linux/Documentation/power/runtime_pm.txt
<<
>>
Prefs
   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(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
   6
   71. Introduction
   8
   9Support for runtime power management (runtime PM) of I/O devices is provided
  10at the power management core (PM core) level by means of:
  11
  12* The power management workqueue pm_wq in which bus types and device drivers can
  13  put their PM-related work items.  It is strongly recommended that pm_wq be
  14  used for queuing all work items related to runtime PM, because this allows
  15  them to be synchronized with system-wide power transitions (suspend to RAM,
  16  hibernation and resume from system sleep states).  pm_wq is declared in
  17  include/linux/pm_runtime.h and defined in kernel/power/main.c.
  18
  19* A number of runtime PM fields in the 'power' member of 'struct device' (which
  20  is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
  21  be used for synchronizing runtime PM operations with one another.
  22
  23* Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in
  24  include/linux/pm.h).
  25
  26* A set of helper functions defined in drivers/base/power/runtime.c that can be
  27  used for carrying out runtime PM operations in such a way that the
  28  synchronization between them is taken care of by the PM core.  Bus types and
  29  device drivers are encouraged to use these functions.
  30
  31The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
  32fields of 'struct dev_pm_info' and the core helper functions provided for
  33runtime PM are described below.
  34
  352. Device Runtime PM Callbacks
  36
  37There are three device runtime PM callbacks defined in 'struct dev_pm_ops':
  38
  39struct dev_pm_ops {
  40        ...
  41        int (*runtime_suspend)(struct device *dev);
  42        int (*runtime_resume)(struct device *dev);
  43        int (*runtime_idle)(struct device *dev);
  44        ...
  45};
  46
  47The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks
  48are executed by the PM core for the device's subsystem that may be either of
  49the following:
  50
  51  1. PM domain of the device, if the device's PM domain object, dev->pm_domain,
  52     is present.
  53
  54  2. Device type of the device, if both dev->type and dev->type->pm are present.
  55
  56  3. Device class of the device, if both dev->class and dev->class->pm are
  57     present.
  58
  59  4. Bus type of the device, if both dev->bus and dev->bus->pm are present.
  60
  61If the subsystem chosen by applying the above rules doesn't provide the relevant
  62callback, the PM core will invoke the corresponding driver callback stored in
  63dev->driver->pm directly (if present).
  64
  65The PM core always checks which callback to use in the order given above, so the
  66priority order of callbacks from high to low is: PM domain, device type, class
  67and bus type.  Moreover, the high-priority one will always take precedence over
  68a low-priority one.  The PM domain, bus type, device type and class callbacks
  69are referred to as subsystem-level callbacks in what follows.
  70
  71By default, the callbacks are always invoked in process context with interrupts
  72enabled.  However, the pm_runtime_irq_safe() helper function can be used to tell
  73the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
  74and ->runtime_idle() callbacks for the given device in atomic context with
  75interrupts disabled.  This implies that the callback routines in question must
  76not block or sleep, but it also means that the synchronous helper functions
  77listed at the end of Section 4 may be used for that device within an interrupt
  78handler or generally in an atomic context.
  79
  80The subsystem-level suspend callback, if present, is _entirely_ _responsible_
  81for handling the suspend of the device as appropriate, which may, but need not
  82include executing the device driver's own ->runtime_suspend() callback (from the
  83PM core's point of view it is not necessary to implement a ->runtime_suspend()
  84callback in a device driver as long as the subsystem-level suspend callback
  85knows what to do to handle the device).
  86
  87  * Once the subsystem-level suspend callback (or the driver suspend callback,
  88    if invoked directly) has completed successfully for the given device, the PM
  89    core regards the device as suspended, which need not mean that it has been
  90    put into a low power state.  It is supposed to mean, however, that the
  91    device will not process data and will not communicate with the CPU(s) and
  92    RAM until the appropriate resume callback is executed for it.  The runtime
  93    PM status of a device after successful execution of the suspend callback is
  94    'suspended'.
  95
  96  * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM
  97    status remains 'active', which means that the device _must_ be fully
  98    operational afterwards.
  99
 100  * If the suspend callback returns an error code different from -EBUSY and
 101    -EAGAIN, the PM core regards this as a fatal error and will refuse to run
 102    the helper functions described in Section 4 for the device until its status
 103    is directly set to  either'active', or 'suspended' (the PM core provides
 104    special helper functions for this purpose).
 105
 106In particular, if the driver requires remote wakeup capability (i.e. hardware
 107mechanism allowing the device to request a change of its power state, such as
 108PCI PME) for proper functioning and device_run_wake() returns 'false' for the
 109device, then ->runtime_suspend() should return -EBUSY.  On the other hand, if
 110device_run_wake() returns 'true' for the device and the device is put into a
 111low-power state during the execution of the suspend callback, it is expected
 112that remote wakeup will be enabled for the device.  Generally, remote wakeup
 113should be enabled for all input devices put into low-power states at run time.
 114
 115The subsystem-level resume callback, if present, is _entirely_ _responsible_ for
 116handling the resume of the device as appropriate, which may, but need not
 117include executing the device driver's own ->runtime_resume() callback (from the
 118PM core's point of view it is not necessary to implement a ->runtime_resume()
 119callback in a device driver as long as the subsystem-level resume callback knows
 120what to do to handle the device).
 121
 122  * Once the subsystem-level resume callback (or the driver resume callback, if
 123    invoked directly) has completed successfully, the PM core regards the device
 124    as fully operational, which means that the device _must_ be able to complete
 125    I/O operations as needed.  The runtime PM status of the device is then
 126    'active'.
 127
 128  * If the resume callback returns an error code, the PM core regards this as a
 129    fatal error and will refuse to run the helper functions described in Section
 130    4 for the device, until its status is directly set to either 'active', or
 131    'suspended' (by means of special helper functions provided by the PM core
 132    for this purpose).
 133
 134The idle callback (a subsystem-level one, if present, or the driver one) is
 135executed by the PM core whenever the device appears to be idle, which is
 136indicated to the PM core by two counters, the device's usage counter and the
 137counter of 'active' children of the device.
 138
 139  * If any of these counters is decreased using a helper function provided by
 140    the PM core and it turns out to be equal to zero, the other counter is
 141    checked.  If that counter also is equal to zero, the PM core executes the
 142    idle callback with the device as its argument.
 143
 144The action performed by the idle callback is totally dependent on the subsystem
 145(or driver) in question, but the expected and recommended action is to check
 146if the device can be suspended (i.e. if all of the conditions necessary for
 147suspending the device are satisfied) and to queue up a suspend request for the
 148device in that case.  If there is no idle callback, or if the callback returns
 1490, then the PM core will attempt to carry out a runtime suspend of the device,
 150also respecting devices configured for autosuspend.  In essence this means a
 151call to pm_runtime_autosuspend() (do note that drivers needs to update the
 152device last busy mark, pm_runtime_mark_last_busy(), to control the delay under
 153this circumstance).  To prevent this (for example, if the callback routine has
 154started a delayed suspend), the routine must return a non-zero value.  Negative
 155error return codes are ignored by the PM core.
 156
 157The helper functions provided by the PM core, described in Section 4, guarantee
 158that the following constraints are met with respect to runtime PM callbacks for
 159one device:
 160
 161(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
 162    ->runtime_suspend() in parallel with ->runtime_resume() or with another
 163    instance of ->runtime_suspend() for the same device) with the exception that
 164    ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
 165    ->runtime_idle() (although ->runtime_idle() will not be started while any
 166    of the other callbacks is being executed for the same device).
 167
 168(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
 169    devices (i.e. the PM core will only execute ->runtime_idle() or
 170    ->runtime_suspend() for the devices the runtime PM status of which is
 171    'active').
 172
 173(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
 174    the usage counter of which is equal to zero _and_ either the counter of
 175    'active' children of which is equal to zero, or the 'power.ignore_children'
 176    flag of which is set.
 177
 178(4) ->runtime_resume() can only be executed for 'suspended' devices  (i.e. the
 179    PM core will only execute ->runtime_resume() for the devices the runtime
 180    PM status of which is 'suspended').
 181
 182Additionally, the helper functions provided by the PM core obey the following
 183rules:
 184
 185  * If ->runtime_suspend() is about to be executed or there's a pending request
 186    to execute it, ->runtime_idle() will not be executed for the same device.
 187
 188  * A request to execute or to schedule the execution of ->runtime_suspend()
 189    will cancel any pending requests to execute ->runtime_idle() for the same
 190    device.
 191
 192  * If ->runtime_resume() is about to be executed or there's a pending request
 193    to execute it, the other callbacks will not be executed for the same device.
 194
 195  * A request to execute ->runtime_resume() will cancel any pending or
 196    scheduled requests to execute the other callbacks for the same device,
 197    except for scheduled autosuspends.
 198
 1993. Runtime PM Device Fields
 200
 201The following device runtime PM fields are present in 'struct dev_pm_info', as
 202defined in include/linux/pm.h:
 203
 204  struct timer_list suspend_timer;
 205    - timer used for scheduling (delayed) suspend and autosuspend requests
 206
 207  unsigned long timer_expires;
 208    - timer expiration time, in jiffies (if this is different from zero, the
 209      timer is running and will expire at that time, otherwise the timer is not
 210      running)
 211
 212  struct work_struct work;
 213    - work structure used for queuing up requests (i.e. work items in pm_wq)
 214
 215  wait_queue_head_t wait_queue;
 216    - wait queue used if any of the helper functions needs to wait for another
 217      one to complete
 218
 219  spinlock_t lock;
 220    - lock used for synchronisation
 221
 222  atomic_t usage_count;
 223    - the usage counter of the device
 224
 225  atomic_t child_count;
 226    - the count of 'active' children of the device
 227
 228  unsigned int ignore_children;
 229    - if set, the value of child_count is ignored (but still updated)
 230
 231  unsigned int disable_depth;
 232    - used for disabling the helper functions (they work normally if this is
 233      equal to zero); the initial value of it is 1 (i.e. runtime PM is
 234      initially disabled for all devices)
 235
 236  int runtime_error;
 237    - if set, there was a fatal error (one of the callbacks returned error code
 238      as described in Section 2), so the helper functions will not work until
 239      this flag is cleared; this is the error code returned by the failing
 240      callback
 241
 242  unsigned int idle_notification;
 243    - if set, ->runtime_idle() is being executed
 244
 245  unsigned int request_pending;
 246    - if set, there's a pending request (i.e. a work item queued up into pm_wq)
 247
 248  enum rpm_request request;
 249    - type of request that's pending (valid if request_pending is set)
 250
 251  unsigned int deferred_resume;
 252    - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
 253      being executed for that device and it is not practical to wait for the
 254      suspend to complete; means "start a resume as soon as you've suspended"
 255
 256  unsigned int run_wake;
 257    - set if the device is capable of generating runtime wake-up events
 258
 259  enum rpm_status runtime_status;
 260    - the runtime PM status of the device; this field's initial value is
 261      RPM_SUSPENDED, which means that each device is initially regarded by the
 262      PM core as 'suspended', regardless of its real hardware status
 263
 264  unsigned int runtime_auto;
 265    - if set, indicates that the user space has allowed the device driver to
 266      power manage the device at run time via the /sys/devices/.../power/control
 267      interface; it may only be modified with the help of the pm_runtime_allow()
 268      and pm_runtime_forbid() helper functions
 269
 270  unsigned int no_callbacks;
 271    - indicates that the device does not use the runtime PM callbacks (see
 272      Section 8); it may be modified only by the pm_runtime_no_callbacks()
 273      helper function
 274
 275  unsigned int irq_safe;
 276    - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
 277      will be invoked with the spinlock held and interrupts disabled
 278
 279  unsigned int use_autosuspend;
 280    - indicates that the device's driver supports delayed autosuspend (see
 281      Section 9); it may be modified only by the
 282      pm_runtime{_dont}_use_autosuspend() helper functions
 283
 284  unsigned int timer_autosuspends;
 285    - indicates that the PM core should attempt to carry out an autosuspend
 286      when the timer expires rather than a normal suspend
 287
 288  int autosuspend_delay;
 289    - the delay time (in milliseconds) to be used for autosuspend
 290
 291  unsigned long last_busy;
 292    - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
 293      function was last called for this device; used in calculating inactivity
 294      periods for autosuspend
 295
 296All of the above fields are members of the 'power' member of 'struct device'.
 297
 2984. Runtime PM Device Helper Functions
 299
 300The following runtime PM helper functions are defined in
 301drivers/base/power/runtime.c and include/linux/pm_runtime.h:
 302
 303  void pm_runtime_init(struct device *dev);
 304    - initialize the device runtime PM fields in 'struct dev_pm_info'
 305
 306  void pm_runtime_remove(struct device *dev);
 307    - make sure that the runtime PM of the device will be disabled after
 308      removing the device from device hierarchy
 309
 310  int pm_runtime_idle(struct device *dev);
 311    - execute the subsystem-level idle callback for the device; returns an
 312      error code on failure, where -EINPROGRESS means that ->runtime_idle() is
 313      already being executed; if there is no callback or the callback returns 0
 314      then run pm_runtime_autosuspend(dev) and return its result
 315
 316  int pm_runtime_suspend(struct device *dev);
 317    - execute the subsystem-level suspend callback for the device; returns 0 on
 318      success, 1 if the device's runtime PM status was already 'suspended', or
 319      error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
 320      to suspend the device again in future and -EACCES means that
 321      'power.disable_depth' is different from 0
 322
 323  int pm_runtime_autosuspend(struct device *dev);
 324    - same as pm_runtime_suspend() except that the autosuspend delay is taken
 325      into account; if pm_runtime_autosuspend_expiration() says the delay has
 326      not yet expired then an autosuspend is scheduled for the appropriate time
 327      and 0 is returned
 328
 329  int pm_runtime_resume(struct device *dev);
 330    - execute the subsystem-level resume callback for the device; returns 0 on
 331      success, 1 if the device's runtime PM status was already 'active' or
 332      error code on failure, where -EAGAIN means it may be safe to attempt to
 333      resume the device again in future, but 'power.runtime_error' should be
 334      checked additionally, and -EACCES means that 'power.disable_depth' is
 335      different from 0
 336
 337  int pm_request_idle(struct device *dev);
 338    - submit a request to execute the subsystem-level idle callback for the
 339      device (the request is represented by a work item in pm_wq); returns 0 on
 340      success or error code if the request has not been queued up
 341
 342  int pm_request_autosuspend(struct device *dev);
 343    - schedule the execution of the subsystem-level suspend callback for the
 344      device when the autosuspend delay has expired; if the delay has already
 345      expired then the work item is queued up immediately
 346
 347  int pm_schedule_suspend(struct device *dev, unsigned int delay);
 348    - schedule the execution of the subsystem-level suspend callback for the
 349      device in future, where 'delay' is the time to wait before queuing up a
 350      suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
 351      item is queued up immediately); returns 0 on success, 1 if the device's PM
 352      runtime status was already 'suspended', or error code if the request
 353      hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
 354      ->runtime_suspend() is already scheduled and not yet expired, the new
 355      value of 'delay' will be used as the time to wait
 356
 357  int pm_request_resume(struct device *dev);
 358    - submit a request to execute the subsystem-level resume callback for the
 359      device (the request is represented by a work item in pm_wq); returns 0 on
 360      success, 1 if the device's runtime PM status was already 'active', or
 361      error code if the request hasn't been queued up
 362
 363  void pm_runtime_get_noresume(struct device *dev);
 364    - increment the device's usage counter
 365
 366  int pm_runtime_get(struct device *dev);
 367    - increment the device's usage counter, run pm_request_resume(dev) and
 368      return its result
 369
 370  int pm_runtime_get_sync(struct device *dev);
 371    - increment the device's usage counter, run pm_runtime_resume(dev) and
 372      return its result
 373
 374  int pm_runtime_get_if_in_use(struct device *dev);
 375    - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
 376      runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
 377      nonzero, increment the counter and return 1; otherwise return 0 without
 378      changing the counter
 379
 380  void pm_runtime_put_noidle(struct device *dev);
 381    - decrement the device's usage counter
 382
 383  int pm_runtime_put(struct device *dev);
 384    - decrement the device's usage counter; if the result is 0 then run
 385      pm_request_idle(dev) and return its result
 386
 387  int pm_runtime_put_autosuspend(struct device *dev);
 388    - decrement the device's usage counter; if the result is 0 then run
 389      pm_request_autosuspend(dev) and return its result
 390
 391  int pm_runtime_put_sync(struct device *dev);
 392    - decrement the device's usage counter; if the result is 0 then run
 393      pm_runtime_idle(dev) and return its result
 394
 395  int pm_runtime_put_sync_suspend(struct device *dev);
 396    - decrement the device's usage counter; if the result is 0 then run
 397      pm_runtime_suspend(dev) and return its result
 398
 399  int pm_runtime_put_sync_autosuspend(struct device *dev);
 400    - decrement the device's usage counter; if the result is 0 then run
 401      pm_runtime_autosuspend(dev) and return its result
 402
 403  void pm_runtime_enable(struct device *dev);
 404    - decrement the device's 'power.disable_depth' field; if that field is equal
 405      to zero, the runtime PM helper functions can execute subsystem-level
 406      callbacks described in Section 2 for the device
 407
 408  int pm_runtime_disable(struct device *dev);
 409    - increment the device's 'power.disable_depth' field (if the value of that
 410      field was previously zero, this prevents subsystem-level runtime PM
 411      callbacks from being run for the device), make sure that all of the
 412      pending runtime PM operations on the device are either completed or
 413      canceled; returns 1 if there was a resume request pending and it was
 414      necessary to execute the subsystem-level resume callback for the device
 415      to satisfy that request, otherwise 0 is returned
 416
 417  int pm_runtime_barrier(struct device *dev);
 418    - check if there's a resume request pending for the device and resume it
 419      (synchronously) in that case, cancel any other pending runtime PM requests
 420      regarding it and wait for all runtime PM operations on it in progress to
 421      complete; returns 1 if there was a resume request pending and it was
 422      necessary to execute the subsystem-level resume callback for the device to
 423      satisfy that request, otherwise 0 is returned
 424
 425  void pm_suspend_ignore_children(struct device *dev, bool enable);
 426    - set/unset the power.ignore_children flag of the device
 427
 428  int pm_runtime_set_active(struct device *dev);
 429    - clear the device's 'power.runtime_error' flag, set the device's runtime
 430      PM status to 'active' and update its parent's counter of 'active'
 431      children as appropriate (it is only valid to use this function if
 432      'power.runtime_error' is set or 'power.disable_depth' is greater than
 433      zero); it will fail and return error code if the device has a parent
 434      which is not active and the 'power.ignore_children' flag of which is unset
 435
 436  void pm_runtime_set_suspended(struct device *dev);
 437    - clear the device's 'power.runtime_error' flag, set the device's runtime
 438      PM status to 'suspended' and update its parent's counter of 'active'
 439      children as appropriate (it is only valid to use this function if
 440      'power.runtime_error' is set or 'power.disable_depth' is greater than
 441      zero)
 442
 443  bool pm_runtime_active(struct device *dev);
 444    - return true if the device's runtime PM status is 'active' or its
 445      'power.disable_depth' field is not equal to zero, or false otherwise
 446
 447  bool pm_runtime_suspended(struct device *dev);
 448    - return true if the device's runtime PM status is 'suspended' and its
 449      'power.disable_depth' field is equal to zero, or false otherwise
 450
 451  bool pm_runtime_status_suspended(struct device *dev);
 452    - return true if the device's runtime PM status is 'suspended'
 453
 454  void pm_runtime_allow(struct device *dev);
 455    - set the power.runtime_auto flag for the device and decrease its usage
 456      counter (used by the /sys/devices/.../power/control interface to
 457      effectively allow the device to be power managed at run time)
 458
 459  void pm_runtime_forbid(struct device *dev);
 460    - unset the power.runtime_auto flag for the device and increase its usage
 461      counter (used by the /sys/devices/.../power/control interface to
 462      effectively prevent the device from being power managed at run time)
 463
 464  void pm_runtime_no_callbacks(struct device *dev);
 465    - set the power.no_callbacks flag for the device and remove the runtime
 466      PM attributes from /sys/devices/.../power (or prevent them from being
 467      added when the device is registered)
 468
 469  void pm_runtime_irq_safe(struct device *dev);
 470    - set the power.irq_safe flag for the device, causing the runtime-PM
 471      callbacks to be invoked with interrupts off
 472
 473  bool pm_runtime_is_irq_safe(struct device *dev);
 474    - return true if power.irq_safe flag was set for the device, causing
 475      the runtime-PM callbacks to be invoked with interrupts off
 476
 477  void pm_runtime_mark_last_busy(struct device *dev);
 478    - set the power.last_busy field to the current time
 479
 480  void pm_runtime_use_autosuspend(struct device *dev);
 481    - set the power.use_autosuspend flag, enabling autosuspend delays
 482
 483  void pm_runtime_dont_use_autosuspend(struct device *dev);
 484    - clear the power.use_autosuspend flag, disabling autosuspend delays
 485
 486  void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
 487    - set the power.autosuspend_delay value to 'delay' (expressed in
 488      milliseconds); if 'delay' is negative then runtime suspends are
 489      prevented
 490
 491  unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
 492    - calculate the time when the current autosuspend delay period will expire,
 493      based on power.last_busy and power.autosuspend_delay; if the delay time
 494      is 1000 ms or larger then the expiration time is rounded up to the
 495      nearest second; returns 0 if the delay period has already expired or
 496      power.use_autosuspend isn't set, otherwise returns the expiration time
 497      in jiffies
 498
 499It is safe to execute the following helper functions from interrupt context:
 500
 501pm_request_idle()
 502pm_request_autosuspend()
 503pm_schedule_suspend()
 504pm_request_resume()
 505pm_runtime_get_noresume()
 506pm_runtime_get()
 507pm_runtime_put_noidle()
 508pm_runtime_put()
 509pm_runtime_put_autosuspend()
 510pm_runtime_enable()
 511pm_suspend_ignore_children()
 512pm_runtime_set_active()
 513pm_runtime_set_suspended()
 514pm_runtime_suspended()
 515pm_runtime_mark_last_busy()
 516pm_runtime_autosuspend_expiration()
 517
 518If pm_runtime_irq_safe() has been called for a device then the following helper
 519functions may also be used in interrupt context:
 520
 521pm_runtime_idle()
 522pm_runtime_suspend()
 523pm_runtime_autosuspend()
 524pm_runtime_resume()
 525pm_runtime_get_sync()
 526pm_runtime_put_sync()
 527pm_runtime_put_sync_suspend()
 528pm_runtime_put_sync_autosuspend()
 529
 5305. Runtime PM Initialization, Device Probing and Removal
 531
 532Initially, the runtime PM is disabled for all devices, which means that the
 533majority of the runtime PM helper functions described in Section 4 will return
 534-EAGAIN until pm_runtime_enable() is called for the device.
 535
 536In addition to that, the initial runtime PM status of all devices is
 537'suspended', but it need not reflect the actual physical state of the device.
 538Thus, if the device is initially active (i.e. it is able to process I/O), its
 539runtime PM status must be changed to 'active', with the help of
 540pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
 541
 542However, if the device has a parent and the parent's runtime PM is enabled,
 543calling pm_runtime_set_active() for the device will affect the parent, unless
 544the parent's 'power.ignore_children' flag is set.  Namely, in that case the
 545parent won't be able to suspend at run time, using the PM core's helper
 546functions, as long as the child's status is 'active', even if the child's
 547runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
 548the child yet or pm_runtime_disable() has been called for it).  For this reason,
 549once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
 550should be called for it too as soon as reasonably possible or its runtime PM
 551status should be changed back to 'suspended' with the help of
 552pm_runtime_set_suspended().
 553
 554If the default initial runtime PM status of the device (i.e. 'suspended')
 555reflects the actual state of the device, its bus type's or its driver's
 556->probe() callback will likely need to wake it up using one of the PM core's
 557helper functions described in Section 4.  In that case, pm_runtime_resume()
 558should be used.  Of course, for this purpose the device's runtime PM has to be
 559enabled earlier by calling pm_runtime_enable().
 560
 561Note, if the device may execute pm_runtime calls during the probe (such as
 562if it is registers with a subsystem that may call back in) then the
 563pm_runtime_get_sync() call paired with a pm_runtime_put() call will be
 564appropriate to ensure that the device is not put back to sleep during the
 565probe. This can happen with systems such as the network device layer.
 566
 567It may be desirable to suspend the device once ->probe() has finished.
 568Therefore the driver core uses the asyncronous pm_request_idle() to submit a
 569request to execute the subsystem-level idle callback for the device at that
 570time.  A driver that makes use of the runtime autosuspend feature, may want to
 571update the last busy mark before returning from ->probe().
 572
 573Moreover, the driver core prevents runtime PM callbacks from racing with the bus
 574notifier callback in __device_release_driver(), which is necessary, because the
 575notifier is used by some subsystems to carry out operations affecting the
 576runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
 577driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications.  This
 578resumes the device if it's in the suspended state and prevents it from
 579being suspended again while those routines are being executed.
 580
 581To allow bus types and drivers to put devices into the suspended state by
 582calling pm_runtime_suspend() from their ->remove() routines, the driver core
 583executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
 584notifications in __device_release_driver().  This requires bus types and
 585drivers to make their ->remove() callbacks avoid races with runtime PM directly,
 586but also it allows of more flexibility in the handling of devices during the
 587removal of their drivers.
 588
 589Drivers in ->remove() callback should undo the runtime PM changes done
 590in ->probe(). Usually this means calling pm_runtime_disable(),
 591pm_runtime_dont_use_autosuspend() etc.
 592
 593The user space can effectively disallow the driver of the device to power manage
 594it at run time by changing the value of its /sys/devices/.../power/control
 595attribute to "on", which causes pm_runtime_forbid() to be called.  In principle,
 596this mechanism may also be used by the driver to effectively turn off the
 597runtime power management of the device until the user space turns it on.
 598Namely, during the initialization the driver can make sure that the runtime PM
 599status of the device is 'active' and call pm_runtime_forbid().  It should be
 600noted, however, that if the user space has already intentionally changed the
 601value of /sys/devices/.../power/control to "auto" to allow the driver to power
 602manage the device at run time, the driver may confuse it by using
 603pm_runtime_forbid() this way.
 604
 6056. Runtime PM and System Sleep
 606
 607Runtime PM and system sleep (i.e., system suspend and hibernation, also known
 608as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
 609ways.  If a device is active when a system sleep starts, everything is
 610straightforward.  But what should happen if the device is already suspended?
 611
 612The device may have different wake-up settings for runtime PM and system sleep.
 613For example, remote wake-up may be enabled for runtime suspend but disallowed
 614for system sleep (device_may_wakeup(dev) returns 'false').  When this happens,
 615the subsystem-level system suspend callback is responsible for changing the
 616device's wake-up setting (it may leave that to the device driver's system
 617suspend routine).  It may be necessary to resume the device and suspend it again
 618in order to do so.  The same is true if the driver uses different power levels
 619or other settings for runtime suspend and system sleep.
 620
 621During system resume, the simplest approach is to bring all devices back to full
 622power, even if they had been suspended before the system suspend began.  There
 623are several reasons for this, including:
 624
 625  * The device might need to switch power levels, wake-up settings, etc.
 626
 627  * Remote wake-up events might have been lost by the firmware.
 628
 629  * The device's children may need the device to be at full power in order
 630    to resume themselves.
 631
 632  * The driver's idea of the device state may not agree with the device's
 633    physical state.  This can happen during resume from hibernation.
 634
 635  * The device might need to be reset.
 636
 637  * Even though the device was suspended, if its usage counter was > 0 then most
 638    likely it would need a runtime resume in the near future anyway.
 639
 640If the device had been suspended before the system suspend began and it's
 641brought back to full power during resume, then its runtime PM status will have
 642to be updated to reflect the actual post-system sleep status.  The way to do
 643this is:
 644
 645        pm_runtime_disable(dev);
 646        pm_runtime_set_active(dev);
 647        pm_runtime_enable(dev);
 648
 649The PM core always increments the runtime usage counter before calling the
 650->suspend() callback and decrements it after calling the ->resume() callback.
 651Hence disabling runtime PM temporarily like this will not cause any runtime
 652suspend attempts to be permanently lost.  If the usage count goes to zero
 653following the return of the ->resume() callback, the ->runtime_idle() callback
 654will be invoked as usual.
 655
 656On some systems, however, system sleep is not entered through a global firmware
 657or hardware operation.  Instead, all hardware components are put into low-power
 658states directly by the kernel in a coordinated way.  Then, the system sleep
 659state effectively follows from the states the hardware components end up in
 660and the system is woken up from that state by a hardware interrupt or a similar
 661mechanism entirely under the kernel's control.  As a result, the kernel never
 662gives control away and the states of all devices during resume are precisely
 663known to it.  If that is the case and none of the situations listed above takes
 664place (in particular, if the system is not waking up from hibernation), it may
 665be more efficient to leave the devices that had been suspended before the system
 666suspend began in the suspended state.
 667
 668To this end, the PM core provides a mechanism allowing some coordination between
 669different levels of device hierarchy.  Namely, if a system suspend .prepare()
 670callback returns a positive number for a device, that indicates to the PM core
 671that the device appears to be runtime-suspended and its state is fine, so it
 672may be left in runtime suspend provided that all of its descendants are also
 673left in runtime suspend.  If that happens, the PM core will not execute any
 674system suspend and resume callbacks for all of those devices, except for the
 675complete callback, which is then entirely responsible for handling the device
 676as appropriate.  This only applies to system suspend transitions that are not
 677related to hibernation (see Documentation/power/admin-guide/devices.rst for more
 678information).
 679
 680The PM core does its best to reduce the probability of race conditions between
 681the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
 682out the following operations:
 683
 684  * During system suspend pm_runtime_get_noresume() is called for every device
 685    right before executing the subsystem-level .prepare() callback for it and
 686    pm_runtime_barrier() is called for every device right before executing the
 687    subsystem-level .suspend() callback for it.  In addition to that the PM core
 688    calls  __pm_runtime_disable() with 'false' as the second argument for every
 689    device right before executing the subsystem-level .suspend_late() callback
 690    for it.
 691
 692  * During system resume pm_runtime_enable() and pm_runtime_put() are called for
 693    every device right after executing the subsystem-level .resume_early()
 694    callback and right after executing the subsystem-level .complete() callback
 695    for it, respectively.
 696
 6977. Generic subsystem callbacks
 698
 699Subsystems may wish to conserve code space by using the set of generic power
 700management callbacks provided by the PM core, defined in
 701driver/base/power/generic_ops.c:
 702
 703  int pm_generic_runtime_suspend(struct device *dev);
 704    - invoke the ->runtime_suspend() callback provided by the driver of this
 705      device and return its result, or return 0 if not defined
 706
 707  int pm_generic_runtime_resume(struct device *dev);
 708    - invoke the ->runtime_resume() callback provided by the driver of this
 709      device and return its result, or return 0 if not defined
 710
 711  int pm_generic_suspend(struct device *dev);
 712    - if the device has not been suspended at run time, invoke the ->suspend()
 713      callback provided by its driver and return its result, or return 0 if not
 714      defined
 715
 716  int pm_generic_suspend_noirq(struct device *dev);
 717    - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
 718      callback provided by the device's driver and return its result, or return
 719      0 if not defined
 720
 721  int pm_generic_resume(struct device *dev);
 722    - invoke the ->resume() callback provided by the driver of this device and,
 723      if successful, change the device's runtime PM status to 'active'
 724
 725  int pm_generic_resume_noirq(struct device *dev);
 726    - invoke the ->resume_noirq() callback provided by the driver of this device
 727
 728  int pm_generic_freeze(struct device *dev);
 729    - if the device has not been suspended at run time, invoke the ->freeze()
 730      callback provided by its driver and return its result, or return 0 if not
 731      defined
 732
 733  int pm_generic_freeze_noirq(struct device *dev);
 734    - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
 735      callback provided by the device's driver and return its result, or return
 736      0 if not defined
 737
 738  int pm_generic_thaw(struct device *dev);
 739    - if the device has not been suspended at run time, invoke the ->thaw()
 740      callback provided by its driver and return its result, or return 0 if not
 741      defined
 742
 743  int pm_generic_thaw_noirq(struct device *dev);
 744    - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
 745      callback provided by the device's driver and return its result, or return
 746      0 if not defined
 747
 748  int pm_generic_poweroff(struct device *dev);
 749    - if the device has not been suspended at run time, invoke the ->poweroff()
 750      callback provided by its driver and return its result, or return 0 if not
 751      defined
 752
 753  int pm_generic_poweroff_noirq(struct device *dev);
 754    - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
 755      callback provided by the device's driver and return its result, or return
 756      0 if not defined
 757
 758  int pm_generic_restore(struct device *dev);
 759    - invoke the ->restore() callback provided by the driver of this device and,
 760      if successful, change the device's runtime PM status to 'active'
 761
 762  int pm_generic_restore_noirq(struct device *dev);
 763    - invoke the ->restore_noirq() callback provided by the device's driver
 764
 765These functions are the defaults used by the PM core, if a subsystem doesn't
 766provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
 767->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
 768->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
 769->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
 770subsystem-level dev_pm_ops structure.
 771
 772Device drivers that wish to use the same function as a system suspend, freeze,
 773poweroff and runtime suspend callback, and similarly for system resume, thaw,
 774restore, and runtime resume, can achieve this with the help of the
 775UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
 776last argument to NULL).
 777
 7788. "No-Callback" Devices
 779
 780Some "devices" are only logical sub-devices of their parent and cannot be
 781power-managed on their own.  (The prototype example is a USB interface.  Entire
 782USB devices can go into low-power mode or send wake-up requests, but neither is
 783possible for individual interfaces.)  The drivers for these devices have no
 784need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
 785and ->runtime_resume() would always return 0 without doing anything else and
 786->runtime_idle() would always call pm_runtime_suspend().
 787
 788Subsystems can tell the PM core about these devices by calling
 789pm_runtime_no_callbacks().  This should be done after the device structure is
 790initialized and before it is registered (although after device registration is
 791also okay).  The routine will set the device's power.no_callbacks flag and
 792prevent the non-debugging runtime PM sysfs attributes from being created.
 793
 794When power.no_callbacks is set, the PM core will not invoke the
 795->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
 796Instead it will assume that suspends and resumes always succeed and that idle
 797devices should be suspended.
 798
 799As a consequence, the PM core will never directly inform the device's subsystem
 800or driver about runtime power changes.  Instead, the driver for the device's
 801parent must take responsibility for telling the device's driver when the
 802parent's power state changes.
 803
 8049. Autosuspend, or automatically-delayed suspends
 805
 806Changing a device's power state isn't free; it requires both time and energy.
 807A device should be put in a low-power state only when there's some reason to
 808think it will remain in that state for a substantial time.  A common heuristic
 809says that a device which hasn't been used for a while is liable to remain
 810unused; following this advice, drivers should not allow devices to be suspended
 811at runtime until they have been inactive for some minimum period.  Even when
 812the heuristic ends up being non-optimal, it will still prevent devices from
 813"bouncing" too rapidly between low-power and full-power states.
 814
 815The term "autosuspend" is an historical remnant.  It doesn't mean that the
 816device is automatically suspended (the subsystem or driver still has to call
 817the appropriate PM routines); rather it means that runtime suspends will
 818automatically be delayed until the desired period of inactivity has elapsed.
 819
 820Inactivity is determined based on the power.last_busy field.  Drivers should
 821call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
 822typically just before calling pm_runtime_put_autosuspend().  The desired length
 823of the inactivity period is a matter of policy.  Subsystems can set this length
 824initially by calling pm_runtime_set_autosuspend_delay(), but after device
 825registration the length should be controlled by user space, using the
 826/sys/devices/.../power/autosuspend_delay_ms attribute.
 827
 828In order to use autosuspend, subsystems or drivers must call
 829pm_runtime_use_autosuspend() (preferably before registering the device), and
 830thereafter they should use the various *_autosuspend() helper functions instead
 831of the non-autosuspend counterparts:
 832
 833        Instead of: pm_runtime_suspend    use: pm_runtime_autosuspend;
 834        Instead of: pm_schedule_suspend   use: pm_request_autosuspend;
 835        Instead of: pm_runtime_put        use: pm_runtime_put_autosuspend;
 836        Instead of: pm_runtime_put_sync   use: pm_runtime_put_sync_autosuspend.
 837
 838Drivers may also continue to use the non-autosuspend helper functions; they
 839will behave normally, not taking the autosuspend delay into account.
 840Similarly, if the power.use_autosuspend field isn't set then the autosuspend
 841helper functions will behave just like the non-autosuspend counterparts.
 842
 843Under some circumstances a driver or subsystem may want to prevent a device
 844from autosuspending immediately, even though the usage counter is zero and the
 845autosuspend delay time has expired.  If the ->runtime_suspend() callback
 846returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
 847in the future (as it normally would be if the callback invoked
 848pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
 849autosuspend.  The ->runtime_suspend() callback can't do this rescheduling
 850itself because no suspend requests of any kind are accepted while the device is
 851suspending (i.e., while the callback is running).
 852
 853The implementation is well suited for asynchronous use in interrupt contexts.
 854However such use inevitably involves races, because the PM core can't
 855synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
 856This synchronization must be handled by the driver, using its private lock.
 857Here is a schematic pseudo-code example:
 858
 859        foo_read_or_write(struct foo_priv *foo, void *data)
 860        {
 861                lock(&foo->private_lock);
 862                add_request_to_io_queue(foo, data);
 863                if (foo->num_pending_requests++ == 0)
 864                        pm_runtime_get(&foo->dev);
 865                if (!foo->is_suspended)
 866                        foo_process_next_request(foo);
 867                unlock(&foo->private_lock);
 868        }
 869
 870        foo_io_completion(struct foo_priv *foo, void *req)
 871        {
 872                lock(&foo->private_lock);
 873                if (--foo->num_pending_requests == 0) {
 874                        pm_runtime_mark_last_busy(&foo->dev);
 875                        pm_runtime_put_autosuspend(&foo->dev);
 876                } else {
 877                        foo_process_next_request(foo);
 878                }
 879                unlock(&foo->private_lock);
 880                /* Send req result back to the user ... */
 881        }
 882
 883        int foo_runtime_suspend(struct device *dev)
 884        {
 885                struct foo_priv foo = container_of(dev, ...);
 886                int ret = 0;
 887
 888                lock(&foo->private_lock);
 889                if (foo->num_pending_requests > 0) {
 890                        ret = -EBUSY;
 891                } else {
 892                        /* ... suspend the device ... */
 893                        foo->is_suspended = 1;
 894                }
 895                unlock(&foo->private_lock);
 896                return ret;
 897        }
 898
 899        int foo_runtime_resume(struct device *dev)
 900        {
 901                struct foo_priv foo = container_of(dev, ...);
 902
 903                lock(&foo->private_lock);
 904                /* ... resume the device ... */
 905                foo->is_suspended = 0;
 906                pm_runtime_mark_last_busy(&foo->dev);
 907                if (foo->num_pending_requests > 0)
 908                        foo_process_next_request(foo);
 909                unlock(&foo->private_lock);
 910                return 0;
 911        }
 912
 913The important point is that after foo_io_completion() asks for an autosuspend,
 914the foo_runtime_suspend() callback may race with foo_read_or_write().
 915Therefore foo_runtime_suspend() has to check whether there are any pending I/O
 916requests (while holding the private lock) before allowing the suspend to
 917proceed.
 918
 919In addition, the power.autosuspend_delay field can be changed by user space at
 920any time.  If a driver cares about this, it can call
 921pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
 922callback while holding its private lock.  If the function returns a nonzero
 923value then the delay has not yet expired and the callback should return
 924-EAGAIN.
 925
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.