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62052ab1 | 1 | Runtime Power Management Framework for I/O Devices |
5e928f77 | 2 | |
9659cc06 | 3 | (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. |
7490e442 | 4 | (C) 2010 Alan Stern <stern@rowland.harvard.edu> |
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5 | |
6 | 1. Introduction | |
7 | ||
62052ab1 | 8 | Support for runtime power management (runtime PM) of I/O devices is provided |
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9 | at the power management core (PM core) level by means of: |
10 | ||
11 | * The power management workqueue pm_wq in which bus types and device drivers can | |
12 | put their PM-related work items. It is strongly recommended that pm_wq be | |
62052ab1 | 13 | used for queuing all work items related to runtime PM, because this allows |
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14 | them to be synchronized with system-wide power transitions (suspend to RAM, |
15 | hibernation and resume from system sleep states). pm_wq is declared in | |
16 | include/linux/pm_runtime.h and defined in kernel/power/main.c. | |
17 | ||
62052ab1 | 18 | * A number of runtime PM fields in the 'power' member of 'struct device' (which |
5e928f77 | 19 | is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can |
62052ab1 | 20 | be used for synchronizing runtime PM operations with one another. |
5e928f77 | 21 | |
62052ab1 | 22 | * Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in |
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23 | include/linux/pm.h). |
24 | ||
25 | * A set of helper functions defined in drivers/base/power/runtime.c that can be | |
62052ab1 | 26 | used for carrying out runtime PM operations in such a way that the |
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27 | synchronization between them is taken care of by the PM core. Bus types and |
28 | device drivers are encouraged to use these functions. | |
29 | ||
62052ab1 | 30 | The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM |
5e928f77 | 31 | fields of 'struct dev_pm_info' and the core helper functions provided for |
62052ab1 | 32 | runtime PM are described below. |
5e928f77 | 33 | |
62052ab1 | 34 | 2. Device Runtime PM Callbacks |
5e928f77 | 35 | |
62052ab1 | 36 | There are three device runtime PM callbacks defined in 'struct dev_pm_ops': |
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37 | |
38 | struct dev_pm_ops { | |
39 | ... | |
40 | int (*runtime_suspend)(struct device *dev); | |
41 | int (*runtime_resume)(struct device *dev); | |
e1b1903e | 42 | int (*runtime_idle)(struct device *dev); |
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43 | ... |
44 | }; | |
45 | ||
2fb242ad | 46 | The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks |
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47 | are executed by the PM core for the device's subsystem that may be either of |
48 | the following: | |
49 | ||
50 | 1. PM domain of the device, if the device's PM domain object, dev->pm_domain, | |
51 | is present. | |
52 | ||
53 | 2. Device type of the device, if both dev->type and dev->type->pm are present. | |
54 | ||
55 | 3. Device class of the device, if both dev->class and dev->class->pm are | |
56 | present. | |
57 | ||
58 | 4. Bus type of the device, if both dev->bus and dev->bus->pm are present. | |
59 | ||
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60 | If the subsystem chosen by applying the above rules doesn't provide the relevant |
61 | callback, the PM core will invoke the corresponding driver callback stored in | |
62 | dev->driver->pm directly (if present). | |
63 | ||
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64 | The PM core always checks which callback to use in the order given above, so the |
65 | priority order of callbacks from high to low is: PM domain, device type, class | |
66 | and bus type. Moreover, the high-priority one will always take precedence over | |
67 | a low-priority one. The PM domain, bus type, device type and class callbacks | |
68 | are referred to as subsystem-level callbacks in what follows. | |
a6ab7aa9 | 69 | |
c7b61de5 | 70 | By default, the callbacks are always invoked in process context with interrupts |
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71 | enabled. However, the pm_runtime_irq_safe() helper function can be used to tell |
72 | the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume() | |
73 | and ->runtime_idle() callbacks for the given device in atomic context with | |
74 | interrupts disabled. This implies that the callback routines in question must | |
75 | not block or sleep, but it also means that the synchronous helper functions | |
76 | listed at the end of Section 4 may be used for that device within an interrupt | |
77 | handler or generally in an atomic context. | |
78 | ||
79 | The subsystem-level suspend callback, if present, is _entirely_ _responsible_ | |
80 | for handling the suspend of the device as appropriate, which may, but need not | |
81 | include executing the device driver's own ->runtime_suspend() callback (from the | |
5e928f77 | 82 | PM core's point of view it is not necessary to implement a ->runtime_suspend() |
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83 | callback in a device driver as long as the subsystem-level suspend callback |
84 | knows what to do to handle the device). | |
5e928f77 | 85 | |
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86 | * Once the subsystem-level suspend callback (or the driver suspend callback, |
87 | if invoked directly) has completed successfully for the given device, the PM | |
88 | core regards the device as suspended, which need not mean that it has been | |
89 | put into a low power state. It is supposed to mean, however, that the | |
90 | device will not process data and will not communicate with the CPU(s) and | |
91 | RAM until the appropriate resume callback is executed for it. The runtime | |
92 | PM status of a device after successful execution of the suspend callback is | |
93 | 'suspended'. | |
94 | ||
95 | * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM | |
96 | status remains 'active', which means that the device _must_ be fully | |
97 | operational afterwards. | |
98 | ||
99 | * If the suspend callback returns an error code different from -EBUSY and | |
100 | -EAGAIN, the PM core regards this as a fatal error and will refuse to run | |
101 | the helper functions described in Section 4 for the device until its status | |
102 | is directly set to either'active', or 'suspended' (the PM core provides | |
103 | special helper functions for this purpose). | |
104 | ||
105 | In particular, if the driver requires remote wakeup capability (i.e. hardware | |
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106 | mechanism allowing the device to request a change of its power state, such as |
107 | PCI PME) for proper functioning and device_run_wake() returns 'false' for the | |
108 | device, then ->runtime_suspend() should return -EBUSY. On the other hand, if | |
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109 | device_run_wake() returns 'true' for the device and the device is put into a |
110 | low-power state during the execution of the suspend callback, it is expected | |
111 | that remote wakeup will be enabled for the device. Generally, remote wakeup | |
112 | should be enabled for all input devices put into low-power states at run time. | |
113 | ||
114 | The subsystem-level resume callback, if present, is _entirely_ _responsible_ for | |
115 | handling the resume of the device as appropriate, which may, but need not | |
116 | include executing the device driver's own ->runtime_resume() callback (from the | |
117 | PM core's point of view it is not necessary to implement a ->runtime_resume() | |
118 | callback in a device driver as long as the subsystem-level resume callback knows | |
119 | what to do to handle the device). | |
120 | ||
121 | * Once the subsystem-level resume callback (or the driver resume callback, if | |
122 | invoked directly) has completed successfully, the PM core regards the device | |
123 | as fully operational, which means that the device _must_ be able to complete | |
124 | I/O operations as needed. The runtime PM status of the device is then | |
125 | 'active'. | |
126 | ||
127 | * If the resume callback returns an error code, the PM core regards this as a | |
128 | fatal error and will refuse to run the helper functions described in Section | |
129 | 4 for the device, until its status is directly set to either 'active', or | |
130 | 'suspended' (by means of special helper functions provided by the PM core | |
131 | for this purpose). | |
132 | ||
133 | The idle callback (a subsystem-level one, if present, or the driver one) is | |
134 | executed by the PM core whenever the device appears to be idle, which is | |
135 | indicated to the PM core by two counters, the device's usage counter and the | |
136 | counter of 'active' children of the device. | |
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137 | |
138 | * If any of these counters is decreased using a helper function provided by | |
139 | the PM core and it turns out to be equal to zero, the other counter is | |
140 | checked. If that counter also is equal to zero, the PM core executes the | |
35cd133c | 141 | idle callback with the device as its argument. |
5e928f77 | 142 | |
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143 | The action performed by the idle callback is totally dependent on the subsystem |
144 | (or driver) in question, but the expected and recommended action is to check | |
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145 | if the device can be suspended (i.e. if all of the conditions necessary for |
146 | suspending the device are satisfied) and to queue up a suspend request for the | |
43d51af4 | 147 | device in that case. If there is no idle callback, or if the callback returns |
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148 | 0, then the PM core will attempt to carry out a runtime suspend of the device, |
149 | also respecting devices configured for autosuspend. In essence this means a | |
150 | call to pm_runtime_autosuspend() (do note that drivers needs to update the | |
151 | device last busy mark, pm_runtime_mark_last_busy(), to control the delay under | |
152 | this circumstance). To prevent this (for example, if the callback routine has | |
153 | started a delayed suspend), the routine must return a non-zero value. Negative | |
154 | error return codes are ignored by the PM core. | |
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155 | |
156 | The helper functions provided by the PM core, described in Section 4, guarantee | |
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157 | that the following constraints are met with respect to runtime PM callbacks for |
158 | one device: | |
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159 | |
160 | (1) The callbacks are mutually exclusive (e.g. it is forbidden to execute | |
161 | ->runtime_suspend() in parallel with ->runtime_resume() or with another | |
162 | instance of ->runtime_suspend() for the same device) with the exception that | |
163 | ->runtime_suspend() or ->runtime_resume() can be executed in parallel with | |
164 | ->runtime_idle() (although ->runtime_idle() will not be started while any | |
165 | of the other callbacks is being executed for the same device). | |
166 | ||
167 | (2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active' | |
168 | devices (i.e. the PM core will only execute ->runtime_idle() or | |
62052ab1 | 169 | ->runtime_suspend() for the devices the runtime PM status of which is |
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170 | 'active'). |
171 | ||
172 | (3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device | |
173 | the usage counter of which is equal to zero _and_ either the counter of | |
174 | 'active' children of which is equal to zero, or the 'power.ignore_children' | |
175 | flag of which is set. | |
176 | ||
177 | (4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the | |
62052ab1 | 178 | PM core will only execute ->runtime_resume() for the devices the runtime |
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179 | PM status of which is 'suspended'). |
180 | ||
181 | Additionally, the helper functions provided by the PM core obey the following | |
182 | rules: | |
183 | ||
184 | * If ->runtime_suspend() is about to be executed or there's a pending request | |
185 | to execute it, ->runtime_idle() will not be executed for the same device. | |
186 | ||
187 | * A request to execute or to schedule the execution of ->runtime_suspend() | |
188 | will cancel any pending requests to execute ->runtime_idle() for the same | |
189 | device. | |
190 | ||
191 | * If ->runtime_resume() is about to be executed or there's a pending request | |
192 | to execute it, the other callbacks will not be executed for the same device. | |
193 | ||
194 | * A request to execute ->runtime_resume() will cancel any pending or | |
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195 | scheduled requests to execute the other callbacks for the same device, |
196 | except for scheduled autosuspends. | |
5e928f77 | 197 | |
62052ab1 | 198 | 3. Runtime PM Device Fields |
5e928f77 | 199 | |
62052ab1 | 200 | The following device runtime PM fields are present in 'struct dev_pm_info', as |
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201 | defined in include/linux/pm.h: |
202 | ||
203 | struct timer_list suspend_timer; | |
15bcb91d | 204 | - timer used for scheduling (delayed) suspend and autosuspend requests |
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205 | |
206 | unsigned long timer_expires; | |
207 | - timer expiration time, in jiffies (if this is different from zero, the | |
208 | timer is running and will expire at that time, otherwise the timer is not | |
209 | running) | |
210 | ||
211 | struct work_struct work; | |
212 | - work structure used for queuing up requests (i.e. work items in pm_wq) | |
213 | ||
214 | wait_queue_head_t wait_queue; | |
215 | - wait queue used if any of the helper functions needs to wait for another | |
216 | one to complete | |
217 | ||
218 | spinlock_t lock; | |
219 | - lock used for synchronisation | |
220 | ||
221 | atomic_t usage_count; | |
222 | - the usage counter of the device | |
223 | ||
224 | atomic_t child_count; | |
225 | - the count of 'active' children of the device | |
226 | ||
227 | unsigned int ignore_children; | |
228 | - if set, the value of child_count is ignored (but still updated) | |
229 | ||
230 | unsigned int disable_depth; | |
231 | - used for disabling the helper funcions (they work normally if this is | |
62052ab1 | 232 | equal to zero); the initial value of it is 1 (i.e. runtime PM is |
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233 | initially disabled for all devices) |
234 | ||
235 | unsigned int runtime_error; | |
236 | - if set, there was a fatal error (one of the callbacks returned error code | |
237 | as described in Section 2), so the helper funtions will not work until | |
238 | this flag is cleared; this is the error code returned by the failing | |
239 | callback | |
240 | ||
241 | unsigned int idle_notification; | |
242 | - if set, ->runtime_idle() is being executed | |
243 | ||
244 | unsigned int request_pending; | |
245 | - if set, there's a pending request (i.e. a work item queued up into pm_wq) | |
246 | ||
247 | enum rpm_request request; | |
248 | - type of request that's pending (valid if request_pending is set) | |
249 | ||
250 | unsigned int deferred_resume; | |
251 | - set if ->runtime_resume() is about to be run while ->runtime_suspend() is | |
252 | being executed for that device and it is not practical to wait for the | |
253 | suspend to complete; means "start a resume as soon as you've suspended" | |
254 | ||
7a1a8eb5 | 255 | unsigned int run_wake; |
62052ab1 | 256 | - set if the device is capable of generating runtime wake-up events |
7a1a8eb5 | 257 | |
5e928f77 | 258 | enum rpm_status runtime_status; |
62052ab1 | 259 | - the runtime PM status of the device; this field's initial value is |
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260 | RPM_SUSPENDED, which means that each device is initially regarded by the |
261 | PM core as 'suspended', regardless of its real hardware status | |
262 | ||
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263 | unsigned int runtime_auto; |
264 | - if set, indicates that the user space has allowed the device driver to | |
265 | power manage the device at run time via the /sys/devices/.../power/control | |
266 | interface; it may only be modified with the help of the pm_runtime_allow() | |
267 | and pm_runtime_forbid() helper functions | |
268 | ||
7490e442 | 269 | unsigned int no_callbacks; |
62052ab1 | 270 | - indicates that the device does not use the runtime PM callbacks (see |
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271 | Section 8); it may be modified only by the pm_runtime_no_callbacks() |
272 | helper function | |
273 | ||
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274 | unsigned int irq_safe; |
275 | - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks | |
276 | will be invoked with the spinlock held and interrupts disabled | |
277 | ||
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278 | unsigned int use_autosuspend; |
279 | - indicates that the device's driver supports delayed autosuspend (see | |
280 | Section 9); it may be modified only by the | |
281 | pm_runtime{_dont}_use_autosuspend() helper functions | |
282 | ||
283 | unsigned int timer_autosuspends; | |
284 | - indicates that the PM core should attempt to carry out an autosuspend | |
285 | when the timer expires rather than a normal suspend | |
286 | ||
287 | int autosuspend_delay; | |
288 | - the delay time (in milliseconds) to be used for autosuspend | |
289 | ||
290 | unsigned long last_busy; | |
291 | - the time (in jiffies) when the pm_runtime_mark_last_busy() helper | |
292 | function was last called for this device; used in calculating inactivity | |
293 | periods for autosuspend | |
294 | ||
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295 | All of the above fields are members of the 'power' member of 'struct device'. |
296 | ||
62052ab1 | 297 | 4. Runtime PM Device Helper Functions |
5e928f77 | 298 | |
62052ab1 | 299 | The following runtime PM helper functions are defined in |
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300 | drivers/base/power/runtime.c and include/linux/pm_runtime.h: |
301 | ||
302 | void pm_runtime_init(struct device *dev); | |
62052ab1 | 303 | - initialize the device runtime PM fields in 'struct dev_pm_info' |
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304 | |
305 | void pm_runtime_remove(struct device *dev); | |
62052ab1 | 306 | - make sure that the runtime PM of the device will be disabled after |
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307 | removing the device from device hierarchy |
308 | ||
309 | int pm_runtime_idle(struct device *dev); | |
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310 | - execute the subsystem-level idle callback for the device; returns an |
311 | error code on failure, where -EINPROGRESS means that ->runtime_idle() is | |
312 | already being executed; if there is no callback or the callback returns 0 | |
d66e6db2 | 313 | then run pm_runtime_autosuspend(dev) and return its result |
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314 | |
315 | int pm_runtime_suspend(struct device *dev); | |
a6ab7aa9 | 316 | - execute the subsystem-level suspend callback for the device; returns 0 on |
62052ab1 | 317 | success, 1 if the device's runtime PM status was already 'suspended', or |
5e928f77 | 318 | error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt |
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319 | to suspend the device again in future and -EACCES means that |
320 | 'power.disable_depth' is different from 0 | |
5e928f77 | 321 | |
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322 | int pm_runtime_autosuspend(struct device *dev); |
323 | - same as pm_runtime_suspend() except that the autosuspend delay is taken | |
324 | into account; if pm_runtime_autosuspend_expiration() says the delay has | |
325 | not yet expired then an autosuspend is scheduled for the appropriate time | |
326 | and 0 is returned | |
327 | ||
5e928f77 | 328 | int pm_runtime_resume(struct device *dev); |
de8164fb | 329 | - execute the subsystem-level resume callback for the device; returns 0 on |
62052ab1 | 330 | success, 1 if the device's runtime PM status was already 'active' or |
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331 | error code on failure, where -EAGAIN means it may be safe to attempt to |
332 | resume the device again in future, but 'power.runtime_error' should be | |
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333 | checked additionally, and -EACCES means that 'power.disable_depth' is |
334 | different from 0 | |
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335 | |
336 | int pm_request_idle(struct device *dev); | |
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337 | - submit a request to execute the subsystem-level idle callback for the |
338 | device (the request is represented by a work item in pm_wq); returns 0 on | |
339 | success or error code if the request has not been queued up | |
5e928f77 | 340 | |
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341 | int pm_request_autosuspend(struct device *dev); |
342 | - schedule the execution of the subsystem-level suspend callback for the | |
343 | device when the autosuspend delay has expired; if the delay has already | |
344 | expired then the work item is queued up immediately | |
345 | ||
5e928f77 | 346 | int pm_schedule_suspend(struct device *dev, unsigned int delay); |
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347 | - schedule the execution of the subsystem-level suspend callback for the |
348 | device in future, where 'delay' is the time to wait before queuing up a | |
349 | suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work | |
350 | item is queued up immediately); returns 0 on success, 1 if the device's PM | |
62052ab1 | 351 | runtime status was already 'suspended', or error code if the request |
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352 | hasn't been scheduled (or queued up if 'delay' is 0); if the execution of |
353 | ->runtime_suspend() is already scheduled and not yet expired, the new | |
354 | value of 'delay' will be used as the time to wait | |
355 | ||
356 | int pm_request_resume(struct device *dev); | |
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357 | - submit a request to execute the subsystem-level resume callback for the |
358 | device (the request is represented by a work item in pm_wq); returns 0 on | |
62052ab1 | 359 | success, 1 if the device's runtime PM status was already 'active', or |
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360 | error code if the request hasn't been queued up |
361 | ||
362 | void pm_runtime_get_noresume(struct device *dev); | |
363 | - increment the device's usage counter | |
364 | ||
365 | int pm_runtime_get(struct device *dev); | |
366 | - increment the device's usage counter, run pm_request_resume(dev) and | |
367 | return its result | |
368 | ||
369 | int pm_runtime_get_sync(struct device *dev); | |
370 | - increment the device's usage counter, run pm_runtime_resume(dev) and | |
371 | return its result | |
372 | ||
373 | void pm_runtime_put_noidle(struct device *dev); | |
374 | - decrement the device's usage counter | |
375 | ||
376 | int pm_runtime_put(struct device *dev); | |
15bcb91d AS |
377 | - decrement the device's usage counter; if the result is 0 then run |
378 | pm_request_idle(dev) and return its result | |
379 | ||
380 | int pm_runtime_put_autosuspend(struct device *dev); | |
381 | - decrement the device's usage counter; if the result is 0 then run | |
382 | pm_request_autosuspend(dev) and return its result | |
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383 | |
384 | int pm_runtime_put_sync(struct device *dev); | |
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385 | - decrement the device's usage counter; if the result is 0 then run |
386 | pm_runtime_idle(dev) and return its result | |
387 | ||
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388 | int pm_runtime_put_sync_suspend(struct device *dev); |
389 | - decrement the device's usage counter; if the result is 0 then run | |
390 | pm_runtime_suspend(dev) and return its result | |
391 | ||
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392 | int pm_runtime_put_sync_autosuspend(struct device *dev); |
393 | - decrement the device's usage counter; if the result is 0 then run | |
394 | pm_runtime_autosuspend(dev) and return its result | |
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395 | |
396 | void pm_runtime_enable(struct device *dev); | |
e358bad7 | 397 | - decrement the device's 'power.disable_depth' field; if that field is equal |
62052ab1 | 398 | to zero, the runtime PM helper functions can execute subsystem-level |
e358bad7 | 399 | callbacks described in Section 2 for the device |
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400 | |
401 | int pm_runtime_disable(struct device *dev); | |
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402 | - increment the device's 'power.disable_depth' field (if the value of that |
403 | field was previously zero, this prevents subsystem-level runtime PM | |
404 | callbacks from being run for the device), make sure that all of the pending | |
62052ab1 | 405 | runtime PM operations on the device are either completed or canceled; |
a6ab7aa9 RW |
406 | returns 1 if there was a resume request pending and it was necessary to |
407 | execute the subsystem-level resume callback for the device to satisfy that | |
408 | request, otherwise 0 is returned | |
5e928f77 | 409 | |
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410 | int pm_runtime_barrier(struct device *dev); |
411 | - check if there's a resume request pending for the device and resume it | |
412 | (synchronously) in that case, cancel any other pending runtime PM requests | |
413 | regarding it and wait for all runtime PM operations on it in progress to | |
414 | complete; returns 1 if there was a resume request pending and it was | |
415 | necessary to execute the subsystem-level resume callback for the device to | |
416 | satisfy that request, otherwise 0 is returned | |
417 | ||
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418 | void pm_suspend_ignore_children(struct device *dev, bool enable); |
419 | - set/unset the power.ignore_children flag of the device | |
420 | ||
421 | int pm_runtime_set_active(struct device *dev); | |
62052ab1 | 422 | - clear the device's 'power.runtime_error' flag, set the device's runtime |
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423 | PM status to 'active' and update its parent's counter of 'active' |
424 | children as appropriate (it is only valid to use this function if | |
425 | 'power.runtime_error' is set or 'power.disable_depth' is greater than | |
426 | zero); it will fail and return error code if the device has a parent | |
427 | which is not active and the 'power.ignore_children' flag of which is unset | |
428 | ||
429 | void pm_runtime_set_suspended(struct device *dev); | |
62052ab1 | 430 | - clear the device's 'power.runtime_error' flag, set the device's runtime |
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431 | PM status to 'suspended' and update its parent's counter of 'active' |
432 | children as appropriate (it is only valid to use this function if | |
433 | 'power.runtime_error' is set or 'power.disable_depth' is greater than | |
434 | zero) | |
435 | ||
fbadc58d SL |
436 | bool pm_runtime_active(struct device *dev); |
437 | - return true if the device's runtime PM status is 'active' or its | |
438 | 'power.disable_depth' field is not equal to zero, or false otherwise | |
439 | ||
d690b2cd | 440 | bool pm_runtime_suspended(struct device *dev); |
f08f5a0a RW |
441 | - return true if the device's runtime PM status is 'suspended' and its |
442 | 'power.disable_depth' field is equal to zero, or false otherwise | |
d690b2cd | 443 | |
f3393b62 KH |
444 | bool pm_runtime_status_suspended(struct device *dev); |
445 | - return true if the device's runtime PM status is 'suspended' | |
446 | ||
87d1b3e6 RW |
447 | void pm_runtime_allow(struct device *dev); |
448 | - set the power.runtime_auto flag for the device and decrease its usage | |
449 | counter (used by the /sys/devices/.../power/control interface to | |
450 | effectively allow the device to be power managed at run time) | |
451 | ||
452 | void pm_runtime_forbid(struct device *dev); | |
453 | - unset the power.runtime_auto flag for the device and increase its usage | |
454 | counter (used by the /sys/devices/.../power/control interface to | |
455 | effectively prevent the device from being power managed at run time) | |
456 | ||
7490e442 | 457 | void pm_runtime_no_callbacks(struct device *dev); |
62052ab1 | 458 | - set the power.no_callbacks flag for the device and remove the runtime |
7490e442 AS |
459 | PM attributes from /sys/devices/.../power (or prevent them from being |
460 | added when the device is registered) | |
461 | ||
c7b61de5 AS |
462 | void pm_runtime_irq_safe(struct device *dev); |
463 | - set the power.irq_safe flag for the device, causing the runtime-PM | |
64584eb9 | 464 | callbacks to be invoked with interrupts off |
c7b61de5 | 465 | |
15bcb91d AS |
466 | void pm_runtime_mark_last_busy(struct device *dev); |
467 | - set the power.last_busy field to the current time | |
468 | ||
469 | void pm_runtime_use_autosuspend(struct device *dev); | |
470 | - set the power.use_autosuspend flag, enabling autosuspend delays | |
471 | ||
472 | void pm_runtime_dont_use_autosuspend(struct device *dev); | |
473 | - clear the power.use_autosuspend flag, disabling autosuspend delays | |
474 | ||
475 | void pm_runtime_set_autosuspend_delay(struct device *dev, int delay); | |
476 | - set the power.autosuspend_delay value to 'delay' (expressed in | |
62052ab1 | 477 | milliseconds); if 'delay' is negative then runtime suspends are |
15bcb91d AS |
478 | prevented |
479 | ||
480 | unsigned long pm_runtime_autosuspend_expiration(struct device *dev); | |
481 | - calculate the time when the current autosuspend delay period will expire, | |
482 | based on power.last_busy and power.autosuspend_delay; if the delay time | |
483 | is 1000 ms or larger then the expiration time is rounded up to the | |
484 | nearest second; returns 0 if the delay period has already expired or | |
485 | power.use_autosuspend isn't set, otherwise returns the expiration time | |
486 | in jiffies | |
487 | ||
5e928f77 RW |
488 | It is safe to execute the following helper functions from interrupt context: |
489 | ||
490 | pm_request_idle() | |
15bcb91d | 491 | pm_request_autosuspend() |
5e928f77 RW |
492 | pm_schedule_suspend() |
493 | pm_request_resume() | |
494 | pm_runtime_get_noresume() | |
495 | pm_runtime_get() | |
496 | pm_runtime_put_noidle() | |
497 | pm_runtime_put() | |
15bcb91d AS |
498 | pm_runtime_put_autosuspend() |
499 | pm_runtime_enable() | |
5e928f77 RW |
500 | pm_suspend_ignore_children() |
501 | pm_runtime_set_active() | |
502 | pm_runtime_set_suspended() | |
15bcb91d AS |
503 | pm_runtime_suspended() |
504 | pm_runtime_mark_last_busy() | |
505 | pm_runtime_autosuspend_expiration() | |
5e928f77 | 506 | |
c7b61de5 AS |
507 | If pm_runtime_irq_safe() has been called for a device then the following helper |
508 | functions may also be used in interrupt context: | |
509 | ||
2e6ba515 | 510 | pm_runtime_idle() |
c7b61de5 AS |
511 | pm_runtime_suspend() |
512 | pm_runtime_autosuspend() | |
513 | pm_runtime_resume() | |
514 | pm_runtime_get_sync() | |
02b26774 | 515 | pm_runtime_put_sync() |
c7b61de5 | 516 | pm_runtime_put_sync_suspend() |
311aab73 | 517 | pm_runtime_put_sync_autosuspend() |
c7b61de5 | 518 | |
62052ab1 | 519 | 5. Runtime PM Initialization, Device Probing and Removal |
5e928f77 | 520 | |
62052ab1 RW |
521 | Initially, the runtime PM is disabled for all devices, which means that the |
522 | majority of the runtime PM helper funtions described in Section 4 will return | |
5e928f77 RW |
523 | -EAGAIN until pm_runtime_enable() is called for the device. |
524 | ||
62052ab1 | 525 | In addition to that, the initial runtime PM status of all devices is |
5e928f77 RW |
526 | 'suspended', but it need not reflect the actual physical state of the device. |
527 | Thus, if the device is initially active (i.e. it is able to process I/O), its | |
62052ab1 | 528 | runtime PM status must be changed to 'active', with the help of |
5e928f77 RW |
529 | pm_runtime_set_active(), before pm_runtime_enable() is called for the device. |
530 | ||
62052ab1 | 531 | However, if the device has a parent and the parent's runtime PM is enabled, |
5e928f77 RW |
532 | calling pm_runtime_set_active() for the device will affect the parent, unless |
533 | the parent's 'power.ignore_children' flag is set. Namely, in that case the | |
534 | parent won't be able to suspend at run time, using the PM core's helper | |
535 | functions, as long as the child's status is 'active', even if the child's | |
62052ab1 | 536 | runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for |
5e928f77 RW |
537 | the child yet or pm_runtime_disable() has been called for it). For this reason, |
538 | once pm_runtime_set_active() has been called for the device, pm_runtime_enable() | |
62052ab1 | 539 | should be called for it too as soon as reasonably possible or its runtime PM |
5e928f77 RW |
540 | status should be changed back to 'suspended' with the help of |
541 | pm_runtime_set_suspended(). | |
542 | ||
62052ab1 | 543 | If the default initial runtime PM status of the device (i.e. 'suspended') |
5e928f77 RW |
544 | reflects the actual state of the device, its bus type's or its driver's |
545 | ->probe() callback will likely need to wake it up using one of the PM core's | |
546 | helper functions described in Section 4. In that case, pm_runtime_resume() | |
62052ab1 | 547 | should be used. Of course, for this purpose the device's runtime PM has to be |
5e928f77 RW |
548 | enabled earlier by calling pm_runtime_enable(). |
549 | ||
f5da24db | 550 | If the device bus type's or driver's ->probe() callback runs |
5e928f77 RW |
551 | pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts, |
552 | they will fail returning -EAGAIN, because the device's usage counter is | |
f5da24db RW |
553 | incremented by the driver core before executing ->probe(). Still, it may be |
554 | desirable to suspend the device as soon as ->probe() has finished, so the driver | |
555 | core uses pm_runtime_put_sync() to invoke the subsystem-level idle callback for | |
556 | the device at that time. | |
557 | ||
558 | Moreover, the driver core prevents runtime PM callbacks from racing with the bus | |
559 | notifier callback in __device_release_driver(), which is necessary, because the | |
560 | notifier is used by some subsystems to carry out operations affecting the | |
561 | runtime PM functionality. It does so by calling pm_runtime_get_sync() before | |
562 | driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This | |
563 | resumes the device if it's in the suspended state and prevents it from | |
564 | being suspended again while those routines are being executed. | |
565 | ||
566 | To allow bus types and drivers to put devices into the suspended state by | |
567 | calling pm_runtime_suspend() from their ->remove() routines, the driver core | |
568 | executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER | |
569 | notifications in __device_release_driver(). This requires bus types and | |
570 | drivers to make their ->remove() callbacks avoid races with runtime PM directly, | |
571 | but also it allows of more flexibility in the handling of devices during the | |
572 | removal of their drivers. | |
f1212ae1 | 573 | |
87d1b3e6 RW |
574 | The user space can effectively disallow the driver of the device to power manage |
575 | it at run time by changing the value of its /sys/devices/.../power/control | |
576 | attribute to "on", which causes pm_runtime_forbid() to be called. In principle, | |
577 | this mechanism may also be used by the driver to effectively turn off the | |
62052ab1 RW |
578 | runtime power management of the device until the user space turns it on. |
579 | Namely, during the initialization the driver can make sure that the runtime PM | |
87d1b3e6 RW |
580 | status of the device is 'active' and call pm_runtime_forbid(). It should be |
581 | noted, however, that if the user space has already intentionally changed the | |
582 | value of /sys/devices/.../power/control to "auto" to allow the driver to power | |
583 | manage the device at run time, the driver may confuse it by using | |
584 | pm_runtime_forbid() this way. | |
585 | ||
62052ab1 | 586 | 6. Runtime PM and System Sleep |
f1212ae1 | 587 | |
62052ab1 | 588 | Runtime PM and system sleep (i.e., system suspend and hibernation, also known |
f1212ae1 AS |
589 | as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of |
590 | ways. If a device is active when a system sleep starts, everything is | |
591 | straightforward. But what should happen if the device is already suspended? | |
592 | ||
62052ab1 RW |
593 | The device may have different wake-up settings for runtime PM and system sleep. |
594 | For example, remote wake-up may be enabled for runtime suspend but disallowed | |
f1212ae1 AS |
595 | for system sleep (device_may_wakeup(dev) returns 'false'). When this happens, |
596 | the subsystem-level system suspend callback is responsible for changing the | |
597 | device's wake-up setting (it may leave that to the device driver's system | |
598 | suspend routine). It may be necessary to resume the device and suspend it again | |
599 | in order to do so. The same is true if the driver uses different power levels | |
62052ab1 | 600 | or other settings for runtime suspend and system sleep. |
f1212ae1 | 601 | |
455716e9 RW |
602 | During system resume, the simplest approach is to bring all devices back to full |
603 | power, even if they had been suspended before the system suspend began. There | |
604 | are several reasons for this, including: | |
f1212ae1 AS |
605 | |
606 | * The device might need to switch power levels, wake-up settings, etc. | |
607 | ||
608 | * Remote wake-up events might have been lost by the firmware. | |
609 | ||
610 | * The device's children may need the device to be at full power in order | |
611 | to resume themselves. | |
612 | ||
613 | * The driver's idea of the device state may not agree with the device's | |
614 | physical state. This can happen during resume from hibernation. | |
615 | ||
616 | * The device might need to be reset. | |
617 | ||
618 | * Even though the device was suspended, if its usage counter was > 0 then most | |
62052ab1 | 619 | likely it would need a runtime resume in the near future anyway. |
f1212ae1 | 620 | |
455716e9 | 621 | If the device had been suspended before the system suspend began and it's |
62052ab1 | 622 | brought back to full power during resume, then its runtime PM status will have |
455716e9 RW |
623 | to be updated to reflect the actual post-system sleep status. The way to do |
624 | this is: | |
f1212ae1 AS |
625 | |
626 | pm_runtime_disable(dev); | |
627 | pm_runtime_set_active(dev); | |
628 | pm_runtime_enable(dev); | |
629 | ||
62052ab1 | 630 | The PM core always increments the runtime usage counter before calling the |
1e2ef05b | 631 | ->suspend() callback and decrements it after calling the ->resume() callback. |
62052ab1 | 632 | Hence disabling runtime PM temporarily like this will not cause any runtime |
1e2ef05b RW |
633 | suspend attempts to be permanently lost. If the usage count goes to zero |
634 | following the return of the ->resume() callback, the ->runtime_idle() callback | |
635 | will be invoked as usual. | |
636 | ||
455716e9 RW |
637 | On some systems, however, system sleep is not entered through a global firmware |
638 | or hardware operation. Instead, all hardware components are put into low-power | |
639 | states directly by the kernel in a coordinated way. Then, the system sleep | |
640 | state effectively follows from the states the hardware components end up in | |
641 | and the system is woken up from that state by a hardware interrupt or a similar | |
642 | mechanism entirely under the kernel's control. As a result, the kernel never | |
643 | gives control away and the states of all devices during resume are precisely | |
644 | known to it. If that is the case and none of the situations listed above takes | |
645 | place (in particular, if the system is not waking up from hibernation), it may | |
646 | be more efficient to leave the devices that had been suspended before the system | |
647 | suspend began in the suspended state. | |
648 | ||
1e2ef05b RW |
649 | The PM core does its best to reduce the probability of race conditions between |
650 | the runtime PM and system suspend/resume (and hibernation) callbacks by carrying | |
651 | out the following operations: | |
652 | ||
653 | * During system suspend it calls pm_runtime_get_noresume() and | |
654 | pm_runtime_barrier() for every device right before executing the | |
655 | subsystem-level .suspend() callback for it. In addition to that it calls | |
9f6d8f6a RW |
656 | __pm_runtime_disable() with 'false' as the second argument for every device |
657 | right before executing the subsystem-level .suspend_late() callback for it. | |
1e2ef05b RW |
658 | |
659 | * During system resume it calls pm_runtime_enable() and pm_runtime_put_sync() | |
9f6d8f6a RW |
660 | for every device right after executing the subsystem-level .resume_early() |
661 | callback and right after executing the subsystem-level .resume() callback | |
662 | for it, respectively. | |
1e2ef05b | 663 | |
d690b2cd RW |
664 | 7. Generic subsystem callbacks |
665 | ||
666 | Subsystems may wish to conserve code space by using the set of generic power | |
667 | management callbacks provided by the PM core, defined in | |
668 | driver/base/power/generic_ops.c: | |
669 | ||
d690b2cd RW |
670 | int pm_generic_runtime_suspend(struct device *dev); |
671 | - invoke the ->runtime_suspend() callback provided by the driver of this | |
672 | device and return its result, or return -EINVAL if not defined | |
673 | ||
674 | int pm_generic_runtime_resume(struct device *dev); | |
675 | - invoke the ->runtime_resume() callback provided by the driver of this | |
676 | device and return its result, or return -EINVAL if not defined | |
677 | ||
678 | int pm_generic_suspend(struct device *dev); | |
679 | - if the device has not been suspended at run time, invoke the ->suspend() | |
680 | callback provided by its driver and return its result, or return 0 if not | |
681 | defined | |
682 | ||
e5291928 RW |
683 | int pm_generic_suspend_noirq(struct device *dev); |
684 | - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq() | |
685 | callback provided by the device's driver and return its result, or return | |
686 | 0 if not defined | |
687 | ||
d690b2cd RW |
688 | int pm_generic_resume(struct device *dev); |
689 | - invoke the ->resume() callback provided by the driver of this device and, | |
690 | if successful, change the device's runtime PM status to 'active' | |
691 | ||
e5291928 RW |
692 | int pm_generic_resume_noirq(struct device *dev); |
693 | - invoke the ->resume_noirq() callback provided by the driver of this device | |
694 | ||
d690b2cd RW |
695 | int pm_generic_freeze(struct device *dev); |
696 | - if the device has not been suspended at run time, invoke the ->freeze() | |
697 | callback provided by its driver and return its result, or return 0 if not | |
698 | defined | |
699 | ||
e5291928 RW |
700 | int pm_generic_freeze_noirq(struct device *dev); |
701 | - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq() | |
702 | callback provided by the device's driver and return its result, or return | |
703 | 0 if not defined | |
704 | ||
d690b2cd RW |
705 | int pm_generic_thaw(struct device *dev); |
706 | - if the device has not been suspended at run time, invoke the ->thaw() | |
707 | callback provided by its driver and return its result, or return 0 if not | |
708 | defined | |
709 | ||
e5291928 RW |
710 | int pm_generic_thaw_noirq(struct device *dev); |
711 | - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq() | |
712 | callback provided by the device's driver and return its result, or return | |
713 | 0 if not defined | |
714 | ||
d690b2cd RW |
715 | int pm_generic_poweroff(struct device *dev); |
716 | - if the device has not been suspended at run time, invoke the ->poweroff() | |
717 | callback provided by its driver and return its result, or return 0 if not | |
718 | defined | |
719 | ||
e5291928 RW |
720 | int pm_generic_poweroff_noirq(struct device *dev); |
721 | - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq() | |
722 | callback provided by the device's driver and return its result, or return | |
723 | 0 if not defined | |
724 | ||
d690b2cd RW |
725 | int pm_generic_restore(struct device *dev); |
726 | - invoke the ->restore() callback provided by the driver of this device and, | |
727 | if successful, change the device's runtime PM status to 'active' | |
728 | ||
e5291928 RW |
729 | int pm_generic_restore_noirq(struct device *dev); |
730 | - invoke the ->restore_noirq() callback provided by the device's driver | |
731 | ||
d690b2cd | 732 | These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(), |
e5291928 RW |
733 | ->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(), |
734 | ->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(), | |
735 | ->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() callback | |
736 | pointers in the subsystem-level dev_pm_ops structures. | |
d690b2cd RW |
737 | |
738 | If a subsystem wishes to use all of them at the same time, it can simply assign | |
739 | the GENERIC_SUBSYS_PM_OPS macro, defined in include/linux/pm.h, to its | |
740 | dev_pm_ops structure pointer. | |
741 | ||
742 | Device drivers that wish to use the same function as a system suspend, freeze, | |
62052ab1 RW |
743 | poweroff and runtime suspend callback, and similarly for system resume, thaw, |
744 | restore, and runtime resume, can achieve this with the help of the | |
d690b2cd RW |
745 | UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its |
746 | last argument to NULL). | |
7490e442 AS |
747 | |
748 | 8. "No-Callback" Devices | |
749 | ||
750 | Some "devices" are only logical sub-devices of their parent and cannot be | |
751 | power-managed on their own. (The prototype example is a USB interface. Entire | |
752 | USB devices can go into low-power mode or send wake-up requests, but neither is | |
753 | possible for individual interfaces.) The drivers for these devices have no | |
62052ab1 | 754 | need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend() |
7490e442 AS |
755 | and ->runtime_resume() would always return 0 without doing anything else and |
756 | ->runtime_idle() would always call pm_runtime_suspend(). | |
757 | ||
758 | Subsystems can tell the PM core about these devices by calling | |
759 | pm_runtime_no_callbacks(). This should be done after the device structure is | |
760 | initialized and before it is registered (although after device registration is | |
761 | also okay). The routine will set the device's power.no_callbacks flag and | |
62052ab1 | 762 | prevent the non-debugging runtime PM sysfs attributes from being created. |
7490e442 AS |
763 | |
764 | When power.no_callbacks is set, the PM core will not invoke the | |
765 | ->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks. | |
766 | Instead it will assume that suspends and resumes always succeed and that idle | |
767 | devices should be suspended. | |
768 | ||
769 | As a consequence, the PM core will never directly inform the device's subsystem | |
62052ab1 | 770 | or driver about runtime power changes. Instead, the driver for the device's |
7490e442 AS |
771 | parent must take responsibility for telling the device's driver when the |
772 | parent's power state changes. | |
15bcb91d AS |
773 | |
774 | 9. Autosuspend, or automatically-delayed suspends | |
775 | ||
776 | Changing a device's power state isn't free; it requires both time and energy. | |
777 | A device should be put in a low-power state only when there's some reason to | |
778 | think it will remain in that state for a substantial time. A common heuristic | |
779 | says that a device which hasn't been used for a while is liable to remain | |
780 | unused; following this advice, drivers should not allow devices to be suspended | |
62052ab1 | 781 | at runtime until they have been inactive for some minimum period. Even when |
15bcb91d AS |
782 | the heuristic ends up being non-optimal, it will still prevent devices from |
783 | "bouncing" too rapidly between low-power and full-power states. | |
784 | ||
785 | The term "autosuspend" is an historical remnant. It doesn't mean that the | |
786 | device is automatically suspended (the subsystem or driver still has to call | |
62052ab1 | 787 | the appropriate PM routines); rather it means that runtime suspends will |
15bcb91d AS |
788 | automatically be delayed until the desired period of inactivity has elapsed. |
789 | ||
790 | Inactivity is determined based on the power.last_busy field. Drivers should | |
791 | call pm_runtime_mark_last_busy() to update this field after carrying out I/O, | |
792 | typically just before calling pm_runtime_put_autosuspend(). The desired length | |
793 | of the inactivity period is a matter of policy. Subsystems can set this length | |
794 | initially by calling pm_runtime_set_autosuspend_delay(), but after device | |
795 | registration the length should be controlled by user space, using the | |
796 | /sys/devices/.../power/autosuspend_delay_ms attribute. | |
797 | ||
798 | In order to use autosuspend, subsystems or drivers must call | |
799 | pm_runtime_use_autosuspend() (preferably before registering the device), and | |
800 | thereafter they should use the various *_autosuspend() helper functions instead | |
801 | of the non-autosuspend counterparts: | |
802 | ||
803 | Instead of: pm_runtime_suspend use: pm_runtime_autosuspend; | |
804 | Instead of: pm_schedule_suspend use: pm_request_autosuspend; | |
805 | Instead of: pm_runtime_put use: pm_runtime_put_autosuspend; | |
806 | Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend. | |
807 | ||
808 | Drivers may also continue to use the non-autosuspend helper functions; they | |
809 | will behave normally, not taking the autosuspend delay into account. | |
810 | Similarly, if the power.use_autosuspend field isn't set then the autosuspend | |
811 | helper functions will behave just like the non-autosuspend counterparts. | |
812 | ||
886486b7 AS |
813 | Under some circumstances a driver or subsystem may want to prevent a device |
814 | from autosuspending immediately, even though the usage counter is zero and the | |
815 | autosuspend delay time has expired. If the ->runtime_suspend() callback | |
816 | returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is | |
817 | in the future (as it normally would be if the callback invoked | |
818 | pm_runtime_mark_last_busy()), the PM core will automatically reschedule the | |
819 | autosuspend. The ->runtime_suspend() callback can't do this rescheduling | |
820 | itself because no suspend requests of any kind are accepted while the device is | |
821 | suspending (i.e., while the callback is running). | |
822 | ||
15bcb91d AS |
823 | The implementation is well suited for asynchronous use in interrupt contexts. |
824 | However such use inevitably involves races, because the PM core can't | |
825 | synchronize ->runtime_suspend() callbacks with the arrival of I/O requests. | |
826 | This synchronization must be handled by the driver, using its private lock. | |
827 | Here is a schematic pseudo-code example: | |
828 | ||
829 | foo_read_or_write(struct foo_priv *foo, void *data) | |
830 | { | |
831 | lock(&foo->private_lock); | |
832 | add_request_to_io_queue(foo, data); | |
833 | if (foo->num_pending_requests++ == 0) | |
834 | pm_runtime_get(&foo->dev); | |
835 | if (!foo->is_suspended) | |
836 | foo_process_next_request(foo); | |
837 | unlock(&foo->private_lock); | |
838 | } | |
839 | ||
840 | foo_io_completion(struct foo_priv *foo, void *req) | |
841 | { | |
842 | lock(&foo->private_lock); | |
843 | if (--foo->num_pending_requests == 0) { | |
844 | pm_runtime_mark_last_busy(&foo->dev); | |
845 | pm_runtime_put_autosuspend(&foo->dev); | |
846 | } else { | |
847 | foo_process_next_request(foo); | |
848 | } | |
849 | unlock(&foo->private_lock); | |
850 | /* Send req result back to the user ... */ | |
851 | } | |
852 | ||
853 | int foo_runtime_suspend(struct device *dev) | |
854 | { | |
855 | struct foo_priv foo = container_of(dev, ...); | |
856 | int ret = 0; | |
857 | ||
858 | lock(&foo->private_lock); | |
859 | if (foo->num_pending_requests > 0) { | |
860 | ret = -EBUSY; | |
861 | } else { | |
862 | /* ... suspend the device ... */ | |
863 | foo->is_suspended = 1; | |
864 | } | |
865 | unlock(&foo->private_lock); | |
866 | return ret; | |
867 | } | |
868 | ||
869 | int foo_runtime_resume(struct device *dev) | |
870 | { | |
871 | struct foo_priv foo = container_of(dev, ...); | |
872 | ||
873 | lock(&foo->private_lock); | |
874 | /* ... resume the device ... */ | |
875 | foo->is_suspended = 0; | |
876 | pm_runtime_mark_last_busy(&foo->dev); | |
877 | if (foo->num_pending_requests > 0) | |
878 | foo_process_requests(foo); | |
879 | unlock(&foo->private_lock); | |
880 | return 0; | |
881 | } | |
882 | ||
883 | The important point is that after foo_io_completion() asks for an autosuspend, | |
884 | the foo_runtime_suspend() callback may race with foo_read_or_write(). | |
885 | Therefore foo_runtime_suspend() has to check whether there are any pending I/O | |
886 | requests (while holding the private lock) before allowing the suspend to | |
887 | proceed. | |
888 | ||
889 | In addition, the power.autosuspend_delay field can be changed by user space at | |
890 | any time. If a driver cares about this, it can call | |
891 | pm_runtime_autosuspend_expiration() from within the ->runtime_suspend() | |
892 | callback while holding its private lock. If the function returns a nonzero | |
893 | value then the delay has not yet expired and the callback should return | |
894 | -EAGAIN. |