 then run 383 pm_runtime_suspend(dev) and return its result 384 385 int pm_runtime_put_sync_autosuspend(struct device *dev); 386 - decrement the device's usage counter; if the result is 0 then run 387 pm_runtime_autosuspend(dev) and return its result 388 389 void pm_runtime_enable(struct device *dev); 390 - decrement the device's 'power.disable_depth' field; if that field is equal 391 to zero, the runtime PM helper functions can execute subsystem-level 392 callbacks described in Section 2 for the device 393 394 int pm_runtime_disable(struct device *dev); 395 - increment the device's 'power.disable_depth' field (if the value of that 396 field was previously zero, this prevents subsystem-level runtime PM 397 callbacks from being run for the device), make sure that all of the pending 398 runtime PM operations on the device are either completed or canceled; 399 returns 1 if there was a resume request pending and it was necessary to 400 execute the subsystem-level resume callback for the device to satisfy that 401 request, otherwise 0 is returned 402 403 int pm_runtime_barrier(struct device *dev); 404 - check if there's a resume request pending for the device and resume it 405 (synchronously) in that case, cancel any other pending runtime PM requests 406 regarding it and wait for all runtime PM operations on it in progress to 407 complete; returns 1 if there was a resume request pending and it was 408 necessary to execute the subsystem-level resume callback for the device to 409 satisfy that request, otherwise 0 is returned 410 411 void pm_suspend_ignore_children(struct device *dev, bool enable); 412 - set/unset the power.ignore_children flag of the device 413 414 int pm_runtime_set_active(struct device *dev); 415 - clear the device's 'power.runtime_error' flag, set the device's runtime 416 PM status to 'active' and update its parent's counter of 'active' 417 children as appropriate (it is only valid to use this function if 418 'power.runtime_error' is set or 'power.disable_depth' is greater than 419 zero); it will fail and return error code if the device has a parent 420 which is not active and the 'power.ignore_children' flag of which is unset 421 422 void pm_runtime_set_suspended(struct device *dev); 423 - clear the device's 'power.runtime_error' flag, set the device's runtime 424 PM status to 'suspended' and update its parent's counter of 'active' 425 children as appropriate (it is only valid to use this function if 426 'power.runtime_error' is set or 'power.disable_depth' is greater than 427 zero) 428 429 bool pm_runtime_suspended(struct device *dev); 430 - return true if the device's runtime PM status is 'suspended' and its 431 'power.disable_depth' field is equal to zero, or false otherwise 432 433 bool pm_runtime_status_suspended(struct device *dev); 434 - return true if the device's runtime PM status is 'suspended' 435 436 void pm_runtime_allow(struct device *dev); 437 - set the power.runtime_auto flag for the device and decrease its usage 438 counter (used by the /sys/devices/.../power/control interface to 439 effectively allow the device to be power managed at run time) 440 441 void pm_runtime_forbid(struct device *dev); 442 - unset the power.runtime_auto flag for the device and increase its usage 443 counter (used by the /sys/devices/.../power/control interface to 444 effectively prevent the device from being power managed at run time) 445 446 void pm_runtime_no_callbacks(struct device *dev); 447 - set the power.no_callbacks flag for the device and remove the runtime 448 PM attributes from /sys/devices/.../power (or prevent them from being 449 added when the device is registered) 450 451 void pm_runtime_irq_safe(struct device *dev); 452 - set the power.irq_safe flag for the device, causing the runtime-PM 453 callbacks to be invoked with interrupts off 454 455 void pm_runtime_mark_last_busy(struct device *dev); 456 - set the power.last_busy field to the current time 457 458 void pm_runtime_use_autosuspend(struct device *dev); 459 - set the power.use_autosuspend flag, enabling autosuspend delays 460 461 void pm_runtime_dont_use_autosuspend(struct device *dev); 462 - clear the power.use_autosuspend flag, disabling autosuspend delays 463 464 void pm_runtime_set_autosuspend_delay(struct device *dev, int delay); 465 - set the power.autosuspend_delay value to 'delay' (expressed in 466 milliseconds); if 'delay' is negative then runtime suspends are 467 prevented 468 469 unsigned long pm_runtime_autosuspend_expiration(struct device *dev); 470 - calculate the time when the current autosuspend delay period will expire, 471 based on power.last_busy and power.autosuspend_delay; if the delay time 472 is 1000 ms or larger then the expiration time is rounded up to the 473 nearest second; returns 0 if the delay period has already expired or 474 power.use_autosuspend isn't set, otherwise returns the expiration time 475 in jiffies 476 477It is safe to execute the following helper functions from interrupt context: 478 479pm_request_idle() 480pm_request_autosuspend() 481pm_schedule_suspend() 482pm_request_resume() 483pm_runtime_get_noresume() 484pm_runtime_get() 485pm_runtime_put_noidle() 486pm_runtime_put() 487pm_runtime_put_autosuspend() 488pm_runtime_enable() 489pm_suspend_ignore_children() 490pm_runtime_set_active() 491pm_runtime_set_suspended() 492pm_runtime_suspended() 493pm_runtime_mark_last_busy() 494pm_runtime_autosuspend_expiration() 495 496If pm_runtime_irq_safe() has been called for a device then the following helper 497functions may also be used in interrupt context: 498 499pm_runtime_idle() 500pm_runtime_suspend() 501pm_runtime_autosuspend() 502pm_runtime_resume() 503pm_runtime_get_sync() 504pm_runtime_put_sync() 505pm_runtime_put_sync_suspend() 506pm_runtime_put_sync_autosuspend() 507 5085. Runtime PM Initialization, Device Probing and Removal 509 510Initially, the runtime PM is disabled for all devices, which means that the 511majority of the runtime PM helper funtions described in Section 4 will return 512-EAGAIN until pm_runtime_enable() is called for the device. 513 514In addition to that, the initial runtime PM status of all devices is 515'suspended', but it need not reflect the actual physical state of the device. 516Thus, if the device is initially active (i.e. it is able to process I/O), its 517runtime PM status must be changed to 'active', with the help of 518pm_runtime_set_active(), before pm_runtime_enable() is called for the device. 519 520However, if the device has a parent and the parent's runtime PM is enabled, 521calling pm_runtime_set_active() for the device will affect the parent, unless 522the parent's 'power.ignore_children' flag is set. Namely, in that case the 523parent won't be able to suspend at run time, using the PM core's helper 524functions, as long as the child's status is 'active', even if the child's 525runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for 526the child yet or pm_runtime_disable() has been called for it). For this reason, 527once pm_runtime_set_active() has been called for the device, pm_runtime_enable() 528should be called for it too as soon as reasonably possible or its runtime PM 529status should be changed back to 'suspended' with the help of 530pm_runtime_set_suspended(). 531 532If the default initial runtime PM status of the device (i.e. 'suspended') 533reflects the actual state of the device, its bus type's or its driver's 534->probe() callback will likely need to wake it up using one of the PM core's 535helper functions described in Section 4. In that case, pm_runtime_resume() 536should be used. Of course, for this purpose the device's runtime PM has to be 537enabled earlier by calling pm_runtime_enable(). 538 539If the device bus type's or driver's ->probe() callback runs 540pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts, 541they will fail returning -EAGAIN, because the device's usage counter is 542incremented by the driver core before executing ->probe(). Still, it may be 543desirable to suspend the device as soon as ->probe() has finished, so the driver 544core uses pm_runtime_put_sync() to invoke the subsystem-level idle callback for 545the device at that time. 546 547Moreover, the driver core prevents runtime PM callbacks from racing with the bus 548notifier callback in __device_release_driver(), which is necessary, because the 549notifier is used by some subsystems to carry out operations affecting the 550runtime PM functionality. It does so by calling pm_runtime_get_sync() before 551driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This 552resumes the device if it's in the suspended state and prevents it from 553being suspended again while those routines are being executed. 554 555To allow bus types and drivers to put devices into the suspended state by 556calling pm_runtime_suspend() from their ->remove() routines, the driver core 557executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER 558notifications in __device_release_driver(). This requires bus types and 559drivers to make their ->remove() callbacks avoid races with runtime PM directly, 560but also it allows of more flexibility in the handling of devices during the 561removal of their drivers. 562 563The user space can effectively disallow the driver of the device to power manage 564it at run time by changing the value of its /sys/devices/.../power/control 565attribute to "on", which causes pm_runtime_forbid() to be called. In principle, 566this mechanism may also be used by the driver to effectively turn off the 567runtime power management of the device until the user space turns it on. 568Namely, during the initialization the driver can make sure that the runtime PM 569status of the device is 'active' and call pm_runtime_forbid(). It should be 570noted, however, that if the user space has already intentionally changed the 571value of /sys/devices/.../power/control to "auto" to allow the driver to power 572manage the device at run time, the driver may confuse it by using 573pm_runtime_forbid() this way. 574 5756. Runtime PM and System Sleep 576 577Runtime PM and system sleep (i.e., system suspend and hibernation, also known 578as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of 579ways. If a device is active when a system sleep starts, everything is 580straightforward. But what should happen if the device is already suspended? 581 582The device may have different wake-up settings for runtime PM and system sleep. 583For example, remote wake-up may be enabled for runtime suspend but disallowed 584for system sleep (device_may_wakeup(dev) returns 'false'). When this happens, 585the subsystem-level system suspend callback is responsible for changing the 586device's wake-up setting (it may leave that to the device driver's system 587suspend routine). It may be necessary to resume the device and suspend it again 588in order to do so. The same is true if the driver uses different power levels 589or other settings for runtime suspend and system sleep. 590 591During system resume, the simplest approach is to bring all devices back to full 592power, even if they had been suspended before the system suspend began. There 593are several reasons for this, including: 594 595 * The device might need to switch power levels, wake-up settings, etc. 596 597 * Remote wake-up events might have been lost by the firmware. 598 599 * The device's children may need the device to be at full power in order 600 to resume themselves. 601 602 * The driver's idea of the device state may not agree with the device's 603 physical state. This can happen during resume from hibernation. 604 605 * The device might need to be reset. 606 607 * Even though the device was suspended, if its usage counter was > 0 then most 608 likely it would need a runtime resume in the near future anyway. 609 610If the device had been suspended before the system suspend began and it's 611brought back to full power during resume, then its runtime PM status will have 612to be updated to reflect the actual post-system sleep status. The way to do 613this is: 614 615 pm_runtime_disable(dev); 616 pm_runtime_set_active(dev); 617 pm_runtime_enable(dev); 618 619The PM core always increments the runtime usage counter before calling the 620->suspend() callback and decrements it after calling the ->resume() callback. 621Hence disabling runtime PM temporarily like this will not cause any runtime 622suspend attempts to be permanently lost. If the usage count goes to zero 623following the return of the ->resume() callback, the ->runtime_idle() callback 624will be invoked as usual. 625 626On some systems, however, system sleep is not entered through a global firmware 627or hardware operation. Instead, all hardware components are put into low-power 628states directly by the kernel in a coordinated way. Then, the system sleep 629state effectively follows from the states the hardware components end up in 630and the system is woken up from that state by a hardware interrupt or a similar 631mechanism entirely under the kernel's control. As a result, the kernel never 632gives control away and the states of all devices during resume are precisely 633known to it. If that is the case and none of the situations listed above takes 634place (in particular, if the system is not waking up from hibernation), it may 635be more efficient to leave the devices that had been suspended before the system 636suspend began in the suspended state. 637 638The PM core does its best to reduce the probability of race conditions between 639the runtime PM and system suspend/resume (and hibernation) callbacks by carrying 640out the following operations: 641 642 * During system suspend it calls pm_runtime_get_noresume() and 643 pm_runtime_barrier() for every device right before executing the 644 subsystem-level .suspend() callback for it. In addition to that it calls 645 pm_runtime_disable() for every device right after executing the 646 subsystem-level .suspend() callback for it. 647 648 * During system resume it calls pm_runtime_enable() and pm_runtime_put_sync() 649 for every device right before and right after executing the subsystem-level 650 .resume() callback for it, respectively. 651 6527. Generic subsystem callbacks 653 654Subsystems may wish to conserve code space by using the set of generic power 655management callbacks provided by the PM core, defined in 656driver/base/power/generic_ops.c: 657 658 int pm_generic_runtime_idle(struct device *dev); 659 - invoke the ->runtime_idle() callback provided by the driver of this 660 device, if defined, and call pm_runtime_suspend() for this device if the 661 return value is 0 or the callback is not defined 662 663 int pm_generic_runtime_suspend(struct device *dev); 664 - invoke the ->runtime_suspend() callback provided by the driver of this 665 device and return its result, or return -EINVAL if not defined 666 667 int pm_generic_runtime_resume(struct device *dev); 668 - invoke the ->runtime_resume() callback provided by the driver of this 669 device and return its result, or return -EINVAL if not defined 670 671 int pm_generic_suspend(struct device *dev); 672 - if the device has not been suspended at run time, invoke the ->suspend() 673 callback provided by its driver and return its result, or return 0 if not 674 defined 675 676 int pm_generic_suspend_noirq(struct device *dev); 677 - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq() 678 callback provided by the device's driver and return its result, or return 679 0 if not defined 680 681 int pm_generic_resume(struct device *dev); 682 - invoke the ->resume() callback provided by the driver of this device and, 683 if successful, change the device's runtime PM status to 'active' 684 685 int pm_generic_resume_noirq(struct device *dev); 686 - invoke the ->resume_noirq() callback provided by the driver of this device 687 688 int pm_generic_freeze(struct device *dev); 689 - if the device has not been suspended at run time, invoke the ->freeze() 690 callback provided by its driver and return its result, or return 0 if not 691 defined 692 693 int pm_generic_freeze_noirq(struct device *dev); 694 - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq() 695 callback provided by the device's driver and return its result, or return 696 0 if not defined 697 698 int pm_generic_thaw(struct device *dev); 699 - if the device has not been suspended at run time, invoke the ->thaw() 700 callback provided by its driver and return its result, or return 0 if not 701 defined 702 703 int pm_generic_thaw_noirq(struct device *dev); 704 - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq() 705 callback provided by the device's driver and return its result, or return 706 0 if not defined 707 708 int pm_generic_poweroff(struct device *dev); 709 - if the device has not been suspended at run time, invoke the ->poweroff() 710 callback provided by its driver and return its result, or return 0 if not 711 defined 712 713 int pm_generic_poweroff_noirq(struct device *dev); 714 - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq() 715 callback provided by the device's driver and return its result, or return 716 0 if not defined 717 718 int pm_generic_restore(struct device *dev); 719 - invoke the ->restore() callback provided by the driver of this device and, 720 if successful, change the device's runtime PM status to 'active' 721 722 int pm_generic_restore_noirq(struct device *dev); 723 - invoke the ->restore_noirq() callback provided by the device's driver 724 725These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(), 726->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(), 727->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(), 728->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() callback 729pointers in the subsystem-level dev_pm_ops structures. 730 731If a subsystem wishes to use all of them at the same time, it can simply assign 732the GENERIC_SUBSYS_PM_OPS macro, defined in include/linux/pm.h, to its 733dev_pm_ops structure pointer. 734 735Device drivers that wish to use the same function as a system suspend, freeze, 736poweroff and runtime suspend callback, and similarly for system resume, thaw, 737restore, and runtime resume, can achieve this with the help of the 738UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its 739last argument to NULL). 740 7418. "No-Callback" Devices 742 743Some "devices" are only logical sub-devices of their parent and cannot be 744power-managed on their own. (The prototype example is a USB interface. Entire 745USB devices can go into low-power mode or send wake-up requests, but neither is 746possible for individual interfaces.) The drivers for these devices have no 747need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend() 748and ->runtime_resume() would always return 0 without doing anything else and 749->runtime_idle() would always call pm_runtime_suspend(). 750 751Subsystems can tell the PM core about these devices by calling 752pm_runtime_no_callbacks(). This should be done after the device structure is 753initialized and before it is registered (although after device registration is 754also okay). The routine will set the device's power.no_callbacks flag and 755prevent the non-debugging runtime PM sysfs attributes from being created. 756 757When power.no_callbacks is set, the PM core will not invoke the 758->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks. 759Instead it will assume that suspends and resumes always succeed and that idle 760devices should be suspended. 761 762As a consequence, the PM core will never directly inform the device's subsystem 763or driver about runtime power changes. Instead, the driver for the device's 764parent must take responsibility for telling the device's driver when the 765parent's power state changes. 766 7679. Autosuspend, or automatically-delayed suspends 768 769Changing a device's power state isn't free; it requires both time and energy. 770A device should be put in a low-power state only when there's some reason to 771think it will remain in that state for a substantial time. A common heuristic 772says that a device which hasn't been used for a while is liable to remain 773unused; following this advice, drivers should not allow devices to be suspended 774at runtime until they have been inactive for some minimum period. Even when 775the heuristic ends up being non-optimal, it will still prevent devices from 776"bouncing" too rapidly between low-power and full-power states. 777 778The term "autosuspend" is an historical remnant. It doesn't mean that the 779device is automatically suspended (the subsystem or driver still has to call 780the appropriate PM routines); rather it means that runtime suspends will 781automatically be delayed until the desired period of inactivity has elapsed. 782 783Inactivity is determined based on the power.last_busy field. Drivers should 784call pm_runtime_mark_last_busy() to update this field after carrying out I/O, 785typically just before calling pm_runtime_put_autosuspend(). The desired length 786of the inactivity period is a matter of policy. Subsystems can set this length 787initially by calling pm_runtime_set_autosuspend_delay(), but after device 788registration the length should be controlled by user space, using the 789/sys/devices/.../power/autosuspend_delay_ms attribute. 790 791In order to use autosuspend, subsystems or drivers must call 792pm_runtime_use_autosuspend() (preferably before registering the device), and 793thereafter they should use the various *_autosuspend() helper functions instead 794of the non-autosuspend counterparts: 795 796 Instead of: pm_runtime_suspend use: pm_runtime_autosuspend; 797 Instead of: pm_schedule_suspend use: pm_request_autosuspend; 798 Instead of: pm_runtime_put use: pm_runtime_put_autosuspend; 799 Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend. 800 801Drivers may also continue to use the non-autosuspend helper functions; they 802will behave normally, not taking the autosuspend delay into account. 803Similarly, if the power.use_autosuspend field isn't set then the autosuspend 804helper functions will behave just like the non-autosuspend counterparts. 805 806Under some circumstances a driver or subsystem may want to prevent a device 807from autosuspending immediately, even though the usage counter is zero and the 808autosuspend delay time has expired. If the ->runtime_suspend() callback 809returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is 810in the future (as it normally would be if the callback invoked 811pm_runtime_mark_last_busy()), the PM core will automatically reschedule the 812autosuspend. The ->runtime_suspend() callback can't do this rescheduling 813itself because no suspend requests of any kind are accepted while the device is 814suspending (i.e., while the callback is running). 815 816The implementation is well suited for asynchronous use in interrupt contexts. 817However such use inevitably involves races, because the PM core can't 818synchronize ->runtime_suspend() callbacks with the arrival of I/O requests. 819This synchronization must be handled by the driver, using its private lock. 820Here is a schematic pseudo-code example: 821 822 foo_read_or_write(struct foo_priv *foo, void *data) 823 { 824 lock(&foo->private_lock); 825 add_request_to_io_queue(foo, data); 826 if (foo->num_pending_requests++ == 0) 827 pm_runtime_get(&foo->dev); 828 if (!foo->is_suspended) 829 foo_process_next_request(foo); 830 unlock(&foo->private_lock); 831 } 832 833 foo_io_completion(struct foo_priv *foo, void *req) 834 { 835 lock(&foo->private_lock); 836 if (--foo->num_pending_requests == 0) { 837 pm_runtime_mark_last_busy(&foo->dev); 838 pm_runtime_put_autosuspend(&foo->dev); 839 } else { 840 foo_process_next_request(foo); 841 } 842 unlock(&foo->private_lock); 843 /* Send req result back to the user ... */ 844 } 845 846 int foo_runtime_suspend(struct device *dev) 847 { 848 struct foo_priv foo = container_of(dev, ...); 849 int ret = 0; 850 851 lock(&foo->private_lock); 852 if (foo->num_pending_requests > 0) { 853 ret = -EBUSY; 854 } else { 855 /* ... suspend the device ... */ 856 foo->is_suspended = 1; 857 } 858 unlock(&foo->private_lock); 859 return ret; 860 } 861 862 int foo_runtime_resume(struct device *dev) 863 { 864 struct foo_priv foo = container_of(dev, ...); 865 866 lock(&foo->private_lock); 867 /* ... resume the device ... */ 868 foo->is_suspended = 0; 869 pm_runtime_mark_last_busy(&foo->dev); 870 if (foo->num_pending_requests > 0) 871 foo_process_requests(foo); 872 unlock(&foo->private_lock); 873 return 0; 874 } 875 876The important point is that after foo_io_completion() asks for an autosuspend, 877the foo_runtime_suspend() callback may race with foo_read_or_write(). 878Therefore foo_runtime_suspend() has to check whether there are any pending I/O 879requests (while holding the private lock) before allowing the suspend to 880proceed. 881 882In addition, the power.autosuspend_delay field can be changed by user space at 883any time. If a driver cares about this, it can call 884pm_runtime_autosuspend_expiration() from within the ->runtime_suspend() 885callback while holding its private lock. If the function returns a nonzero 886value then the delay has not yet expired and the callback should return 887-EAGAIN. 888