1================================================== 2Runtime Power Management Framework for I/O Devices 3================================================== 4 5(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. 6 7(C) 2010 Alan Stern <stern@rowland.harvard.edu> 8 9(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> 10 111. Introduction 12=============== 13 14Support for runtime power management (runtime PM) of I/O devices is provided 15at the power management core (PM core) level by means of: 16 17* The power management workqueue pm_wq in which bus types and device drivers can 18 put their PM-related work items. It is strongly recommended that pm_wq be 19 used for queuing all work items related to runtime PM, because this allows 20 them to be synchronized with system-wide power transitions (suspend to RAM, 21 hibernation and resume from system sleep states). pm_wq is declared in 22 include/linux/pm_runtime.h and defined in kernel/power/main.c. 23 24* A number of runtime PM fields in the 'power' member of 'struct device' (which 25 is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can 26 be used for synchronizing runtime PM operations with one another. 27 28* Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in 29 include/linux/pm.h). 30 31* A set of helper functions defined in drivers/base/power/runtime.c that can be 32 used for carrying out runtime PM operations in such a way that the 33 synchronization between them is taken care of by the PM core. Bus types and 34 device drivers are encouraged to use these functions. 35 36The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM 37fields of 'struct dev_pm_info' and the core helper functions provided for 38runtime PM are described below. 39 402. Device Runtime PM Callbacks 41============================== 42 43There are three device runtime PM callbacks defined in 'struct dev_pm_ops':: 44 45 struct dev_pm_ops { 46 ... 47 int (*runtime_suspend)(struct device *dev); 48 int (*runtime_resume)(struct device *dev); 49 int (*runtime_idle)(struct device *dev); 50 ... 51 }; 52 53The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks 54are executed by the PM core for the device's subsystem that may be either of 55the following: 56 57 1. PM domain of the device, if the device's PM domain object, dev->pm_domain, 58 is present. 59 60 2. Device type of the device, if both dev->type and dev->type->pm are present. 61 62 3. Device class of the device, if both dev->class and dev->class->pm are 63 present. 64 65 4. Bus type of the device, if both dev->bus and dev->bus->pm are present. 66 67If the subsystem chosen by applying the above rules doesn't provide the relevant 68callback, the PM core will invoke the corresponding driver callback stored in 69dev->driver->pm directly (if present). 70 71The PM core always checks which callback to use in the order given above, so the 72priority order of callbacks from high to low is: PM domain, device type, class 73and bus type. Moreover, the high-priority one will always take precedence over 74a low-priority one. The PM domain, bus type, device type and class callbacks 75are referred to as subsystem-level callbacks in what follows. 76 77By default, the callbacks are always invoked in process context with interrupts 78enabled. However, the pm_runtime_irq_safe() helper function can be used to tell 79the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume() 80and ->runtime_idle() callbacks for the given device in atomic context with 81interrupts disabled. This implies that the callback routines in question must 82not block or sleep, but it also means that the synchronous helper functions 83listed at the end of Section 4 may be used for that device within an interrupt 84handler or generally in an atomic context. 85 86The subsystem-level suspend callback, if present, is _entirely_ _responsible_ 87for handling the suspend of the device as appropriate, which may, but need not 88include executing the device driver's own ->runtime_suspend() callback (from the 89PM core's point of view it is not necessary to implement a ->runtime_suspend() 90callback in a device driver as long as the subsystem-level suspend callback 91knows what to do to handle the device). 92 93 * Once the subsystem-level suspend callback (or the driver suspend callback, 94 if invoked directly) has completed successfully for the given device, the PM 95 core regards the device as suspended, which need not mean that it has been 96 put into a low power state. It is supposed to mean, however, that the 97 device will not process data and will not communicate with the CPU(s) and 98 RAM until the appropriate resume callback is executed for it. The runtime 99 PM status of a device after successful execution of the suspend callback is 100 'suspended'. 101 102 * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM 103 status remains 'active', which means that the device _must_ be fully 104 operational afterwards. 105 106 * If the suspend callback returns an error code different from -EBUSY and 107 -EAGAIN, the PM core regards this as a fatal error and will refuse to run 108 the helper functions described in Section 4 for the device until its status 109 is directly set to either 'active', or 'suspended' (the PM core provides 110 special helper functions for this purpose). 111 112In particular, if the driver requires remote wakeup capability (i.e. hardware 113mechanism allowing the device to request a change of its power state, such as 114PCI PME) for proper functioning and device_can_wakeup() returns 'false' for the 115device, then ->runtime_suspend() should return -EBUSY. On the other hand, if 116device_can_wakeup() returns 'true' for the device and the device is put into a 117low-power state during the execution of the suspend callback, it is expected 118that remote wakeup will be enabled for the device. Generally, remote wakeup 119should be enabled for all input devices put into low-power states at run time. 120 121The subsystem-level resume callback, if present, is **entirely responsible** for 122handling the resume of the device as appropriate, which may, but need not 123include executing the device driver's own ->runtime_resume() callback (from the 124PM core's point of view it is not necessary to implement a ->runtime_resume() 125callback in a device driver as long as the subsystem-level resume callback knows 126what to do to handle the device). 127 128 * Once the subsystem-level resume callback (or the driver resume callback, if 129 invoked directly) has completed successfully, the PM core regards the device 130 as fully operational, which means that the device _must_ be able to complete 131 I/O operations as needed. The runtime PM status of the device is then 132 'active'. 133 134 * If the resume callback returns an error code, the PM core regards this as a 135 fatal error and will refuse to run the helper functions described in Section 136 4 for the device, until its status is directly set to either 'active', or 137 'suspended' (by means of special helper functions provided by the PM core 138 for this purpose). 139 140The idle callback (a subsystem-level one, if present, or the driver one) is 141executed by the PM core whenever the device appears to be idle, which is 142indicated to the PM core by two counters, the device's usage counter and the 143counter of 'active' children of the device. 144 145 * If any of these counters is decreased using a helper function provided by 146 the PM core and it turns out to be equal to zero, the other counter is 147 checked. If that counter also is equal to zero, the PM core executes the 148 idle callback with the device as its argument. 149 150The action performed by the idle callback is totally dependent on the subsystem 151(or driver) in question, but the expected and recommended action is to check 152if the device can be suspended (i.e. if all of the conditions necessary for 153suspending the device are satisfied) and to queue up a suspend request for the 154device in that case. If there is no idle callback, or if the callback returns 1550, then the PM core will attempt to carry out a runtime suspend of the device, 156also respecting devices configured for autosuspend. In essence this means a 157call to pm_runtime_autosuspend() (do note that drivers needs to update the 158device last busy mark, pm_runtime_mark_last_busy(), to control the delay under 159this circumstance). To prevent this (for example, if the callback routine has 160started a delayed suspend), the routine must return a non-zero value. Negative 161error return codes are ignored by the PM core. 162 163The helper functions provided by the PM core, described in Section 4, guarantee 164that the following constraints are met with respect to runtime PM callbacks for 165one device: 166 167(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute 168 ->runtime_suspend() in parallel with ->runtime_resume() or with another 169 instance of ->runtime_suspend() for the same device) with the exception that 170 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with 171 ->runtime_idle() (although ->runtime_idle() will not be started while any 172 of the other callbacks is being executed for the same device). 173 174(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active' 175 devices (i.e. the PM core will only execute ->runtime_idle() or 176 ->runtime_suspend() for the devices the runtime PM status of which is 177 'active'). 178 179(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device 180 the usage counter of which is equal to zero _and_ either the counter of 181 'active' children of which is equal to zero, or the 'power.ignore_children' 182 flag of which is set. 183 184(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the 185 PM core will only execute ->runtime_resume() for the devices the runtime 186 PM status of which is 'suspended'). 187 188Additionally, the helper functions provided by the PM core obey the following 189rules: 190 191 * If ->runtime_suspend() is about to be executed or there's a pending request 192 to execute it, ->runtime_idle() will not be executed for the same device. 193 194 * A request to execute or to schedule the execution of ->runtime_suspend() 195 will cancel any pending requests to execute ->runtime_idle() for the same 196 device. 197 198 * If ->runtime_resume() is about to be executed or there's a pending request 199 to execute it, the other callbacks will not be executed for the same device. 200 201 * A request to execute ->runtime_resume() will cancel any pending or 202 scheduled requests to execute the other callbacks for the same device, 203 except for scheduled autosuspends. 204 2053. Runtime PM Device Fields 206=========================== 207 208The following device runtime PM fields are present in 'struct dev_pm_info', as 209defined in include/linux/pm.h: 210 211 `struct timer_list suspend_timer;` 212 - timer used for scheduling (delayed) suspend and autosuspend requests 213 214 `unsigned long timer_expires;` 215 - timer expiration time, in jiffies (if this is different from zero, the 216 timer is running and will expire at that time, otherwise the timer is not 217 running) 218 219 `struct work_struct work;` 220 - work structure used for queuing up requests (i.e. work items in pm_wq) 221 222 `wait_queue_head_t wait_queue;` 223 - wait queue used if any of the helper functions needs to wait for another 224 one to complete 225 226 `spinlock_t lock;` 227 - lock used for synchronization 228 229 `atomic_t usage_count;` 230 - the usage counter of the device 231 232 `atomic_t child_count;` 233 - the count of 'active' children of the device 234 235 `unsigned int ignore_children;` 236 - if set, the value of child_count is ignored (but still updated) 237 238 `unsigned int disable_depth;` 239 - used for disabling the helper functions (they work normally if this is 240 equal to zero); the initial value of it is 1 (i.e. runtime PM is 241 initially disabled for all devices) 242 243 `int runtime_error;` 244 - if set, there was a fatal error (one of the callbacks returned error code 245 as described in Section 2), so the helper functions will not work until 246 this flag is cleared; this is the error code returned by the failing 247 callback 248 249 `unsigned int idle_notification;` 250 - if set, ->runtime_idle() is being executed 251 252 `unsigned int request_pending;` 253 - if set, there's a pending request (i.e. a work item queued up into pm_wq) 254 255 `enum rpm_request request;` 256 - type of request that's pending (valid if request_pending is set) 257 258 `unsigned int deferred_resume;` 259 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is 260 being executed for that device and it is not practical to wait for the 261 suspend to complete; means "start a resume as soon as you've suspended" 262 263 `enum rpm_status runtime_status;` 264 - the runtime PM status of the device; this field's initial value is 265 RPM_SUSPENDED, which means that each device is initially regarded by the 266 PM core as 'suspended', regardless of its real hardware status 267 268 `unsigned int runtime_auto;` 269 - if set, indicates that the user space has allowed the device driver to 270 power manage the device at run time via the /sys/devices/.../power/control 271 `interface;` it may only be modified with the help of the 272 pm_runtime_allow() and pm_runtime_forbid() helper functions 273 274 `unsigned int no_callbacks;` 275 - indicates that the device does not use the runtime PM callbacks (see 276 Section 8); it may be modified only by the pm_runtime_no_callbacks() 277 helper function 278 279 `unsigned int irq_safe;` 280 - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks 281 will be invoked with the spinlock held and interrupts disabled 282 283 `unsigned int use_autosuspend;` 284 - indicates that the device's driver supports delayed autosuspend (see 285 Section 9); it may be modified only by the 286 pm_runtime{_dont}_use_autosuspend() helper functions 287 288 `unsigned int timer_autosuspends;` 289 - indicates that the PM core should attempt to carry out an autosuspend 290 when the timer expires rather than a normal suspend 291 292 `int autosuspend_delay;` 293 - the delay time (in milliseconds) to be used for autosuspend 294 295 `unsigned long last_busy;` 296 - the time (in jiffies) when the pm_runtime_mark_last_busy() helper 297 function was last called for this device; used in calculating inactivity 298 periods for autosuspend 299 300All of the above fields are members of the 'power' member of 'struct device'. 301 3024. Runtime PM Device Helper Functions 303===================================== 304 305The following runtime PM helper functions are defined in 306drivers/base/power/runtime.c and include/linux/pm_runtime.h: 307 308 `void pm_runtime_init(struct device *dev);` 309 - initialize the device runtime PM fields in 'struct dev_pm_info' 310 311 `void pm_runtime_remove(struct device *dev);` 312 - make sure that the runtime PM of the device will be disabled after 313 removing the device from device hierarchy 314 315 `int pm_runtime_idle(struct device *dev);` 316 - execute the subsystem-level idle callback for the device; returns an 317 error code on failure, where -EINPROGRESS means that ->runtime_idle() is 318 already being executed; if there is no callback or the callback returns 0 319 then run pm_runtime_autosuspend(dev) and return its result 320 321 `int pm_runtime_suspend(struct device *dev);` 322 - execute the subsystem-level suspend callback for the device; returns 0 on 323 success, 1 if the device's runtime PM status was already 'suspended', or 324 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt 325 to suspend the device again in future and -EACCES means that 326 'power.disable_depth' is different from 0 327 328 `int pm_runtime_autosuspend(struct device *dev);` 329 - same as pm_runtime_suspend() except that the autosuspend delay is taken 330 `into account;` if pm_runtime_autosuspend_expiration() says the delay has 331 not yet expired then an autosuspend is scheduled for the appropriate time 332 and 0 is returned 333 334 `int pm_runtime_resume(struct device *dev);` 335 - execute the subsystem-level resume callback for the device; returns 0 on 336 success, 1 if the device's runtime PM status was already 'active' or 337 error code on failure, where -EAGAIN means it may be safe to attempt to 338 resume the device again in future, but 'power.runtime_error' should be 339 checked additionally, and -EACCES means that 'power.disable_depth' is 340 different from 0 341 342 `int pm_request_idle(struct device *dev);` 343 - submit a request to execute the subsystem-level idle callback for the 344 device (the request is represented by a work item in pm_wq); returns 0 on 345 success or error code if the request has not been queued up 346 347 `int pm_request_autosuspend(struct device *dev);` 348 - schedule the execution of the subsystem-level suspend callback for the 349 device when the autosuspend delay has expired; if the delay has already 350 expired then the work item is queued up immediately 351 352 `int pm_schedule_suspend(struct device *dev, unsigned int delay);` 353 - schedule the execution of the subsystem-level suspend callback for the 354 device in future, where 'delay' is the time to wait before queuing up a 355 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work 356 item is queued up immediately); returns 0 on success, 1 if the device's PM 357 runtime status was already 'suspended', or error code if the request 358 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of 359 ->runtime_suspend() is already scheduled and not yet expired, the new 360 value of 'delay' will be used as the time to wait 361 362 `int pm_request_resume(struct device *dev);` 363 - submit a request to execute the subsystem-level resume callback for the 364 device (the request is represented by a work item in pm_wq); returns 0 on 365 success, 1 if the device's runtime PM status was already 'active', or 366 error code if the request hasn't been queued up 367 368 `void pm_runtime_get_noresume(struct device *dev);` 369 - increment the device's usage counter 370 371 `int pm_runtime_get(struct device *dev);` 372 - increment the device's usage counter, run pm_request_resume(dev) and 373 return its result 374 375 `int pm_runtime_get_sync(struct device *dev);` 376 - increment the device's usage counter, run pm_runtime_resume(dev) and 377 return its result 378 379 `int pm_runtime_get_if_in_use(struct device *dev);` 380 - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the 381 runtime PM status is RPM_ACTIVE and the runtime PM usage counter is 382 nonzero, increment the counter and return 1; otherwise return 0 without 383 changing the counter 384 385 `int pm_runtime_get_if_active(struct device *dev, bool ign_usage_count);` 386 - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the 387 runtime PM status is RPM_ACTIVE, and either ign_usage_count is true 388 or the device's usage_count is non-zero, increment the counter and 389 return 1; otherwise return 0 without changing the counter 390 391 `void pm_runtime_put_noidle(struct device *dev);` 392 - decrement the device's usage counter 393 394 `int pm_runtime_put(struct device *dev);` 395 - decrement the device's usage counter; if the result is 0 then run 396 pm_request_idle(dev) and return its result 397 398 `int pm_runtime_put_autosuspend(struct device *dev);` 399 - decrement the device's usage counter; if the result is 0 then run 400 pm_request_autosuspend(dev) and return its result 401 402 `int pm_runtime_put_sync(struct device *dev);` 403 - decrement the device's usage counter; if the result is 0 then run 404 pm_runtime_idle(dev) and return its result 405 406 `int pm_runtime_put_sync_suspend(struct device *dev);` 407 - decrement the device's usage counter; if the result is 0 then run 408 pm_runtime_suspend(dev) and return its result 409 410 `int pm_runtime_put_sync_autosuspend(struct device *dev);` 411 - decrement the device's usage counter; if the result is 0 then run 412 pm_runtime_autosuspend(dev) and return its result 413 414 `void pm_runtime_enable(struct device *dev);` 415 - decrement the device's 'power.disable_depth' field; if that field is equal 416 to zero, the runtime PM helper functions can execute subsystem-level 417 callbacks described in Section 2 for the device 418 419 `int pm_runtime_disable(struct device *dev);` 420 - increment the device's 'power.disable_depth' field (if the value of that 421 field was previously zero, this prevents subsystem-level runtime PM 422 callbacks from being run for the device), make sure that all of the 423 pending runtime PM operations on the device are either completed or 424 canceled; returns 1 if there was a resume request pending and it was 425 necessary to execute the subsystem-level resume callback for the device 426 to satisfy that request, otherwise 0 is returned 427 428 `int pm_runtime_barrier(struct device *dev);` 429 - check if there's a resume request pending for the device and resume it 430 (synchronously) in that case, cancel any other pending runtime PM requests 431 regarding it and wait for all runtime PM operations on it in progress to 432 complete; returns 1 if there was a resume request pending and it was 433 necessary to execute the subsystem-level resume callback for the device to 434 satisfy that request, otherwise 0 is returned 435 436 `void pm_suspend_ignore_children(struct device *dev, bool enable);` 437 - set/unset the power.ignore_children flag of the device 438 439 `int pm_runtime_set_active(struct device *dev);` 440 - clear the device's 'power.runtime_error' flag, set the device's runtime 441 PM status to 'active' and update its parent's counter of 'active' 442 children as appropriate (it is only valid to use this function if 443 'power.runtime_error' is set or 'power.disable_depth' is greater than 444 zero); it will fail and return error code if the device has a parent 445 which is not active and the 'power.ignore_children' flag of which is unset 446 447 `void pm_runtime_set_suspended(struct device *dev);` 448 - clear the device's 'power.runtime_error' flag, set the device's runtime 449 PM status to 'suspended' and update its parent's counter of 'active' 450 children as appropriate (it is only valid to use this function if 451 'power.runtime_error' is set or 'power.disable_depth' is greater than 452 zero) 453 454 `bool pm_runtime_active(struct device *dev);` 455 - return true if the device's runtime PM status is 'active' or its 456 'power.disable_depth' field is not equal to zero, or false otherwise 457 458 `bool pm_runtime_suspended(struct device *dev);` 459 - return true if the device's runtime PM status is 'suspended' and its 460 'power.disable_depth' field is equal to zero, or false otherwise 461 462 `bool pm_runtime_status_suspended(struct device *dev);` 463 - return true if the device's runtime PM status is 'suspended' 464 465 `void pm_runtime_allow(struct device *dev);` 466 - set the power.runtime_auto flag for the device and decrease its usage 467 counter (used by the /sys/devices/.../power/control interface to 468 effectively allow the device to be power managed at run time) 469 470 `void pm_runtime_forbid(struct device *dev);` 471 - unset the power.runtime_auto flag for the device and increase its usage 472 counter (used by the /sys/devices/.../power/control interface to 473 effectively prevent the device from being power managed at run time) 474 475 `void pm_runtime_no_callbacks(struct device *dev);` 476 - set the power.no_callbacks flag for the device and remove the runtime 477 PM attributes from /sys/devices/.../power (or prevent them from being 478 added when the device is registered) 479 480 `void pm_runtime_irq_safe(struct device *dev);` 481 - set the power.irq_safe flag for the device, causing the runtime-PM 482 callbacks to be invoked with interrupts off 483 484 `bool pm_runtime_is_irq_safe(struct device *dev);` 485 - return true if power.irq_safe flag was set for the device, causing 486 the runtime-PM callbacks to be invoked with interrupts off 487 488 `void pm_runtime_mark_last_busy(struct device *dev);` 489 - set the power.last_busy field to the current time 490 491 `void pm_runtime_use_autosuspend(struct device *dev);` 492 - set the power.use_autosuspend flag, enabling autosuspend delays; call 493 pm_runtime_get_sync if the flag was previously cleared and 494 power.autosuspend_delay is negative 495 496 `void pm_runtime_dont_use_autosuspend(struct device *dev);` 497 - clear the power.use_autosuspend flag, disabling autosuspend delays; 498 decrement the device's usage counter if the flag was previously set and 499 power.autosuspend_delay is negative; call pm_runtime_idle 500 501 `void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);` 502 - set the power.autosuspend_delay value to 'delay' (expressed in 503 milliseconds); if 'delay' is negative then runtime suspends are 504 prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be 505 called or the device's usage counter may be decremented and 506 pm_runtime_idle called depending on if power.autosuspend_delay is 507 changed to or from a negative value; if power.use_autosuspend is clear, 508 pm_runtime_idle is called 509 510 `unsigned long pm_runtime_autosuspend_expiration(struct device *dev);` 511 - calculate the time when the current autosuspend delay period will expire, 512 based on power.last_busy and power.autosuspend_delay; if the delay time 513 is 1000 ms or larger then the expiration time is rounded up to the 514 nearest second; returns 0 if the delay period has already expired or 515 power.use_autosuspend isn't set, otherwise returns the expiration time 516 in jiffies 517 518It is safe to execute the following helper functions from interrupt context: 519 520- pm_request_idle() 521- pm_request_autosuspend() 522- pm_schedule_suspend() 523- pm_request_resume() 524- pm_runtime_get_noresume() 525- pm_runtime_get() 526- pm_runtime_put_noidle() 527- pm_runtime_put() 528- pm_runtime_put_autosuspend() 529- pm_runtime_enable() 530- pm_suspend_ignore_children() 531- pm_runtime_set_active() 532- pm_runtime_set_suspended() 533- pm_runtime_suspended() 534- pm_runtime_mark_last_busy() 535- pm_runtime_autosuspend_expiration() 536 537If pm_runtime_irq_safe() has been called for a device then the following helper 538functions may also be used in interrupt context: 539 540- pm_runtime_idle() 541- pm_runtime_suspend() 542- pm_runtime_autosuspend() 543- pm_runtime_resume() 544- pm_runtime_get_sync() 545- pm_runtime_put_sync() 546- pm_runtime_put_sync_suspend() 547- pm_runtime_put_sync_autosuspend() 548 5495. Runtime PM Initialization, Device Probing and Removal 550======================================================== 551 552Initially, the runtime PM is disabled for all devices, which means that the 553majority of the runtime PM helper functions described in Section 4 will return 554-EAGAIN until pm_runtime_enable() is called for the device. 555 556In addition to that, the initial runtime PM status of all devices is 557'suspended', but it need not reflect the actual physical state of the device. 558Thus, if the device is initially active (i.e. it is able to process I/O), its 559runtime PM status must be changed to 'active', with the help of 560pm_runtime_set_active(), before pm_runtime_enable() is called for the device. 561 562However, if the device has a parent and the parent's runtime PM is enabled, 563calling pm_runtime_set_active() for the device will affect the parent, unless 564the parent's 'power.ignore_children' flag is set. Namely, in that case the 565parent won't be able to suspend at run time, using the PM core's helper 566functions, as long as the child's status is 'active', even if the child's 567runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for 568the child yet or pm_runtime_disable() has been called for it). For this reason, 569once pm_runtime_set_active() has been called for the device, pm_runtime_enable() 570should be called for it too as soon as reasonably possible or its runtime PM 571status should be changed back to 'suspended' with the help of 572pm_runtime_set_suspended(). 573 574If the default initial runtime PM status of the device (i.e. 'suspended') 575reflects the actual state of the device, its bus type's or its driver's 576->probe() callback will likely need to wake it up using one of the PM core's 577helper functions described in Section 4. In that case, pm_runtime_resume() 578should be used. Of course, for this purpose the device's runtime PM has to be 579enabled earlier by calling pm_runtime_enable(). 580 581Note, if the device may execute pm_runtime calls during the probe (such as 582if it is registered with a subsystem that may call back in) then the 583pm_runtime_get_sync() call paired with a pm_runtime_put() call will be 584appropriate to ensure that the device is not put back to sleep during the 585probe. This can happen with systems such as the network device layer. 586 587It may be desirable to suspend the device once ->probe() has finished. 588Therefore the driver core uses the asynchronous pm_request_idle() to submit a 589request to execute the subsystem-level idle callback for the device at that 590time. A driver that makes use of the runtime autosuspend feature may want to 591update the last busy mark before returning from ->probe(). 592 593Moreover, the driver core prevents runtime PM callbacks from racing with the bus 594notifier callback in __device_release_driver(), which is necessary because the 595notifier is used by some subsystems to carry out operations affecting the 596runtime PM functionality. It does so by calling pm_runtime_get_sync() before 597driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This 598resumes the device if it's in the suspended state and prevents it from 599being suspended again while those routines are being executed. 600 601To allow bus types and drivers to put devices into the suspended state by 602calling pm_runtime_suspend() from their ->remove() routines, the driver core 603executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER 604notifications in __device_release_driver(). This requires bus types and 605drivers to make their ->remove() callbacks avoid races with runtime PM directly, 606but it also allows more flexibility in the handling of devices during the 607removal of their drivers. 608 609Drivers in ->remove() callback should undo the runtime PM changes done 610in ->probe(). Usually this means calling pm_runtime_disable(), 611pm_runtime_dont_use_autosuspend() etc. 612 613The user space can effectively disallow the driver of the device to power manage 614it at run time by changing the value of its /sys/devices/.../power/control 615attribute to "on", which causes pm_runtime_forbid() to be called. In principle, 616this mechanism may also be used by the driver to effectively turn off the 617runtime power management of the device until the user space turns it on. 618Namely, during the initialization the driver can make sure that the runtime PM 619status of the device is 'active' and call pm_runtime_forbid(). It should be 620noted, however, that if the user space has already intentionally changed the 621value of /sys/devices/.../power/control to "auto" to allow the driver to power 622manage the device at run time, the driver may confuse it by using 623pm_runtime_forbid() this way. 624 6256. Runtime PM and System Sleep 626============================== 627 628Runtime PM and system sleep (i.e., system suspend and hibernation, also known 629as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of 630ways. If a device is active when a system sleep starts, everything is 631straightforward. But what should happen if the device is already suspended? 632 633The device may have different wake-up settings for runtime PM and system sleep. 634For example, remote wake-up may be enabled for runtime suspend but disallowed 635for system sleep (device_may_wakeup(dev) returns 'false'). When this happens, 636the subsystem-level system suspend callback is responsible for changing the 637device's wake-up setting (it may leave that to the device driver's system 638suspend routine). It may be necessary to resume the device and suspend it again 639in order to do so. The same is true if the driver uses different power levels 640or other settings for runtime suspend and system sleep. 641 642During system resume, the simplest approach is to bring all devices back to full 643power, even if they had been suspended before the system suspend began. There 644are several reasons for this, including: 645 646 * The device might need to switch power levels, wake-up settings, etc. 647 648 * Remote wake-up events might have been lost by the firmware. 649 650 * The device's children may need the device to be at full power in order 651 to resume themselves. 652 653 * The driver's idea of the device state may not agree with the device's 654 physical state. This can happen during resume from hibernation. 655 656 * The device might need to be reset. 657 658 * Even though the device was suspended, if its usage counter was > 0 then most 659 likely it would need a runtime resume in the near future anyway. 660 661If the device had been suspended before the system suspend began and it's 662brought back to full power during resume, then its runtime PM status will have 663to be updated to reflect the actual post-system sleep status. The way to do 664this is: 665 666 - pm_runtime_disable(dev); 667 - pm_runtime_set_active(dev); 668 - pm_runtime_enable(dev); 669 670The PM core always increments the runtime usage counter before calling the 671->suspend() callback and decrements it after calling the ->resume() callback. 672Hence disabling runtime PM temporarily like this will not cause any runtime 673suspend attempts to be permanently lost. If the usage count goes to zero 674following the return of the ->resume() callback, the ->runtime_idle() callback 675will be invoked as usual. 676 677On some systems, however, system sleep is not entered through a global firmware 678or hardware operation. Instead, all hardware components are put into low-power 679states directly by the kernel in a coordinated way. Then, the system sleep 680state effectively follows from the states the hardware components end up in 681and the system is woken up from that state by a hardware interrupt or a similar 682mechanism entirely under the kernel's control. As a result, the kernel never 683gives control away and the states of all devices during resume are precisely 684known to it. If that is the case and none of the situations listed above takes 685place (in particular, if the system is not waking up from hibernation), it may 686be more efficient to leave the devices that had been suspended before the system 687suspend began in the suspended state. 688 689To this end, the PM core provides a mechanism allowing some coordination between 690different levels of device hierarchy. Namely, if a system suspend .prepare() 691callback returns a positive number for a device, that indicates to the PM core 692that the device appears to be runtime-suspended and its state is fine, so it 693may be left in runtime suspend provided that all of its descendants are also 694left in runtime suspend. If that happens, the PM core will not execute any 695system suspend and resume callbacks for all of those devices, except for the 696.complete() callback, which is then entirely responsible for handling the device 697as appropriate. This only applies to system suspend transitions that are not 698related to hibernation (see Documentation/driver-api/pm/devices.rst for more 699information). 700 701The PM core does its best to reduce the probability of race conditions between 702the runtime PM and system suspend/resume (and hibernation) callbacks by carrying 703out the following operations: 704 705 * During system suspend pm_runtime_get_noresume() is called for every device 706 right before executing the subsystem-level .prepare() callback for it and 707 pm_runtime_barrier() is called for every device right before executing the 708 subsystem-level .suspend() callback for it. In addition to that the PM core 709 calls __pm_runtime_disable() with 'false' as the second argument for every 710 device right before executing the subsystem-level .suspend_late() callback 711 for it. 712 713 * During system resume pm_runtime_enable() and pm_runtime_put() are called for 714 every device right after executing the subsystem-level .resume_early() 715 callback and right after executing the subsystem-level .complete() callback 716 for it, respectively. 717 7187. Generic subsystem callbacks 719 720Subsystems may wish to conserve code space by using the set of generic power 721management callbacks provided by the PM core, defined in 722driver/base/power/generic_ops.c: 723 724 `int pm_generic_runtime_suspend(struct device *dev);` 725 - invoke the ->runtime_suspend() callback provided by the driver of this 726 device and return its result, or return 0 if not defined 727 728 `int pm_generic_runtime_resume(struct device *dev);` 729 - invoke the ->runtime_resume() callback provided by the driver of this 730 device and return its result, or return 0 if not defined 731 732 `int pm_generic_suspend(struct device *dev);` 733 - if the device has not been suspended at run time, invoke the ->suspend() 734 callback provided by its driver and return its result, or return 0 if not 735 defined 736 737 `int pm_generic_suspend_noirq(struct device *dev);` 738 - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq() 739 callback provided by the device's driver and return its result, or return 740 0 if not defined 741 742 `int pm_generic_resume(struct device *dev);` 743 - invoke the ->resume() callback provided by the driver of this device and, 744 if successful, change the device's runtime PM status to 'active' 745 746 `int pm_generic_resume_noirq(struct device *dev);` 747 - invoke the ->resume_noirq() callback provided by the driver of this device 748 749 `int pm_generic_freeze(struct device *dev);` 750 - if the device has not been suspended at run time, invoke the ->freeze() 751 callback provided by its driver and return its result, or return 0 if not 752 defined 753 754 `int pm_generic_freeze_noirq(struct device *dev);` 755 - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq() 756 callback provided by the device's driver and return its result, or return 757 0 if not defined 758 759 `int pm_generic_thaw(struct device *dev);` 760 - if the device has not been suspended at run time, invoke the ->thaw() 761 callback provided by its driver and return its result, or return 0 if not 762 defined 763 764 `int pm_generic_thaw_noirq(struct device *dev);` 765 - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq() 766 callback provided by the device's driver and return its result, or return 767 0 if not defined 768 769 `int pm_generic_poweroff(struct device *dev);` 770 - if the device has not been suspended at run time, invoke the ->poweroff() 771 callback provided by its driver and return its result, or return 0 if not 772 defined 773 774 `int pm_generic_poweroff_noirq(struct device *dev);` 775 - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq() 776 callback provided by the device's driver and return its result, or return 777 0 if not defined 778 779 `int pm_generic_restore(struct device *dev);` 780 - invoke the ->restore() callback provided by the driver of this device and, 781 if successful, change the device's runtime PM status to 'active' 782 783 `int pm_generic_restore_noirq(struct device *dev);` 784 - invoke the ->restore_noirq() callback provided by the device's driver 785 786These functions are the defaults used by the PM core if a subsystem doesn't 787provide its own callbacks for ->runtime_idle(), ->runtime_suspend(), 788->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(), 789->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(), 790->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the 791subsystem-level dev_pm_ops structure. 792 793Device drivers that wish to use the same function as a system suspend, freeze, 794poweroff and runtime suspend callback, and similarly for system resume, thaw, 795restore, and runtime resume, can achieve this with the help of the 796UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its 797last argument to NULL). 798 7998. "No-Callback" Devices 800======================== 801 802Some "devices" are only logical sub-devices of their parent and cannot be 803power-managed on their own. (The prototype example is a USB interface. Entire 804USB devices can go into low-power mode or send wake-up requests, but neither is 805possible for individual interfaces.) The drivers for these devices have no 806need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend() 807and ->runtime_resume() would always return 0 without doing anything else and 808->runtime_idle() would always call pm_runtime_suspend(). 809 810Subsystems can tell the PM core about these devices by calling 811pm_runtime_no_callbacks(). This should be done after the device structure is 812initialized and before it is registered (although after device registration is 813also okay). The routine will set the device's power.no_callbacks flag and 814prevent the non-debugging runtime PM sysfs attributes from being created. 815 816When power.no_callbacks is set, the PM core will not invoke the 817->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks. 818Instead it will assume that suspends and resumes always succeed and that idle 819devices should be suspended. 820 821As a consequence, the PM core will never directly inform the device's subsystem 822or driver about runtime power changes. Instead, the driver for the device's 823parent must take responsibility for telling the device's driver when the 824parent's power state changes. 825 8269. Autosuspend, or automatically-delayed suspends 827================================================= 828 829Changing a device's power state isn't free; it requires both time and energy. 830A device should be put in a low-power state only when there's some reason to 831think it will remain in that state for a substantial time. A common heuristic 832says that a device which hasn't been used for a while is liable to remain 833unused; following this advice, drivers should not allow devices to be suspended 834at runtime until they have been inactive for some minimum period. Even when 835the heuristic ends up being non-optimal, it will still prevent devices from 836"bouncing" too rapidly between low-power and full-power states. 837 838The term "autosuspend" is an historical remnant. It doesn't mean that the 839device is automatically suspended (the subsystem or driver still has to call 840the appropriate PM routines); rather it means that runtime suspends will 841automatically be delayed until the desired period of inactivity has elapsed. 842 843Inactivity is determined based on the power.last_busy field. Drivers should 844call pm_runtime_mark_last_busy() to update this field after carrying out I/O, 845typically just before calling pm_runtime_put_autosuspend(). The desired length 846of the inactivity period is a matter of policy. Subsystems can set this length 847initially by calling pm_runtime_set_autosuspend_delay(), but after device 848registration the length should be controlled by user space, using the 849/sys/devices/.../power/autosuspend_delay_ms attribute. 850 851In order to use autosuspend, subsystems or drivers must call 852pm_runtime_use_autosuspend() (preferably before registering the device), and 853thereafter they should use the various `*_autosuspend()` helper functions 854instead of the non-autosuspend counterparts:: 855 856 Instead of: pm_runtime_suspend use: pm_runtime_autosuspend; 857 Instead of: pm_schedule_suspend use: pm_request_autosuspend; 858 Instead of: pm_runtime_put use: pm_runtime_put_autosuspend; 859 Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend. 860 861Drivers may also continue to use the non-autosuspend helper functions; they 862will behave normally, which means sometimes taking the autosuspend delay into 863account (see pm_runtime_idle). 864 865Under some circumstances a driver or subsystem may want to prevent a device 866from autosuspending immediately, even though the usage counter is zero and the 867autosuspend delay time has expired. If the ->runtime_suspend() callback 868returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is 869in the future (as it normally would be if the callback invoked 870pm_runtime_mark_last_busy()), the PM core will automatically reschedule the 871autosuspend. The ->runtime_suspend() callback can't do this rescheduling 872itself because no suspend requests of any kind are accepted while the device is 873suspending (i.e., while the callback is running). 874 875The implementation is well suited for asynchronous use in interrupt contexts. 876However such use inevitably involves races, because the PM core can't 877synchronize ->runtime_suspend() callbacks with the arrival of I/O requests. 878This synchronization must be handled by the driver, using its private lock. 879Here is a schematic pseudo-code example:: 880 881 foo_read_or_write(struct foo_priv *foo, void *data) 882 { 883 lock(&foo->private_lock); 884 add_request_to_io_queue(foo, data); 885 if (foo->num_pending_requests++ == 0) 886 pm_runtime_get(&foo->dev); 887 if (!foo->is_suspended) 888 foo_process_next_request(foo); 889 unlock(&foo->private_lock); 890 } 891 892 foo_io_completion(struct foo_priv *foo, void *req) 893 { 894 lock(&foo->private_lock); 895 if (--foo->num_pending_requests == 0) { 896 pm_runtime_mark_last_busy(&foo->dev); 897 pm_runtime_put_autosuspend(&foo->dev); 898 } else { 899 foo_process_next_request(foo); 900 } 901 unlock(&foo->private_lock); 902 /* Send req result back to the user ... */ 903 } 904 905 int foo_runtime_suspend(struct device *dev) 906 { 907 struct foo_priv foo = container_of(dev, ...); 908 int ret = 0; 909 910 lock(&foo->private_lock); 911 if (foo->num_pending_requests > 0) { 912 ret = -EBUSY; 913 } else { 914 /* ... suspend the device ... */ 915 foo->is_suspended = 1; 916 } 917 unlock(&foo->private_lock); 918 return ret; 919 } 920 921 int foo_runtime_resume(struct device *dev) 922 { 923 struct foo_priv foo = container_of(dev, ...); 924 925 lock(&foo->private_lock); 926 /* ... resume the device ... */ 927 foo->is_suspended = 0; 928 pm_runtime_mark_last_busy(&foo->dev); 929 if (foo->num_pending_requests > 0) 930 foo_process_next_request(foo); 931 unlock(&foo->private_lock); 932 return 0; 933 } 934 935The important point is that after foo_io_completion() asks for an autosuspend, 936the foo_runtime_suspend() callback may race with foo_read_or_write(). 937Therefore foo_runtime_suspend() has to check whether there are any pending I/O 938requests (while holding the private lock) before allowing the suspend to 939proceed. 940 941In addition, the power.autosuspend_delay field can be changed by user space at 942any time. If a driver cares about this, it can call 943pm_runtime_autosuspend_expiration() from within the ->runtime_suspend() 944callback while holding its private lock. If the function returns a nonzero 945value then the delay has not yet expired and the callback should return 946-EAGAIN. 947