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Commit | Line | Data |
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cd38c1e1 AS |
1 | Power Management for USB |
2 | ||
3 | Alan Stern <stern@rowland.harvard.edu> | |
4 | ||
5 | October 5, 2007 | |
6 | ||
7 | ||
8 | ||
9 | What is Power Management? | |
10 | ------------------------- | |
11 | ||
12 | Power Management (PM) is the practice of saving energy by suspending | |
13 | parts of a computer system when they aren't being used. While a | |
14 | component is "suspended" it is in a nonfunctional low-power state; it | |
15 | might even be turned off completely. A suspended component can be | |
16 | "resumed" (returned to a functional full-power state) when the kernel | |
17 | needs to use it. (There also are forms of PM in which components are | |
18 | placed in a less functional but still usable state instead of being | |
19 | suspended; an example would be reducing the CPU's clock rate. This | |
20 | document will not discuss those other forms.) | |
21 | ||
22 | When the parts being suspended include the CPU and most of the rest of | |
23 | the system, we speak of it as a "system suspend". When a particular | |
24 | device is turned off while the system as a whole remains running, we | |
25 | call it a "dynamic suspend" (also known as a "runtime suspend" or | |
26 | "selective suspend"). This document concentrates mostly on how | |
27 | dynamic PM is implemented in the USB subsystem, although system PM is | |
28 | covered to some extent (see Documentation/power/*.txt for more | |
29 | information about system PM). | |
30 | ||
31 | Note: Dynamic PM support for USB is present only if the kernel was | |
32 | built with CONFIG_USB_SUSPEND enabled. System PM support is present | |
33 | only if the kernel was built with CONFIG_SUSPEND or CONFIG_HIBERNATION | |
34 | enabled. | |
35 | ||
36 | ||
37 | What is Remote Wakeup? | |
38 | ---------------------- | |
39 | ||
40 | When a device has been suspended, it generally doesn't resume until | |
41 | the computer tells it to. Likewise, if the entire computer has been | |
42 | suspended, it generally doesn't resume until the user tells it to, say | |
43 | by pressing a power button or opening the cover. | |
44 | ||
45 | However some devices have the capability of resuming by themselves, or | |
46 | asking the kernel to resume them, or even telling the entire computer | |
47 | to resume. This capability goes by several names such as "Wake On | |
48 | LAN"; we will refer to it generically as "remote wakeup". When a | |
49 | device is enabled for remote wakeup and it is suspended, it may resume | |
50 | itself (or send a request to be resumed) in response to some external | |
51 | event. Examples include a suspended keyboard resuming when a key is | |
52 | pressed, or a suspended USB hub resuming when a device is plugged in. | |
53 | ||
54 | ||
55 | When is a USB device idle? | |
56 | -------------------------- | |
57 | ||
58 | A device is idle whenever the kernel thinks it's not busy doing | |
59 | anything important and thus is a candidate for being suspended. The | |
60 | exact definition depends on the device's driver; drivers are allowed | |
61 | to declare that a device isn't idle even when there's no actual | |
62 | communication taking place. (For example, a hub isn't considered idle | |
63 | unless all the devices plugged into that hub are already suspended.) | |
64 | In addition, a device isn't considered idle so long as a program keeps | |
65 | its usbfs file open, whether or not any I/O is going on. | |
66 | ||
67 | If a USB device has no driver, its usbfs file isn't open, and it isn't | |
68 | being accessed through sysfs, then it definitely is idle. | |
69 | ||
70 | ||
71 | Forms of dynamic PM | |
72 | ------------------- | |
73 | ||
74 | Dynamic suspends can occur in two ways: manual and automatic. | |
75 | "Manual" means that the user has told the kernel to suspend a device, | |
76 | whereas "automatic" means that the kernel has decided all by itself to | |
77 | suspend a device. Automatic suspend is called "autosuspend" for | |
78 | short. In general, a device won't be autosuspended unless it has been | |
79 | idle for some minimum period of time, the so-called idle-delay time. | |
80 | ||
81 | Of course, nothing the kernel does on its own initiative should | |
82 | prevent the computer or its devices from working properly. If a | |
83 | device has been autosuspended and a program tries to use it, the | |
84 | kernel will automatically resume the device (autoresume). For the | |
85 | same reason, an autosuspended device will usually have remote wakeup | |
86 | enabled, if the device supports remote wakeup. | |
87 | ||
88 | It is worth mentioning that many USB drivers don't support | |
89 | autosuspend. In fact, at the time of this writing (Linux 2.6.23) the | |
90 | only drivers which do support it are the hub driver, kaweth, asix, | |
91 | usblp, usblcd, and usb-skeleton (which doesn't count). If a | |
92 | non-supporting driver is bound to a device, the device won't be | |
93 | autosuspended. In effect, the kernel pretends the device is never | |
94 | idle. | |
95 | ||
96 | We can categorize power management events in two broad classes: | |
97 | external and internal. External events are those triggered by some | |
98 | agent outside the USB stack: system suspend/resume (triggered by | |
99 | userspace), manual dynamic suspend/resume (also triggered by | |
100 | userspace), and remote wakeup (triggered by the device). Internal | |
101 | events are those triggered within the USB stack: autosuspend and | |
102 | autoresume. | |
103 | ||
104 | ||
105 | The user interface for dynamic PM | |
106 | --------------------------------- | |
107 | ||
108 | The user interface for controlling dynamic PM is located in the power/ | |
109 | subdirectory of each USB device's sysfs directory, that is, in | |
110 | /sys/bus/usb/devices/.../power/ where "..." is the device's ID. The | |
111 | relevant attribute files are: wakeup, level, and autosuspend. | |
112 | ||
113 | power/wakeup | |
114 | ||
115 | This file is empty if the device does not support | |
116 | remote wakeup. Otherwise the file contains either the | |
117 | word "enabled" or the word "disabled", and you can | |
118 | write those words to the file. The setting determines | |
119 | whether or not remote wakeup will be enabled when the | |
120 | device is next suspended. (If the setting is changed | |
121 | while the device is suspended, the change won't take | |
122 | effect until the following suspend.) | |
123 | ||
124 | power/level | |
125 | ||
126 | This file contains one of three words: "on", "auto", | |
127 | or "suspend". You can write those words to the file | |
128 | to change the device's setting. | |
129 | ||
130 | "on" means that the device should be resumed and | |
131 | autosuspend is not allowed. (Of course, system | |
132 | suspends are still allowed.) | |
133 | ||
134 | "auto" is the normal state in which the kernel is | |
135 | allowed to autosuspend and autoresume the device. | |
136 | ||
137 | "suspend" means that the device should remain | |
138 | suspended, and autoresume is not allowed. (But remote | |
139 | wakeup may still be allowed, since it is controlled | |
140 | separately by the power/wakeup attribute.) | |
141 | ||
142 | power/autosuspend | |
143 | ||
144 | This file contains an integer value, which is the | |
145 | number of seconds the device should remain idle before | |
146 | the kernel will autosuspend it (the idle-delay time). | |
147 | The default is 2. 0 means to autosuspend as soon as | |
148 | the device becomes idle, and -1 means never to | |
149 | autosuspend. You can write a number to the file to | |
150 | change the autosuspend idle-delay time. | |
151 | ||
152 | Writing "-1" to power/autosuspend and writing "on" to power/level do | |
153 | essentially the same thing -- they both prevent the device from being | |
154 | autosuspended. Yes, this is a redundancy in the API. | |
155 | ||
156 | (In 2.6.21 writing "0" to power/autosuspend would prevent the device | |
157 | from being autosuspended; the behavior was changed in 2.6.22. The | |
158 | power/autosuspend attribute did not exist prior to 2.6.21, and the | |
159 | power/level attribute did not exist prior to 2.6.22.) | |
160 | ||
161 | ||
162 | Changing the default idle-delay time | |
163 | ------------------------------------ | |
164 | ||
165 | The default autosuspend idle-delay time is controlled by a module | |
166 | parameter in usbcore. You can specify the value when usbcore is | |
167 | loaded. For example, to set it to 5 seconds instead of 2 you would | |
168 | do: | |
169 | ||
170 | modprobe usbcore autosuspend=5 | |
171 | ||
172 | Equivalently, you could add to /etc/modprobe.conf a line saying: | |
173 | ||
174 | options usbcore autosuspend=5 | |
175 | ||
176 | Some distributions load the usbcore module very early during the boot | |
177 | process, by means of a program or script running from an initramfs | |
178 | image. To alter the parameter value you would have to rebuild that | |
179 | image. | |
180 | ||
181 | If usbcore is compiled into the kernel rather than built as a loadable | |
182 | module, you can add | |
183 | ||
184 | usbcore.autosuspend=5 | |
185 | ||
186 | to the kernel's boot command line. | |
187 | ||
188 | Finally, the parameter value can be changed while the system is | |
189 | running. If you do: | |
190 | ||
191 | echo 5 >/sys/module/usbcore/parameters/autosuspend | |
192 | ||
193 | then each new USB device will have its autosuspend idle-delay | |
194 | initialized to 5. (The idle-delay values for already existing devices | |
195 | will not be affected.) | |
196 | ||
197 | Setting the initial default idle-delay to -1 will prevent any | |
198 | autosuspend of any USB device. This is a simple alternative to | |
199 | disabling CONFIG_USB_SUSPEND and rebuilding the kernel, and it has the | |
200 | added benefit of allowing you to enable autosuspend for selected | |
201 | devices. | |
202 | ||
203 | ||
204 | Warnings | |
205 | -------- | |
206 | ||
207 | The USB specification states that all USB devices must support power | |
208 | management. Nevertheless, the sad fact is that many devices do not | |
209 | support it very well. You can suspend them all right, but when you | |
210 | try to resume them they disconnect themselves from the USB bus or | |
211 | they stop working entirely. This seems to be especially prevalent | |
212 | among printers and scanners, but plenty of other types of device have | |
213 | the same deficiency. | |
214 | ||
215 | For this reason, by default the kernel disables autosuspend (the | |
216 | power/level attribute is initialized to "on") for all devices other | |
217 | than hubs. Hubs, at least, appear to be reasonably well-behaved in | |
218 | this regard. | |
219 | ||
220 | (In 2.6.21 and 2.6.22 this wasn't the case. Autosuspend was enabled | |
221 | by default for almost all USB devices. A number of people experienced | |
222 | problems as a result.) | |
223 | ||
224 | This means that non-hub devices won't be autosuspended unless the user | |
225 | or a program explicitly enables it. As of this writing there aren't | |
226 | any widespread programs which will do this; we hope that in the near | |
227 | future device managers such as HAL will take on this added | |
228 | responsibility. In the meantime you can always carry out the | |
229 | necessary operations by hand or add them to a udev script. You can | |
230 | also change the idle-delay time; 2 seconds is not the best choice for | |
231 | every device. | |
232 | ||
233 | Sometimes it turns out that even when a device does work okay with | |
234 | autosuspend there are still problems. For example, there are | |
235 | experimental patches adding autosuspend support to the usbhid driver, | |
236 | which manages keyboards and mice, among other things. Tests with a | |
237 | number of keyboards showed that typing on a suspended keyboard, while | |
238 | causing the keyboard to do a remote wakeup all right, would | |
239 | nonetheless frequently result in lost keystrokes. Tests with mice | |
240 | showed that some of them would issue a remote-wakeup request in | |
241 | response to button presses but not to motion, and some in response to | |
242 | neither. | |
243 | ||
244 | The kernel will not prevent you from enabling autosuspend on devices | |
245 | that can't handle it. It is even possible in theory to damage a | |
246 | device by suspending it at the wrong time -- for example, suspending a | |
247 | USB hard disk might cause it to spin down without parking the heads. | |
248 | (Highly unlikely, but possible.) Take care. | |
249 | ||
250 | ||
251 | The driver interface for Power Management | |
252 | ----------------------------------------- | |
253 | ||
254 | The requirements for a USB driver to support external power management | |
255 | are pretty modest; the driver need only define | |
256 | ||
257 | .suspend | |
258 | .resume | |
259 | .reset_resume | |
260 | ||
261 | methods in its usb_driver structure, and the reset_resume method is | |
262 | optional. The methods' jobs are quite simple: | |
263 | ||
264 | The suspend method is called to warn the driver that the | |
265 | device is going to be suspended. If the driver returns a | |
266 | negative error code, the suspend will be aborted. Normally | |
267 | the driver will return 0, in which case it must cancel all | |
268 | outstanding URBs (usb_kill_urb()) and not submit any more. | |
269 | ||
270 | The resume method is called to tell the driver that the | |
271 | device has been resumed and the driver can return to normal | |
272 | operation. URBs may once more be submitted. | |
273 | ||
274 | The reset_resume method is called to tell the driver that | |
275 | the device has been resumed and it also has been reset. | |
276 | The driver should redo any necessary device initialization, | |
277 | since the device has probably lost most or all of its state | |
278 | (although the interfaces will be in the same altsettings as | |
279 | before the suspend). | |
280 | ||
3c886c50 AS |
281 | If the device is disconnected or powered down while it is suspended, |
282 | the disconnect method will be called instead of the resume or | |
283 | reset_resume method. This is also quite likely to happen when | |
284 | waking up from hibernation, as many systems do not maintain suspend | |
285 | current to the USB host controllers during hibernation. (It's | |
286 | possible to work around the hibernation-forces-disconnect problem by | |
287 | using the USB Persist facility.) | |
288 | ||
cd38c1e1 AS |
289 | The reset_resume method is used by the USB Persist facility (see |
290 | Documentation/usb/persist.txt) and it can also be used under certain | |
291 | circumstances when CONFIG_USB_PERSIST is not enabled. Currently, if a | |
292 | device is reset during a resume and the driver does not have a | |
293 | reset_resume method, the driver won't receive any notification about | |
294 | the resume. Later kernels will call the driver's disconnect method; | |
295 | 2.6.23 doesn't do this. | |
296 | ||
297 | USB drivers are bound to interfaces, so their suspend and resume | |
298 | methods get called when the interfaces are suspended or resumed. In | |
299 | principle one might want to suspend some interfaces on a device (i.e., | |
300 | force the drivers for those interface to stop all activity) without | |
301 | suspending the other interfaces. The USB core doesn't allow this; all | |
302 | interfaces are suspended when the device itself is suspended and all | |
303 | interfaces are resumed when the device is resumed. It isn't possible | |
304 | to suspend or resume some but not all of a device's interfaces. The | |
305 | closest you can come is to unbind the interfaces' drivers. | |
306 | ||
307 | ||
308 | The driver interface for autosuspend and autoresume | |
309 | --------------------------------------------------- | |
310 | ||
311 | To support autosuspend and autoresume, a driver should implement all | |
312 | three of the methods listed above. In addition, a driver indicates | |
313 | that it supports autosuspend by setting the .supports_autosuspend flag | |
314 | in its usb_driver structure. It is then responsible for informing the | |
315 | USB core whenever one of its interfaces becomes busy or idle. The | |
316 | driver does so by calling these three functions: | |
317 | ||
318 | int usb_autopm_get_interface(struct usb_interface *intf); | |
319 | void usb_autopm_put_interface(struct usb_interface *intf); | |
320 | int usb_autopm_set_interface(struct usb_interface *intf); | |
321 | ||
322 | The functions work by maintaining a counter in the usb_interface | |
323 | structure. When intf->pm_usage_count is > 0 then the interface is | |
324 | deemed to be busy, and the kernel will not autosuspend the interface's | |
325 | device. When intf->pm_usage_count is <= 0 then the interface is | |
326 | considered to be idle, and the kernel may autosuspend the device. | |
327 | ||
328 | (There is a similar pm_usage_count field in struct usb_device, | |
329 | associated with the device itself rather than any of its interfaces. | |
330 | This field is used only by the USB core.) | |
331 | ||
332 | The driver owns intf->pm_usage_count; it can modify the value however | |
333 | and whenever it likes. A nice aspect of the usb_autopm_* routines is | |
334 | that the changes they make are protected by the usb_device structure's | |
335 | PM mutex (udev->pm_mutex); however drivers may change pm_usage_count | |
336 | without holding the mutex. | |
337 | ||
338 | usb_autopm_get_interface() increments pm_usage_count and | |
339 | attempts an autoresume if the new value is > 0 and the | |
340 | device is suspended. | |
341 | ||
342 | usb_autopm_put_interface() decrements pm_usage_count and | |
343 | attempts an autosuspend if the new value is <= 0 and the | |
344 | device isn't suspended. | |
345 | ||
346 | usb_autopm_set_interface() leaves pm_usage_count alone. | |
347 | It attempts an autoresume if the value is > 0 and the device | |
348 | is suspended, and it attempts an autosuspend if the value is | |
349 | <= 0 and the device isn't suspended. | |
350 | ||
351 | There also are a couple of utility routines drivers can use: | |
352 | ||
81ab5b8e | 353 | usb_autopm_enable() sets pm_usage_cnt to 0 and then calls |
cd38c1e1 AS |
354 | usb_autopm_set_interface(), which will attempt an autosuspend. |
355 | ||
81ab5b8e GL |
356 | usb_autopm_disable() sets pm_usage_cnt to 1 and then calls |
357 | usb_autopm_set_interface(), which will attempt an autoresume. | |
358 | ||
cd38c1e1 AS |
359 | The conventional usage pattern is that a driver calls |
360 | usb_autopm_get_interface() in its open routine and | |
361 | usb_autopm_put_interface() in its close or release routine. But | |
362 | other patterns are possible. | |
363 | ||
364 | The autosuspend attempts mentioned above will often fail for one | |
365 | reason or another. For example, the power/level attribute might be | |
366 | set to "on", or another interface in the same device might not be | |
367 | idle. This is perfectly normal. If the reason for failure was that | |
368 | the device hasn't been idle for long enough, a delayed workqueue | |
369 | routine is automatically set up to carry out the operation when the | |
370 | autosuspend idle-delay has expired. | |
371 | ||
372 | Autoresume attempts also can fail. This will happen if power/level is | |
373 | set to "suspend" or if the device doesn't manage to resume properly. | |
374 | Unlike autosuspend, there's no delay for an autoresume. | |
375 | ||
376 | ||
377 | Other parts of the driver interface | |
378 | ----------------------------------- | |
379 | ||
380 | Sometimes a driver needs to make sure that remote wakeup is enabled | |
381 | during autosuspend. For example, there's not much point | |
382 | autosuspending a keyboard if the user can't cause the keyboard to do a | |
383 | remote wakeup by typing on it. If the driver sets | |
384 | intf->needs_remote_wakeup to 1, the kernel won't autosuspend the | |
385 | device if remote wakeup isn't available or has been disabled through | |
386 | the power/wakeup attribute. (If the device is already autosuspended, | |
387 | though, setting this flag won't cause the kernel to autoresume it. | |
388 | Normally a driver would set this flag in its probe method, at which | |
389 | time the device is guaranteed not to be autosuspended.) | |
390 | ||
391 | The usb_autopm_* routines have to run in a sleepable process context; | |
392 | they must not be called from an interrupt handler or while holding a | |
393 | spinlock. In fact, the entire autosuspend mechanism is not well geared | |
394 | toward interrupt-driven operation. However there is one thing a | |
395 | driver can do in an interrupt handler: | |
396 | ||
397 | usb_mark_last_busy(struct usb_device *udev); | |
398 | ||
399 | This sets udev->last_busy to the current time. udev->last_busy is the | |
400 | field used for idle-delay calculations; updating it will cause any | |
401 | pending autosuspend to be moved back. The usb_autopm_* routines will | |
402 | also set the last_busy field to the current time. | |
403 | ||
404 | Calling urb_mark_last_busy() from within an URB completion handler is | |
405 | subject to races: The kernel may have just finished deciding the | |
406 | device has been idle for long enough but not yet gotten around to | |
407 | calling the driver's suspend method. The driver would have to be | |
408 | responsible for synchronizing its suspend method with its URB | |
409 | completion handler and causing the autosuspend to fail with -EBUSY if | |
410 | an URB had completed too recently. | |
411 | ||
412 | External suspend calls should never be allowed to fail in this way, | |
413 | only autosuspend calls. The driver can tell them apart by checking | |
414 | udev->auto_pm; this flag will be set to 1 for internal PM events | |
415 | (autosuspend or autoresume) and 0 for external PM events. | |
416 | ||
417 | Many of the ingredients in the autosuspend framework are oriented | |
418 | towards interfaces: The usb_interface structure contains the | |
419 | pm_usage_cnt field, and the usb_autopm_* routines take an interface | |
420 | pointer as their argument. But somewhat confusingly, a few of the | |
421 | pieces (usb_mark_last_busy() and udev->auto_pm) use the usb_device | |
422 | structure instead. Drivers need to keep this straight; they can call | |
423 | interface_to_usbdev() to find the device structure for a given | |
424 | interface. | |
425 | ||
426 | ||
427 | Locking requirements | |
428 | -------------------- | |
429 | ||
430 | All three suspend/resume methods are always called while holding the | |
431 | usb_device's PM mutex. For external events -- but not necessarily for | |
432 | autosuspend or autoresume -- the device semaphore (udev->dev.sem) will | |
433 | also be held. This implies that external suspend/resume events are | |
434 | mutually exclusive with calls to probe, disconnect, pre_reset, and | |
435 | post_reset; the USB core guarantees that this is true of internal | |
436 | suspend/resume events as well. | |
437 | ||
438 | If a driver wants to block all suspend/resume calls during some | |
f0fa7463 ON |
439 | critical section, it can simply acquire udev->pm_mutex. Note that |
440 | calls to resume may be triggered indirectly. Block IO due to memory | |
441 | allocations can make the vm subsystem resume a device. Thus while | |
442 | holding this lock you must not allocate memory with GFP_KERNEL or | |
443 | GFP_NOFS. | |
444 | ||
cd38c1e1 AS |
445 | Alternatively, if the critical section might call some of the |
446 | usb_autopm_* routines, the driver can avoid deadlock by doing: | |
447 | ||
448 | down(&udev->dev.sem); | |
449 | rc = usb_autopm_get_interface(intf); | |
450 | ||
451 | and at the end of the critical section: | |
452 | ||
453 | if (!rc) | |
454 | usb_autopm_put_interface(intf); | |
455 | up(&udev->dev.sem); | |
456 | ||
457 | Holding the device semaphore will block all external PM calls, and the | |
458 | usb_autopm_get_interface() will prevent any internal PM calls, even if | |
459 | it fails. (Exercise: Why?) | |
460 | ||
461 | The rules for locking order are: | |
462 | ||
463 | Never acquire any device semaphore while holding any PM mutex. | |
464 | ||
465 | Never acquire udev->pm_mutex while holding the PM mutex for | |
466 | a device that isn't a descendant of udev. | |
467 | ||
468 | In other words, PM mutexes should only be acquired going up the device | |
469 | tree, and they should be acquired only after locking all the device | |
470 | semaphores you need to hold. These rules don't matter to drivers very | |
471 | much; they usually affect just the USB core. | |
472 | ||
473 | Still, drivers do need to be careful. For example, many drivers use a | |
474 | private mutex to synchronize their normal I/O activities with their | |
475 | disconnect method. Now if the driver supports autosuspend then it | |
476 | must call usb_autopm_put_interface() from somewhere -- maybe from its | |
477 | close method. It should make the call while holding the private mutex, | |
478 | since a driver shouldn't call any of the usb_autopm_* functions for an | |
479 | interface from which it has been unbound. | |
480 | ||
481 | But the usb_autpm_* routines always acquire the device's PM mutex, and | |
482 | consequently the locking order has to be: private mutex first, PM | |
483 | mutex second. Since the suspend method is always called with the PM | |
484 | mutex held, it mustn't try to acquire the private mutex. It has to | |
485 | synchronize with the driver's I/O activities in some other way. | |
486 | ||
487 | ||
488 | Interaction between dynamic PM and system PM | |
489 | -------------------------------------------- | |
490 | ||
491 | Dynamic power management and system power management can interact in | |
492 | a couple of ways. | |
493 | ||
494 | Firstly, a device may already be manually suspended or autosuspended | |
495 | when a system suspend occurs. Since system suspends are supposed to | |
496 | be as transparent as possible, the device should remain suspended | |
497 | following the system resume. The 2.6.23 kernel obeys this principle | |
498 | for manually suspended devices but not for autosuspended devices; they | |
499 | do get resumed when the system wakes up. (Presumably they will be | |
500 | autosuspended again after their idle-delay time expires.) In later | |
501 | kernels this behavior will be fixed. | |
502 | ||
503 | (There is an exception. If a device would undergo a reset-resume | |
504 | instead of a normal resume, and the device is enabled for remote | |
505 | wakeup, then the reset-resume takes place even if the device was | |
506 | already suspended when the system suspend began. The justification is | |
507 | that a reset-resume is a kind of remote-wakeup event. Or to put it | |
508 | another way, a device which needs a reset won't be able to generate | |
509 | normal remote-wakeup signals, so it ought to be resumed immediately.) | |
510 | ||
511 | Secondly, a dynamic power-management event may occur as a system | |
512 | suspend is underway. The window for this is short, since system | |
513 | suspends don't take long (a few seconds usually), but it can happen. | |
514 | For example, a suspended device may send a remote-wakeup signal while | |
515 | the system is suspending. The remote wakeup may succeed, which would | |
516 | cause the system suspend to abort. If the remote wakeup doesn't | |
517 | succeed, it may still remain active and thus cause the system to | |
518 | resume as soon as the system suspend is complete. Or the remote | |
519 | wakeup may fail and get lost. Which outcome occurs depends on timing | |
520 | and on the hardware and firmware design. | |
521 | ||
522 | More interestingly, a device might undergo a manual resume or | |
523 | autoresume during system suspend. With current kernels this shouldn't | |
524 | happen, because manual resumes must be initiated by userspace and | |
525 | autoresumes happen in response to I/O requests, but all user processes | |
526 | and I/O should be quiescent during a system suspend -- thanks to the | |
527 | freezer. However there are plans to do away with the freezer, which | |
528 | would mean these things would become possible. If and when this comes | |
529 | about, the USB core will carefully arrange matters so that either type | |
530 | of resume will block until the entire system has resumed. |