linux/Documentation/power/runtime_pm.txt
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   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_can_wakeup() returns 'false' for the
 109device, then ->runtime_suspend() should return -EBUSY.  On the other hand, if
 110device_can_wakeup() 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 synchronization
 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  enum rpm_status runtime_status;
 257    - the runtime PM status of the device; this field's initial value is
 258      RPM_SUSPENDED, which means that each device is initially regarded by the
 259      PM core as 'suspended', regardless of its real hardware status
 260
 261  unsigned int runtime_auto;
 262    - if set, indicates that the user space has allowed the device driver to
 263      power manage the device at run time via the /sys/devices/.../power/control
 264      interface; it may only be modified with the help of the pm_runtime_allow()
 265      and pm_runtime_forbid() helper functions
 266
 267  unsigned int no_callbacks;
 268    - indicates that the device does not use the runtime PM callbacks (see
 269      Section 8); it may be modified only by the pm_runtime_no_callbacks()
 270      helper function
 271
 272  unsigned int irq_safe;
 273    - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
 274      will be invoked with the spinlock held and interrupts disabled
 275
 276  unsigned int use_autosuspend;
 277    - indicates that the device's driver supports delayed autosuspend (see
 278      Section 9); it may be modified only by the
 279      pm_runtime{_dont}_use_autosuspend() helper functions
 280
 281  unsigned int timer_autosuspends;
 282    - indicates that the PM core should attempt to carry out an autosuspend
 283      when the timer expires rather than a normal suspend
 284
 285  int autosuspend_delay;
 286    - the delay time (in milliseconds) to be used for autosuspend
 287
 288  unsigned long last_busy;
 289    - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
 290      function was last called for this device; used in calculating inactivity
 291      periods for autosuspend
 292
 293All of the above fields are members of the 'power' member of 'struct device'.
 294
 2954. Runtime PM Device Helper Functions
 296
 297The following runtime PM helper functions are defined in
 298drivers/base/power/runtime.c and include/linux/pm_runtime.h:
 299
 300  void pm_runtime_init(struct device *dev);
 301    - initialize the device runtime PM fields in 'struct dev_pm_info'
 302
 303  void pm_runtime_remove(struct device *dev);
 304    - make sure that the runtime PM of the device will be disabled after
 305      removing the device from device hierarchy
 306
 307  int pm_runtime_idle(struct device *dev);
 308    - execute the subsystem-level idle callback for the device; returns an
 309      error code on failure, where -EINPROGRESS means that ->runtime_idle() is
 310      already being executed; if there is no callback or the callback returns 0
 311      then run pm_runtime_autosuspend(dev) and return its result
 312
 313  int pm_runtime_suspend(struct device *dev);
 314    - execute the subsystem-level suspend callback for the device; returns 0 on
 315      success, 1 if the device's runtime PM status was already 'suspended', or
 316      error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
 317      to suspend the device again in future and -EACCES means that
 318      'power.disable_depth' is different from 0
 319
 320  int pm_runtime_autosuspend(struct device *dev);
 321    - same as pm_runtime_suspend() except that the autosuspend delay is taken
 322      into account; if pm_runtime_autosuspend_expiration() says the delay has
 323      not yet expired then an autosuspend is scheduled for the appropriate time
 324      and 0 is returned
 325
 326  int pm_runtime_resume(struct device *dev);
 327    - execute the subsystem-level resume callback for the device; returns 0 on
 328      success, 1 if the device's runtime PM status was already 'active' or
 329      error code on failure, where -EAGAIN means it may be safe to attempt to
 330      resume the device again in future, but 'power.runtime_error' should be
 331      checked additionally, and -EACCES means that 'power.disable_depth' is
 332      different from 0
 333
 334  int pm_request_idle(struct device *dev);
 335    - submit a request to execute the subsystem-level idle callback for the
 336      device (the request is represented by a work item in pm_wq); returns 0 on
 337      success or error code if the request has not been queued up
 338
 339  int pm_request_autosuspend(struct device *dev);
 340    - schedule the execution of the subsystem-level suspend callback for the
 341      device when the autosuspend delay has expired; if the delay has already
 342      expired then the work item is queued up immediately
 343
 344  int pm_schedule_suspend(struct device *dev, unsigned int delay);
 345    - schedule the execution of the subsystem-level suspend callback for the
 346      device in future, where 'delay' is the time to wait before queuing up a
 347      suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
 348      item is queued up immediately); returns 0 on success, 1 if the device's PM
 349      runtime status was already 'suspended', or error code if the request
 350      hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
 351      ->runtime_suspend() is already scheduled and not yet expired, the new
 352      value of 'delay' will be used as the time to wait
 353
 354  int pm_request_resume(struct device *dev);
 355    - submit a request to execute the subsystem-level resume callback for the
 356      device (the request is represented by a work item in pm_wq); returns 0 on
 357      success, 1 if the device's runtime PM status was already 'active', or
 358      error code if the request hasn't been queued up
 359
 360  void pm_runtime_get_noresume(struct device *dev);
 361    - increment the device's usage counter
 362
 363  int pm_runtime_get(struct device *dev);
 364    - increment the device's usage counter, run pm_request_resume(dev) and
 365      return its result
 366
 367  int pm_runtime_get_sync(struct device *dev);
 368    - increment the device's usage counter, run pm_runtime_resume(dev) and
 369      return its result
 370
 371  int pm_runtime_get_if_in_use(struct device *dev);
 372    - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
 373      runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
 374      nonzero, increment the counter and return 1; otherwise return 0 without
 375      changing the counter
 376
 377  void pm_runtime_put_noidle(struct device *dev);
 378    - decrement the device's usage counter
 379
 380  int pm_runtime_put(struct device *dev);
 381    - decrement the device's usage counter; if the result is 0 then run
 382      pm_request_idle(dev) and return its result
 383
 384  int pm_runtime_put_autosuspend(struct device *dev);
 385    - decrement the device's usage counter; if the result is 0 then run
 386      pm_request_autosuspend(dev) and return its result
 387
 388  int pm_runtime_put_sync(struct device *dev);
 389    - decrement the device's usage counter; if the result is 0 then run
 390      pm_runtime_idle(dev) and return its result
 391
 392  int pm_runtime_put_sync_suspend(struct device *dev);
 393    - decrement the device's usage counter; if the result is 0 then run
 394      pm_runtime_suspend(dev) and return its result
 395
 396  int pm_runtime_put_sync_autosuspend(struct device *dev);
 397    - decrement the device's usage counter; if the result is 0 then run
 398      pm_runtime_autosuspend(dev) and return its result
 399
 400  void pm_runtime_enable(struct device *dev);
 401    - decrement the device's 'power.disable_depth' field; if that field is equal
 402      to zero, the runtime PM helper functions can execute subsystem-level
 403      callbacks described in Section 2 for the device
 404
 405  int pm_runtime_disable(struct device *dev);
 406    - increment the device's 'power.disable_depth' field (if the value of that
 407      field was previously zero, this prevents subsystem-level runtime PM
 408      callbacks from being run for the device), make sure that all of the
 409      pending runtime PM operations on the device are either completed or
 410      canceled; returns 1 if there was a resume request pending and it was
 411      necessary to execute the subsystem-level resume callback for the device
 412      to satisfy that request, otherwise 0 is returned
 413
 414  int pm_runtime_barrier(struct device *dev);
 415    - check if there's a resume request pending for the device and resume it
 416      (synchronously) in that case, cancel any other pending runtime PM requests
 417      regarding it and wait for all runtime PM operations on it in progress to
 418      complete; returns 1 if there was a resume request pending and it was
 419      necessary to execute the subsystem-level resume callback for the device to
 420      satisfy that request, otherwise 0 is returned
 421
 422  void pm_suspend_ignore_children(struct device *dev, bool enable);
 423    - set/unset the power.ignore_children flag of the device
 424
 425  int pm_runtime_set_active(struct device *dev);
 426    - clear the device's 'power.runtime_error' flag, set the device's runtime
 427      PM status to 'active' and update its parent's counter of 'active'
 428      children as appropriate (it is only valid to use this function if
 429      'power.runtime_error' is set or 'power.disable_depth' is greater than
 430      zero); it will fail and return error code if the device has a parent
 431      which is not active and the 'power.ignore_children' flag of which is unset
 432
 433  void pm_runtime_set_suspended(struct device *dev);
 434    - clear the device's 'power.runtime_error' flag, set the device's runtime
 435      PM status to 'suspended' and update its parent's counter of 'active'
 436      children as appropriate (it is only valid to use this function if
 437      'power.runtime_error' is set or 'power.disable_depth' is greater than
 438      zero)
 439
 440  bool pm_runtime_active(struct device *dev);
 441    - return true if the device's runtime PM status is 'active' or its
 442      'power.disable_depth' field is not equal to zero, or false otherwise
 443
 444  bool pm_runtime_suspended(struct device *dev);
 445    - return true if the device's runtime PM status is 'suspended' and its
 446      'power.disable_depth' field is equal to zero, or false otherwise
 447
 448  bool pm_runtime_status_suspended(struct device *dev);
 449    - return true if the device's runtime PM status is 'suspended'
 450
 451  void pm_runtime_allow(struct device *dev);
 452    - set the power.runtime_auto flag for the device and decrease its usage
 453      counter (used by the /sys/devices/.../power/control interface to
 454      effectively allow the device to be power managed at run time)
 455
 456  void pm_runtime_forbid(struct device *dev);
 457    - unset the power.runtime_auto flag for the device and increase its usage
 458      counter (used by the /sys/devices/.../power/control interface to
 459      effectively prevent the device from being power managed at run time)
 460
 461  void pm_runtime_no_callbacks(struct device *dev);
 462    - set the power.no_callbacks flag for the device and remove the runtime
 463      PM attributes from /sys/devices/.../power (or prevent them from being
 464      added when the device is registered)
 465
 466  void pm_runtime_irq_safe(struct device *dev);
 467    - set the power.irq_safe flag for the device, causing the runtime-PM
 468      callbacks to be invoked with interrupts off
 469
 470  bool pm_runtime_is_irq_safe(struct device *dev);
 471    - return true if power.irq_safe flag was set for the device, causing
 472      the runtime-PM callbacks to be invoked with interrupts off
 473
 474  void pm_runtime_mark_last_busy(struct device *dev);
 475    - set the power.last_busy field to the current time
 476
 477  void pm_runtime_use_autosuspend(struct device *dev);
 478    - set the power.use_autosuspend flag, enabling autosuspend delays; call
 479      pm_runtime_get_sync if the flag was previously cleared and
 480      power.autosuspend_delay is negative
 481
 482  void pm_runtime_dont_use_autosuspend(struct device *dev);
 483    - clear the power.use_autosuspend flag, disabling autosuspend delays;
 484      decrement the device's usage counter if the flag was previously set and
 485      power.autosuspend_delay is negative; call pm_runtime_idle
 486
 487  void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
 488    - set the power.autosuspend_delay value to 'delay' (expressed in
 489      milliseconds); if 'delay' is negative then runtime suspends are
 490      prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be
 491      called or the device's usage counter may be decremented and
 492      pm_runtime_idle called depending on if power.autosuspend_delay is
 493      changed to or from a negative value; if power.use_autosuspend is clear,
 494      pm_runtime_idle is called
 495
 496  unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
 497    - calculate the time when the current autosuspend delay period will expire,
 498      based on power.last_busy and power.autosuspend_delay; if the delay time
 499      is 1000 ms or larger then the expiration time is rounded up to the
 500      nearest second; returns 0 if the delay period has already expired or
 501      power.use_autosuspend isn't set, otherwise returns the expiration time
 502      in jiffies
 503
 504It is safe to execute the following helper functions from interrupt context:
 505
 506pm_request_idle()
 507pm_request_autosuspend()
 508pm_schedule_suspend()
 509pm_request_resume()
 510pm_runtime_get_noresume()
 511pm_runtime_get()
 512pm_runtime_put_noidle()
 513pm_runtime_put()
 514pm_runtime_put_autosuspend()
 515pm_runtime_enable()
 516pm_suspend_ignore_children()
 517pm_runtime_set_active()
 518pm_runtime_set_suspended()
 519pm_runtime_suspended()
 520pm_runtime_mark_last_busy()
 521pm_runtime_autosuspend_expiration()
 522
 523If pm_runtime_irq_safe() has been called for a device then the following helper
 524functions may also be used in interrupt context:
 525
 526pm_runtime_idle()
 527pm_runtime_suspend()
 528pm_runtime_autosuspend()
 529pm_runtime_resume()
 530pm_runtime_get_sync()
 531pm_runtime_put_sync()
 532pm_runtime_put_sync_suspend()
 533pm_runtime_put_sync_autosuspend()
 534
 5355. Runtime PM Initialization, Device Probing and Removal
 536
 537Initially, the runtime PM is disabled for all devices, which means that the
 538majority of the runtime PM helper functions described in Section 4 will return
 539-EAGAIN until pm_runtime_enable() is called for the device.
 540
 541In addition to that, the initial runtime PM status of all devices is
 542'suspended', but it need not reflect the actual physical state of the device.
 543Thus, if the device is initially active (i.e. it is able to process I/O), its
 544runtime PM status must be changed to 'active', with the help of
 545pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
 546
 547However, if the device has a parent and the parent's runtime PM is enabled,
 548calling pm_runtime_set_active() for the device will affect the parent, unless
 549the parent's 'power.ignore_children' flag is set.  Namely, in that case the
 550parent won't be able to suspend at run time, using the PM core's helper
 551functions, as long as the child's status is 'active', even if the child's
 552runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
 553the child yet or pm_runtime_disable() has been called for it).  For this reason,
 554once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
 555should be called for it too as soon as reasonably possible or its runtime PM
 556status should be changed back to 'suspended' with the help of
 557pm_runtime_set_suspended().
 558
 559If the default initial runtime PM status of the device (i.e. 'suspended')
 560reflects the actual state of the device, its bus type's or its driver's
 561->probe() callback will likely need to wake it up using one of the PM core's
 562helper functions described in Section 4.  In that case, pm_runtime_resume()
 563should be used.  Of course, for this purpose the device's runtime PM has to be
 564enabled earlier by calling pm_runtime_enable().
 565
 566Note, if the device may execute pm_runtime calls during the probe (such as
 567if it is registers with a subsystem that may call back in) then the
 568pm_runtime_get_sync() call paired with a pm_runtime_put() call will be
 569appropriate to ensure that the device is not put back to sleep during the
 570probe. This can happen with systems such as the network device layer.
 571
 572It may be desirable to suspend the device once ->probe() has finished.
 573Therefore the driver core uses the asynchronous pm_request_idle() to submit a
 574request to execute the subsystem-level idle callback for the device at that
 575time.  A driver that makes use of the runtime autosuspend feature, may want to
 576update the last busy mark before returning from ->probe().
 577
 578Moreover, the driver core prevents runtime PM callbacks from racing with the bus
 579notifier callback in __device_release_driver(), which is necessary, because the
 580notifier is used by some subsystems to carry out operations affecting the
 581runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
 582driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications.  This
 583resumes the device if it's in the suspended state and prevents it from
 584being suspended again while those routines are being executed.
 585
 586To allow bus types and drivers to put devices into the suspended state by
 587calling pm_runtime_suspend() from their ->remove() routines, the driver core
 588executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
 589notifications in __device_release_driver().  This requires bus types and
 590drivers to make their ->remove() callbacks avoid races with runtime PM directly,
 591but also it allows of more flexibility in the handling of devices during the
 592removal of their drivers.
 593
 594Drivers in ->remove() callback should undo the runtime PM changes done
 595in ->probe(). Usually this means calling pm_runtime_disable(),
 596pm_runtime_dont_use_autosuspend() etc.
 597
 598The user space can effectively disallow the driver of the device to power manage
 599it at run time by changing the value of its /sys/devices/.../power/control
 600attribute to "on", which causes pm_runtime_forbid() to be called.  In principle,
 601this mechanism may also be used by the driver to effectively turn off the
 602runtime power management of the device until the user space turns it on.
 603Namely, during the initialization the driver can make sure that the runtime PM
 604status of the device is 'active' and call pm_runtime_forbid().  It should be
 605noted, however, that if the user space has already intentionally changed the
 606value of /sys/devices/.../power/control to "auto" to allow the driver to power
 607manage the device at run time, the driver may confuse it by using
 608pm_runtime_forbid() this way.
 609
 6106. Runtime PM and System Sleep
 611
 612Runtime PM and system sleep (i.e., system suspend and hibernation, also known
 613as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
 614ways.  If a device is active when a system sleep starts, everything is
 615straightforward.  But what should happen if the device is already suspended?
 616
 617The device may have different wake-up settings for runtime PM and system sleep.
 618For example, remote wake-up may be enabled for runtime suspend but disallowed
 619for system sleep (device_may_wakeup(dev) returns 'false').  When this happens,
 620the subsystem-level system suspend callback is responsible for changing the
 621device's wake-up setting (it may leave that to the device driver's system
 622suspend routine).  It may be necessary to resume the device and suspend it again
 623in order to do so.  The same is true if the driver uses different power levels
 624or other settings for runtime suspend and system sleep.
 625
 626During system resume, the simplest approach is to bring all devices back to full
 627power, even if they had been suspended before the system suspend began.  There
 628are several reasons for this, including:
 629
 630  * The device might need to switch power levels, wake-up settings, etc.
 631
 632  * Remote wake-up events might have been lost by the firmware.
 633
 634  * The device's children may need the device to be at full power in order
 635    to resume themselves.
 636
 637  * The driver's idea of the device state may not agree with the device's
 638    physical state.  This can happen during resume from hibernation.
 639
 640  * The device might need to be reset.
 641
 642  * Even though the device was suspended, if its usage counter was > 0 then most
 643    likely it would need a runtime resume in the near future anyway.
 644
 645If the device had been suspended before the system suspend began and it's
 646brought back to full power during resume, then its runtime PM status will have
 647to be updated to reflect the actual post-system sleep status.  The way to do
 648this is:
 649
 650        pm_runtime_disable(dev);
 651        pm_runtime_set_active(dev);
 652        pm_runtime_enable(dev);
 653
 654The PM core always increments the runtime usage counter before calling the
 655->suspend() callback and decrements it after calling the ->resume() callback.
 656Hence disabling runtime PM temporarily like this will not cause any runtime
 657suspend attempts to be permanently lost.  If the usage count goes to zero
 658following the return of the ->resume() callback, the ->runtime_idle() callback
 659will be invoked as usual.
 660
 661On some systems, however, system sleep is not entered through a global firmware
 662or hardware operation.  Instead, all hardware components are put into low-power
 663states directly by the kernel in a coordinated way.  Then, the system sleep
 664state effectively follows from the states the hardware components end up in
 665and the system is woken up from that state by a hardware interrupt or a similar
 666mechanism entirely under the kernel's control.  As a result, the kernel never
 667gives control away and the states of all devices during resume are precisely
 668known to it.  If that is the case and none of the situations listed above takes
 669place (in particular, if the system is not waking up from hibernation), it may
 670be more efficient to leave the devices that had been suspended before the system
 671suspend began in the suspended state.
 672
 673To this end, the PM core provides a mechanism allowing some coordination between
 674different levels of device hierarchy.  Namely, if a system suspend .prepare()
 675callback returns a positive number for a device, that indicates to the PM core
 676that the device appears to be runtime-suspended and its state is fine, so it
 677may be left in runtime suspend provided that all of its descendants are also
 678left in runtime suspend.  If that happens, the PM core will not execute any
 679system suspend and resume callbacks for all of those devices, except for the
 680complete callback, which is then entirely responsible for handling the device
 681as appropriate.  This only applies to system suspend transitions that are not
 682related to hibernation (see Documentation/driver-api/pm/devices.rst for more
 683information).
 684
 685The PM core does its best to reduce the probability of race conditions between
 686the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
 687out the following operations:
 688
 689  * During system suspend pm_runtime_get_noresume() is called for every device
 690    right before executing the subsystem-level .prepare() callback for it and
 691    pm_runtime_barrier() is called for every device right before executing the
 692    subsystem-level .suspend() callback for it.  In addition to that the PM core
 693    calls  __pm_runtime_disable() with 'false' as the second argument for every
 694    device right before executing the subsystem-level .suspend_late() callback
 695    for it.
 696
 697  * During system resume pm_runtime_enable() and pm_runtime_put() are called for
 698    every device right after executing the subsystem-level .resume_early()
 699    callback and right after executing the subsystem-level .complete() callback
 700    for it, respectively.
 701
 7027. Generic subsystem callbacks
 703
 704Subsystems may wish to conserve code space by using the set of generic power
 705management callbacks provided by the PM core, defined in
 706driver/base/power/generic_ops.c:
 707
 708  int pm_generic_runtime_suspend(struct device *dev);
 709    - invoke the ->runtime_suspend() callback provided by the driver of this
 710      device and return its result, or return 0 if not defined
 711
 712  int pm_generic_runtime_resume(struct device *dev);
 713    - invoke the ->runtime_resume() callback provided by the driver of this
 714      device and return its result, or return 0 if not defined
 715
 716  int pm_generic_suspend(struct device *dev);
 717    - if the device has not been suspended at run time, invoke the ->suspend()
 718      callback provided by its driver and return its result, or return 0 if not
 719      defined
 720
 721  int pm_generic_suspend_noirq(struct device *dev);
 722    - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
 723      callback provided by the device's driver and return its result, or return
 724      0 if not defined
 725
 726  int pm_generic_resume(struct device *dev);
 727    - invoke the ->resume() callback provided by the driver of this device and,
 728      if successful, change the device's runtime PM status to 'active'
 729
 730  int pm_generic_resume_noirq(struct device *dev);
 731    - invoke the ->resume_noirq() callback provided by the driver of this device
 732
 733  int pm_generic_freeze(struct device *dev);
 734    - if the device has not been suspended at run time, invoke the ->freeze()
 735      callback provided by its driver and return its result, or return 0 if not
 736      defined
 737
 738  int pm_generic_freeze_noirq(struct device *dev);
 739    - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
 740      callback provided by the device's driver and return its result, or return
 741      0 if not defined
 742
 743  int pm_generic_thaw(struct device *dev);
 744    - if the device has not been suspended at run time, invoke the ->thaw()
 745      callback provided by its driver and return its result, or return 0 if not
 746      defined
 747
 748  int pm_generic_thaw_noirq(struct device *dev);
 749    - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
 750      callback provided by the device's driver and return its result, or return
 751      0 if not defined
 752
 753  int pm_generic_poweroff(struct device *dev);
 754    - if the device has not been suspended at run time, invoke the ->poweroff()
 755      callback provided by its driver and return its result, or return 0 if not
 756      defined
 757
 758  int pm_generic_poweroff_noirq(struct device *dev);
 759    - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
 760      callback provided by the device's driver and return its result, or return
 761      0 if not defined
 762
 763  int pm_generic_restore(struct device *dev);
 764    - invoke the ->restore() callback provided by the driver of this device and,
 765      if successful, change the device's runtime PM status to 'active'
 766
 767  int pm_generic_restore_noirq(struct device *dev);
 768    - invoke the ->restore_noirq() callback provided by the device's driver
 769
 770These functions are the defaults used by the PM core, if a subsystem doesn't
 771provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
 772->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
 773->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
 774->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
 775subsystem-level dev_pm_ops structure.
 776
 777Device drivers that wish to use the same function as a system suspend, freeze,
 778poweroff and runtime suspend callback, and similarly for system resume, thaw,
 779restore, and runtime resume, can achieve this with the help of the
 780UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
 781last argument to NULL).
 782
 7838. "No-Callback" Devices
 784
 785Some "devices" are only logical sub-devices of their parent and cannot be
 786power-managed on their own.  (The prototype example is a USB interface.  Entire
 787USB devices can go into low-power mode or send wake-up requests, but neither is
 788possible for individual interfaces.)  The drivers for these devices have no
 789need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
 790and ->runtime_resume() would always return 0 without doing anything else and
 791->runtime_idle() would always call pm_runtime_suspend().
 792
 793Subsystems can tell the PM core about these devices by calling
 794pm_runtime_no_callbacks().  This should be done after the device structure is
 795initialized and before it is registered (although after device registration is
 796also okay).  The routine will set the device's power.no_callbacks flag and
 797prevent the non-debugging runtime PM sysfs attributes from being created.
 798
 799When power.no_callbacks is set, the PM core will not invoke the
 800->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
 801Instead it will assume that suspends and resumes always succeed and that idle
 802devices should be suspended.
 803
 804As a consequence, the PM core will never directly inform the device's subsystem
 805or driver about runtime power changes.  Instead, the driver for the device's
 806parent must take responsibility for telling the device's driver when the
 807parent's power state changes.
 808
 8099. Autosuspend, or automatically-delayed suspends
 810
 811Changing a device's power state isn't free; it requires both time and energy.
 812A device should be put in a low-power state only when there's some reason to
 813think it will remain in that state for a substantial time.  A common heuristic
 814says that a device which hasn't been used for a while is liable to remain
 815unused; following this advice, drivers should not allow devices to be suspended
 816at runtime until they have been inactive for some minimum period.  Even when
 817the heuristic ends up being non-optimal, it will still prevent devices from
 818"bouncing" too rapidly between low-power and full-power states.
 819
 820The term "autosuspend" is an historical remnant.  It doesn't mean that the
 821device is automatically suspended (the subsystem or driver still has to call
 822the appropriate PM routines); rather it means that runtime suspends will
 823automatically be delayed until the desired period of inactivity has elapsed.
 824
 825Inactivity is determined based on the power.last_busy field.  Drivers should
 826call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
 827typically just before calling pm_runtime_put_autosuspend().  The desired length
 828of the inactivity period is a matter of policy.  Subsystems can set this length
 829initially by calling pm_runtime_set_autosuspend_delay(), but after device
 830registration the length should be controlled by user space, using the
 831/sys/devices/.../power/autosuspend_delay_ms attribute.
 832
 833In order to use autosuspend, subsystems or drivers must call
 834pm_runtime_use_autosuspend() (preferably before registering the device), and
 835thereafter they should use the various *_autosuspend() helper functions instead
 836of the non-autosuspend counterparts:
 837
 838        Instead of: pm_runtime_suspend    use: pm_runtime_autosuspend;
 839        Instead of: pm_schedule_suspend   use: pm_request_autosuspend;
 840        Instead of: pm_runtime_put        use: pm_runtime_put_autosuspend;
 841        Instead of: pm_runtime_put_sync   use: pm_runtime_put_sync_autosuspend.
 842
 843Drivers may also continue to use the non-autosuspend helper functions; they
 844will behave normally, which means sometimes taking the autosuspend delay into
 845account (see pm_runtime_idle).
 846
 847Under some circumstances a driver or subsystem may want to prevent a device
 848from autosuspending immediately, even though the usage counter is zero and the
 849autosuspend delay time has expired.  If the ->runtime_suspend() callback
 850returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
 851in the future (as it normally would be if the callback invoked
 852pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
 853autosuspend.  The ->runtime_suspend() callback can't do this rescheduling
 854itself because no suspend requests of any kind are accepted while the device is
 855suspending (i.e., while the callback is running).
 856
 857The implementation is well suited for asynchronous use in interrupt contexts.
 858However such use inevitably involves races, because the PM core can't
 859synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
 860This synchronization must be handled by the driver, using its private lock.
 861Here is a schematic pseudo-code example:
 862
 863        foo_read_or_write(struct foo_priv *foo, void *data)
 864        {
 865                lock(&foo->private_lock);
 866                add_request_to_io_queue(foo, data);
 867                if (foo->num_pending_requests++ == 0)
 868                        pm_runtime_get(&foo->dev);
 869                if (!foo->is_suspended)
 870                        foo_process_next_request(foo);
 871                unlock(&foo->private_lock);
 872        }
 873
 874        foo_io_completion(struct foo_priv *foo, void *req)
 875        {
 876                lock(&foo->private_lock);
 877                if (--foo->num_pending_requests == 0) {
 878                        pm_runtime_mark_last_busy(&foo->dev);
 879                        pm_runtime_put_autosuspend(&foo->dev);
 880                } else {
 881                        foo_process_next_request(foo);
 882                }
 883                unlock(&foo->private_lock);
 884                /* Send req result back to the user ... */
 885        }
 886
 887        int foo_runtime_suspend(struct device *dev)
 888        {
 889                struct foo_priv foo = container_of(dev, ...);
 890                int ret = 0;
 891
 892                lock(&foo->private_lock);
 893                if (foo->num_pending_requests > 0) {
 894                        ret = -EBUSY;
 895                } else {
 896                        /* ... suspend the device ... */
 897                        foo->is_suspended = 1;
 898                }
 899                unlock(&foo->private_lock);
 900                return ret;
 901        }
 902
 903        int foo_runtime_resume(struct device *dev)
 904        {
 905                struct foo_priv foo = container_of(dev, ...);
 906
 907                lock(&foo->private_lock);
 908                /* ... resume the device ... */
 909                foo->is_suspended = 0;
 910                pm_runtime_mark_last_busy(&foo->dev);
 911                if (foo->num_pending_requests > 0)
 912                        foo_process_next_request(foo);
 913                unlock(&foo->private_lock);
 914                return 0;
 915        }
 916
 917The important point is that after foo_io_completion() asks for an autosuspend,
 918the foo_runtime_suspend() callback may race with foo_read_or_write().
 919Therefore foo_runtime_suspend() has to check whether there are any pending I/O
 920requests (while holding the private lock) before allowing the suspend to
 921proceed.
 922
 923In addition, the power.autosuspend_delay field can be changed by user space at
 924any time.  If a driver cares about this, it can call
 925pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
 926callback while holding its private lock.  If the function returns a nonzero
 927value then the delay has not yet expired and the callback should return
 928-EAGAIN.
 929
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