1 .. _usb-power-management: 1 .. _usb-power-management:
2 2
3 Power Management for USB 3 Power Management for USB
4 ~~~~~~~~~~~~~~~~~~~~~~~~ 4 ~~~~~~~~~~~~~~~~~~~~~~~~
5 5
6 :Author: Alan Stern <stern@rowland.harvard.edu> 6 :Author: Alan Stern <stern@rowland.harvard.edu>
7 :Date: Last-updated: February 2014 7 :Date: Last-updated: February 2014
8 8
9 .. 9 ..
10 Contents: 10 Contents:
11 --------- 11 ---------
12 * What is Power Management? 12 * What is Power Management?
13 * What is Remote Wakeup? 13 * What is Remote Wakeup?
14 * When is a USB device idle? 14 * When is a USB device idle?
15 * Forms of dynamic PM 15 * Forms of dynamic PM
16 * The user interface for dynamic PM 16 * The user interface for dynamic PM
17 * Changing the default idle-delay time 17 * Changing the default idle-delay time
18 * Warnings 18 * Warnings
19 * The driver interface for Power Manag 19 * The driver interface for Power Management
20 * The driver interface for autosuspend 20 * The driver interface for autosuspend and autoresume
21 * Other parts of the driver interface 21 * Other parts of the driver interface
22 * Mutual exclusion 22 * Mutual exclusion
23 * Interaction between dynamic PM and s 23 * Interaction between dynamic PM and system PM
24 * xHCI hardware link PM 24 * xHCI hardware link PM
25 * USB Port Power Control 25 * USB Port Power Control
26 * User Interface for Port Power Contro 26 * User Interface for Port Power Control
27 * Suggested Userspace Port Power Polic 27 * Suggested Userspace Port Power Policy
28 28
29 29
30 What is Power Management? 30 What is Power Management?
31 ------------------------- 31 -------------------------
32 32
33 Power Management (PM) is the practice of savin 33 Power Management (PM) is the practice of saving energy by suspending
34 parts of a computer system when they aren't be 34 parts of a computer system when they aren't being used. While a
35 component is ``suspended`` it is in a nonfunct 35 component is ``suspended`` it is in a nonfunctional low-power state; it
36 might even be turned off completely. A suspen 36 might even be turned off completely. A suspended component can be
37 ``resumed`` (returned to a functional full-pow 37 ``resumed`` (returned to a functional full-power state) when the kernel
38 needs to use it. (There also are forms of PM 38 needs to use it. (There also are forms of PM in which components are
39 placed in a less functional but still usable s 39 placed in a less functional but still usable state instead of being
40 suspended; an example would be reducing the CP 40 suspended; an example would be reducing the CPU's clock rate. This
41 document will not discuss those other forms.) 41 document will not discuss those other forms.)
42 42
43 When the parts being suspended include the CPU 43 When the parts being suspended include the CPU and most of the rest of
44 the system, we speak of it as a "system suspen 44 the system, we speak of it as a "system suspend". When a particular
45 device is turned off while the system as a who 45 device is turned off while the system as a whole remains running, we
46 call it a "dynamic suspend" (also known as a " 46 call it a "dynamic suspend" (also known as a "runtime suspend" or
47 "selective suspend"). This document concentra 47 "selective suspend"). This document concentrates mostly on how
48 dynamic PM is implemented in the USB subsystem 48 dynamic PM is implemented in the USB subsystem, although system PM is
49 covered to some extent (see ``Documentation/po !! 49 covered to some extent (see ``Documentation/power/*.txt`` for more
50 information about system PM). 50 information about system PM).
51 51
52 System PM support is present only if the kerne 52 System PM support is present only if the kernel was built with
53 ``CONFIG_SUSPEND`` or ``CONFIG_HIBERNATION`` e 53 ``CONFIG_SUSPEND`` or ``CONFIG_HIBERNATION`` enabled. Dynamic PM support
54 54
55 for USB is present whenever 55 for USB is present whenever
56 the kernel was built with ``CONFIG_PM`` enable 56 the kernel was built with ``CONFIG_PM`` enabled.
57 57
58 [Historically, dynamic PM support for USB was 58 [Historically, dynamic PM support for USB was present only if the
59 kernel had been built with ``CONFIG_USB_SUSPEN 59 kernel had been built with ``CONFIG_USB_SUSPEND`` enabled (which depended on
60 ``CONFIG_PM_RUNTIME``). Starting with the 3.1 60 ``CONFIG_PM_RUNTIME``). Starting with the 3.10 kernel release, dynamic PM
61 support for USB was present whenever the kerne 61 support for USB was present whenever the kernel was built with
62 ``CONFIG_PM_RUNTIME`` enabled. The ``CONFIG_U 62 ``CONFIG_PM_RUNTIME`` enabled. The ``CONFIG_USB_SUSPEND`` option had been
63 eliminated.] 63 eliminated.]
64 64
65 65
66 What is Remote Wakeup? 66 What is Remote Wakeup?
67 ---------------------- 67 ----------------------
68 68
69 When a device has been suspended, it generally 69 When a device has been suspended, it generally doesn't resume until
70 the computer tells it to. Likewise, if the en 70 the computer tells it to. Likewise, if the entire computer has been
71 suspended, it generally doesn't resume until t 71 suspended, it generally doesn't resume until the user tells it to, say
72 by pressing a power button or opening the cove 72 by pressing a power button or opening the cover.
73 73
74 However some devices have the capability of re 74 However some devices have the capability of resuming by themselves, or
75 asking the kernel to resume them, or even tell 75 asking the kernel to resume them, or even telling the entire computer
76 to resume. This capability goes by several na 76 to resume. This capability goes by several names such as "Wake On
77 LAN"; we will refer to it generically as "remo 77 LAN"; we will refer to it generically as "remote wakeup". When a
78 device is enabled for remote wakeup and it is 78 device is enabled for remote wakeup and it is suspended, it may resume
79 itself (or send a request to be resumed) in re 79 itself (or send a request to be resumed) in response to some external
80 event. Examples include a suspended keyboard 80 event. Examples include a suspended keyboard resuming when a key is
81 pressed, or a suspended USB hub resuming when 81 pressed, or a suspended USB hub resuming when a device is plugged in.
82 82
83 83
84 When is a USB device idle? 84 When is a USB device idle?
85 -------------------------- 85 --------------------------
86 86
87 A device is idle whenever the kernel thinks it 87 A device is idle whenever the kernel thinks it's not busy doing
88 anything important and thus is a candidate for 88 anything important and thus is a candidate for being suspended. The
89 exact definition depends on the device's drive 89 exact definition depends on the device's driver; drivers are allowed
90 to declare that a device isn't idle even when 90 to declare that a device isn't idle even when there's no actual
91 communication taking place. (For example, a h 91 communication taking place. (For example, a hub isn't considered idle
92 unless all the devices plugged into that hub a 92 unless all the devices plugged into that hub are already suspended.)
93 In addition, a device isn't considered idle so 93 In addition, a device isn't considered idle so long as a program keeps
94 its usbfs file open, whether or not any I/O is 94 its usbfs file open, whether or not any I/O is going on.
95 95
96 If a USB device has no driver, its usbfs file 96 If a USB device has no driver, its usbfs file isn't open, and it isn't
97 being accessed through sysfs, then it definite 97 being accessed through sysfs, then it definitely is idle.
98 98
99 99
100 Forms of dynamic PM 100 Forms of dynamic PM
101 ------------------- 101 -------------------
102 102
103 Dynamic suspends occur when the kernel decides 103 Dynamic suspends occur when the kernel decides to suspend an idle
104 device. This is called ``autosuspend`` for sh 104 device. This is called ``autosuspend`` for short. In general, a device
105 won't be autosuspended unless it has been idle 105 won't be autosuspended unless it has been idle for some minimum period
106 of time, the so-called idle-delay time. 106 of time, the so-called idle-delay time.
107 107
108 Of course, nothing the kernel does on its own 108 Of course, nothing the kernel does on its own initiative should
109 prevent the computer or its devices from worki 109 prevent the computer or its devices from working properly. If a
110 device has been autosuspended and a program tr 110 device has been autosuspended and a program tries to use it, the
111 kernel will automatically resume the device (a 111 kernel will automatically resume the device (autoresume). For the
112 same reason, an autosuspended device will usua 112 same reason, an autosuspended device will usually have remote wakeup
113 enabled, if the device supports remote wakeup. 113 enabled, if the device supports remote wakeup.
114 114
115 It is worth mentioning that many USB drivers d 115 It is worth mentioning that many USB drivers don't support
116 autosuspend. In fact, at the time of this wri 116 autosuspend. In fact, at the time of this writing (Linux 2.6.23) the
117 only drivers which do support it are the hub d 117 only drivers which do support it are the hub driver, kaweth, asix,
118 usblp, usblcd, and usb-skeleton (which doesn't 118 usblp, usblcd, and usb-skeleton (which doesn't count). If a
119 non-supporting driver is bound to a device, th 119 non-supporting driver is bound to a device, the device won't be
120 autosuspended. In effect, the kernel pretends 120 autosuspended. In effect, the kernel pretends the device is never
121 idle. 121 idle.
122 122
123 We can categorize power management events in t 123 We can categorize power management events in two broad classes:
124 external and internal. External events are th 124 external and internal. External events are those triggered by some
125 agent outside the USB stack: system suspend/re 125 agent outside the USB stack: system suspend/resume (triggered by
126 userspace), manual dynamic resume (also trigge 126 userspace), manual dynamic resume (also triggered by userspace), and
127 remote wakeup (triggered by the device). Inte 127 remote wakeup (triggered by the device). Internal events are those
128 triggered within the USB stack: autosuspend an 128 triggered within the USB stack: autosuspend and autoresume. Note that
129 all dynamic suspend events are internal; exter 129 all dynamic suspend events are internal; external agents are not
130 allowed to issue dynamic suspends. 130 allowed to issue dynamic suspends.
131 131
132 132
133 The user interface for dynamic PM 133 The user interface for dynamic PM
134 --------------------------------- 134 ---------------------------------
135 135
136 The user interface for controlling dynamic PM 136 The user interface for controlling dynamic PM is located in the ``power/``
137 subdirectory of each USB device's sysfs direct 137 subdirectory of each USB device's sysfs directory, that is, in
138 ``/sys/bus/usb/devices/.../power/`` where "... 138 ``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
139 relevant attribute files are: wakeup, control, 139 relevant attribute files are: wakeup, control, and
140 ``autosuspend_delay_ms``. (There may also be 140 ``autosuspend_delay_ms``. (There may also be a file named ``level``; this
141 file was deprecated as of the 2.6.35 kernel an 141 file was deprecated as of the 2.6.35 kernel and replaced by the
142 ``control`` file. In 2.6.38 the ``autosuspend 142 ``control`` file. In 2.6.38 the ``autosuspend`` file will be deprecated
143 and replaced by the ``autosuspend_delay_ms`` f 143 and replaced by the ``autosuspend_delay_ms`` file. The only difference
144 is that the newer file expresses the delay in 144 is that the newer file expresses the delay in milliseconds whereas the
145 older file uses seconds. Confusingly, both fi 145 older file uses seconds. Confusingly, both files are present in 2.6.37
146 but only ``autosuspend`` works.) 146 but only ``autosuspend`` works.)
147 147
148 ``power/wakeup`` 148 ``power/wakeup``
149 149
150 This file is empty if the devi 150 This file is empty if the device does not support
151 remote wakeup. Otherwise the 151 remote wakeup. Otherwise the file contains either the
152 word ``enabled`` or the word ` 152 word ``enabled`` or the word ``disabled``, and you can
153 write those words to the file. 153 write those words to the file. The setting determines
154 whether or not remote wakeup w 154 whether or not remote wakeup will be enabled when the
155 device is next suspended. (If 155 device is next suspended. (If the setting is changed
156 while the device is suspended, 156 while the device is suspended, the change won't take
157 effect until the following sus 157 effect until the following suspend.)
158 158
159 ``power/control`` 159 ``power/control``
160 160
161 This file contains one of two 161 This file contains one of two words: ``on`` or ``auto``.
162 You can write those words to t 162 You can write those words to the file to change the
163 device's setting. 163 device's setting.
164 164
165 - ``on`` means that the device 165 - ``on`` means that the device should be resumed and
166 autosuspend is not allowed. 166 autosuspend is not allowed. (Of course, system
167 suspends are still allowed.) 167 suspends are still allowed.)
168 168
169 - ``auto`` is the normal state 169 - ``auto`` is the normal state in which the kernel is
170 allowed to autosuspend and a 170 allowed to autosuspend and autoresume the device.
171 171
172 (In kernels up to 2.6.32, you 172 (In kernels up to 2.6.32, you could also specify
173 ``suspend``, meaning that the 173 ``suspend``, meaning that the device should remain
174 suspended and autoresume was n 174 suspended and autoresume was not allowed. This
175 setting is no longer supported 175 setting is no longer supported.)
176 176
177 ``power/autosuspend_delay_ms`` 177 ``power/autosuspend_delay_ms``
178 178
179 This file contains an integer 179 This file contains an integer value, which is the
180 number of milliseconds the dev 180 number of milliseconds the device should remain idle
181 before the kernel will autosus 181 before the kernel will autosuspend it (the idle-delay
182 time). The default is 2000. 182 time). The default is 2000. 0 means to autosuspend
183 as soon as the device becomes 183 as soon as the device becomes idle, and negative
184 values mean never to autosuspe 184 values mean never to autosuspend. You can write a
185 number to the file to change t 185 number to the file to change the autosuspend
186 idle-delay time. 186 idle-delay time.
187 187
188 Writing ``-1`` to ``power/autosuspend_delay_ms 188 Writing ``-1`` to ``power/autosuspend_delay_ms`` and writing ``on`` to
189 ``power/control`` do essentially the same thin 189 ``power/control`` do essentially the same thing -- they both prevent the
190 device from being autosuspended. Yes, this is 190 device from being autosuspended. Yes, this is a redundancy in the
191 API. 191 API.
192 192
193 (In 2.6.21 writing ``0`` to ``power/autosuspen 193 (In 2.6.21 writing ``0`` to ``power/autosuspend`` would prevent the device
194 from being autosuspended; the behavior was cha 194 from being autosuspended; the behavior was changed in 2.6.22. The
195 ``power/autosuspend`` attribute did not exist 195 ``power/autosuspend`` attribute did not exist prior to 2.6.21, and the
196 ``power/level`` attribute did not exist prior 196 ``power/level`` attribute did not exist prior to 2.6.22. ``power/control``
197 was added in 2.6.34, and ``power/autosuspend_d 197 was added in 2.6.34, and ``power/autosuspend_delay_ms`` was added in
198 2.6.37 but did not become functional until 2.6 198 2.6.37 but did not become functional until 2.6.38.)
199 199
200 200
201 Changing the default idle-delay time 201 Changing the default idle-delay time
202 ------------------------------------ 202 ------------------------------------
203 203
204 The default autosuspend idle-delay time (in se 204 The default autosuspend idle-delay time (in seconds) is controlled by
205 a module parameter in usbcore. You can specif 205 a module parameter in usbcore. You can specify the value when usbcore
206 is loaded. For example, to set it to 5 second 206 is loaded. For example, to set it to 5 seconds instead of 2 you would
207 do:: 207 do::
208 208
209 modprobe usbcore autosuspend=5 209 modprobe usbcore autosuspend=5
210 210
211 Equivalently, you could add to a configuration 211 Equivalently, you could add to a configuration file in /etc/modprobe.d
212 a line saying:: 212 a line saying::
213 213
214 options usbcore autosuspend=5 214 options usbcore autosuspend=5
215 215
216 Some distributions load the usbcore module ver 216 Some distributions load the usbcore module very early during the boot
217 process, by means of a program or script runni 217 process, by means of a program or script running from an initramfs
218 image. To alter the parameter value you would 218 image. To alter the parameter value you would have to rebuild that
219 image. 219 image.
220 220
221 If usbcore is compiled into the kernel rather 221 If usbcore is compiled into the kernel rather than built as a loadable
222 module, you can add:: 222 module, you can add::
223 223
224 usbcore.autosuspend=5 224 usbcore.autosuspend=5
225 225
226 to the kernel's boot command line. 226 to the kernel's boot command line.
227 227
228 Finally, the parameter value can be changed wh 228 Finally, the parameter value can be changed while the system is
229 running. If you do:: 229 running. If you do::
230 230
231 echo 5 >/sys/module/usbcore/parameters 231 echo 5 >/sys/module/usbcore/parameters/autosuspend
232 232
233 then each new USB device will have its autosus 233 then each new USB device will have its autosuspend idle-delay
234 initialized to 5. (The idle-delay values for 234 initialized to 5. (The idle-delay values for already existing devices
235 will not be affected.) 235 will not be affected.)
236 236
237 Setting the initial default idle-delay to -1 w 237 Setting the initial default idle-delay to -1 will prevent any
238 autosuspend of any USB device. This has the b 238 autosuspend of any USB device. This has the benefit of allowing you
239 then to enable autosuspend for selected device 239 then to enable autosuspend for selected devices.
240 240
241 241
242 Warnings 242 Warnings
243 -------- 243 --------
244 244
245 The USB specification states that all USB devi 245 The USB specification states that all USB devices must support power
246 management. Nevertheless, the sad fact is tha 246 management. Nevertheless, the sad fact is that many devices do not
247 support it very well. You can suspend them al 247 support it very well. You can suspend them all right, but when you
248 try to resume them they disconnect themselves 248 try to resume them they disconnect themselves from the USB bus or
249 they stop working entirely. This seems to be 249 they stop working entirely. This seems to be especially prevalent
250 among printers and scanners, but plenty of oth 250 among printers and scanners, but plenty of other types of device have
251 the same deficiency. 251 the same deficiency.
252 252
253 For this reason, by default the kernel disable 253 For this reason, by default the kernel disables autosuspend (the
254 ``power/control`` attribute is initialized to 254 ``power/control`` attribute is initialized to ``on``) for all devices other
255 than hubs. Hubs, at least, appear to be reaso 255 than hubs. Hubs, at least, appear to be reasonably well-behaved in
256 this regard. 256 this regard.
257 257
258 (In 2.6.21 and 2.6.22 this wasn't the case. A 258 (In 2.6.21 and 2.6.22 this wasn't the case. Autosuspend was enabled
259 by default for almost all USB devices. A numb 259 by default for almost all USB devices. A number of people experienced
260 problems as a result.) 260 problems as a result.)
261 261
262 This means that non-hub devices won't be autos 262 This means that non-hub devices won't be autosuspended unless the user
263 or a program explicitly enables it. As of thi 263 or a program explicitly enables it. As of this writing there aren't
264 any widespread programs which will do this; we 264 any widespread programs which will do this; we hope that in the near
265 future device managers such as HAL will take o 265 future device managers such as HAL will take on this added
266 responsibility. In the meantime you can alway 266 responsibility. In the meantime you can always carry out the
267 necessary operations by hand or add them to a 267 necessary operations by hand or add them to a udev script. You can
268 also change the idle-delay time; 2 seconds is 268 also change the idle-delay time; 2 seconds is not the best choice for
269 every device. 269 every device.
270 270
271 If a driver knows that its device has proper s 271 If a driver knows that its device has proper suspend/resume support,
272 it can enable autosuspend all by itself. For 272 it can enable autosuspend all by itself. For example, the video
273 driver for a laptop's webcam might do this (in 273 driver for a laptop's webcam might do this (in recent kernels they
274 do), since these devices are rarely used and s 274 do), since these devices are rarely used and so should normally be
275 autosuspended. 275 autosuspended.
276 276
277 Sometimes it turns out that even when a device 277 Sometimes it turns out that even when a device does work okay with
278 autosuspend there are still problems. For exa 278 autosuspend there are still problems. For example, the usbhid driver,
279 which manages keyboards and mice, has autosusp 279 which manages keyboards and mice, has autosuspend support. Tests with
280 a number of keyboards show that typing on a su 280 a number of keyboards show that typing on a suspended keyboard, while
281 causing the keyboard to do a remote wakeup all 281 causing the keyboard to do a remote wakeup all right, will nonetheless
282 frequently result in lost keystrokes. Tests w 282 frequently result in lost keystrokes. Tests with mice show that some
283 of them will issue a remote-wakeup request in 283 of them will issue a remote-wakeup request in response to button
284 presses but not to motion, and some in respons 284 presses but not to motion, and some in response to neither.
285 285
286 The kernel will not prevent you from enabling 286 The kernel will not prevent you from enabling autosuspend on devices
287 that can't handle it. It is even possible in 287 that can't handle it. It is even possible in theory to damage a
288 device by suspending it at the wrong time. (H 288 device by suspending it at the wrong time. (Highly unlikely, but
289 possible.) Take care. 289 possible.) Take care.
290 290
291 291
292 The driver interface for Power Management 292 The driver interface for Power Management
293 ----------------------------------------- 293 -----------------------------------------
294 294
295 The requirements for a USB driver to support e 295 The requirements for a USB driver to support external power management
296 are pretty modest; the driver need only define 296 are pretty modest; the driver need only define::
297 297
298 .suspend 298 .suspend
299 .resume 299 .resume
300 .reset_resume 300 .reset_resume
301 301
302 methods in its :c:type:`usb_driver` structure, 302 methods in its :c:type:`usb_driver` structure, and the ``reset_resume`` method
303 is optional. The methods' jobs are quite simp 303 is optional. The methods' jobs are quite simple:
304 304
305 - The ``suspend`` method is called to wa 305 - The ``suspend`` method is called to warn the driver that the
306 device is going to be suspended. If t 306 device is going to be suspended. If the driver returns a
307 negative error code, the suspend will 307 negative error code, the suspend will be aborted. Normally
308 the driver will return 0, in which cas 308 the driver will return 0, in which case it must cancel all
309 outstanding URBs (:c:func:`usb_kill_ur 309 outstanding URBs (:c:func:`usb_kill_urb`) and not submit any more.
310 310
311 - The ``resume`` method is called to tel 311 - The ``resume`` method is called to tell the driver that the
312 device has been resumed and the driver 312 device has been resumed and the driver can return to normal
313 operation. URBs may once more be subm 313 operation. URBs may once more be submitted.
314 314
315 - The ``reset_resume`` method is called 315 - The ``reset_resume`` method is called to tell the driver that
316 the device has been resumed and it als 316 the device has been resumed and it also has been reset.
317 The driver should redo any necessary d 317 The driver should redo any necessary device initialization,
318 since the device has probably lost mos 318 since the device has probably lost most or all of its state
319 (although the interfaces will be in th 319 (although the interfaces will be in the same altsettings as
320 before the suspend). 320 before the suspend).
321 321
322 If the device is disconnected or powered down 322 If the device is disconnected or powered down while it is suspended,
323 the ``disconnect`` method will be called inste 323 the ``disconnect`` method will be called instead of the ``resume`` or
324 ``reset_resume`` method. This is also quite l 324 ``reset_resume`` method. This is also quite likely to happen when
325 waking up from hibernation, as many systems do 325 waking up from hibernation, as many systems do not maintain suspend
326 current to the USB host controllers during hib 326 current to the USB host controllers during hibernation. (It's
327 possible to work around the hibernation-forces 327 possible to work around the hibernation-forces-disconnect problem by
328 using the USB Persist facility.) 328 using the USB Persist facility.)
329 329
330 The ``reset_resume`` method is used by the USB 330 The ``reset_resume`` method is used by the USB Persist facility (see
331 :ref:`usb-persist`) and it can also be used un 331 :ref:`usb-persist`) and it can also be used under certain
332 circumstances when ``CONFIG_USB_PERSIST`` is n 332 circumstances when ``CONFIG_USB_PERSIST`` is not enabled. Currently, if a
333 device is reset during a resume and the driver 333 device is reset during a resume and the driver does not have a
334 ``reset_resume`` method, the driver won't rece 334 ``reset_resume`` method, the driver won't receive any notification about
335 the resume. Later kernels will call the drive 335 the resume. Later kernels will call the driver's ``disconnect`` method;
336 2.6.23 doesn't do this. 336 2.6.23 doesn't do this.
337 337
338 USB drivers are bound to interfaces, so their 338 USB drivers are bound to interfaces, so their ``suspend`` and ``resume``
339 methods get called when the interfaces are sus 339 methods get called when the interfaces are suspended or resumed. In
340 principle one might want to suspend some inter 340 principle one might want to suspend some interfaces on a device (i.e.,
341 force the drivers for those interface to stop 341 force the drivers for those interface to stop all activity) without
342 suspending the other interfaces. The USB core 342 suspending the other interfaces. The USB core doesn't allow this; all
343 interfaces are suspended when the device itsel 343 interfaces are suspended when the device itself is suspended and all
344 interfaces are resumed when the device is resu 344 interfaces are resumed when the device is resumed. It isn't possible
345 to suspend or resume some but not all of a dev 345 to suspend or resume some but not all of a device's interfaces. The
346 closest you can come is to unbind the interfac 346 closest you can come is to unbind the interfaces' drivers.
347 347
348 348
349 The driver interface for autosuspend and autor 349 The driver interface for autosuspend and autoresume
350 ---------------------------------------------- 350 ---------------------------------------------------
351 351
352 To support autosuspend and autoresume, a drive 352 To support autosuspend and autoresume, a driver should implement all
353 three of the methods listed above. In additio 353 three of the methods listed above. In addition, a driver indicates
354 that it supports autosuspend by setting the `` 354 that it supports autosuspend by setting the ``.supports_autosuspend`` flag
355 in its usb_driver structure. It is then respo 355 in its usb_driver structure. It is then responsible for informing the
356 USB core whenever one of its interfaces become 356 USB core whenever one of its interfaces becomes busy or idle. The
357 driver does so by calling these six functions: 357 driver does so by calling these six functions::
358 358
359 int usb_autopm_get_interface(struct u 359 int usb_autopm_get_interface(struct usb_interface *intf);
360 void usb_autopm_put_interface(struct u 360 void usb_autopm_put_interface(struct usb_interface *intf);
361 int usb_autopm_get_interface_async(st 361 int usb_autopm_get_interface_async(struct usb_interface *intf);
362 void usb_autopm_put_interface_async(st 362 void usb_autopm_put_interface_async(struct usb_interface *intf);
363 void usb_autopm_get_interface_no_resum 363 void usb_autopm_get_interface_no_resume(struct usb_interface *intf);
364 void usb_autopm_put_interface_no_suspe 364 void usb_autopm_put_interface_no_suspend(struct usb_interface *intf);
365 365
366 The functions work by maintaining a usage coun 366 The functions work by maintaining a usage counter in the
367 usb_interface's embedded device structure. Wh 367 usb_interface's embedded device structure. When the counter is > 0
368 then the interface is deemed to be busy, and t 368 then the interface is deemed to be busy, and the kernel will not
369 autosuspend the interface's device. When the 369 autosuspend the interface's device. When the usage counter is = 0
370 then the interface is considered to be idle, a 370 then the interface is considered to be idle, and the kernel may
371 autosuspend the device. 371 autosuspend the device.
372 372
373 Drivers must be careful to balance their overa 373 Drivers must be careful to balance their overall changes to the usage
374 counter. Unbalanced "get"s will remain in eff 374 counter. Unbalanced "get"s will remain in effect when a driver is
375 unbound from its interface, preventing the dev 375 unbound from its interface, preventing the device from going into
376 runtime suspend should the interface be bound 376 runtime suspend should the interface be bound to a driver again. On
377 the other hand, drivers are allowed to achieve 377 the other hand, drivers are allowed to achieve this balance by calling
378 the ``usb_autopm_*`` functions even after thei 378 the ``usb_autopm_*`` functions even after their ``disconnect`` routine
379 has returned -- say from within a work-queue r 379 has returned -- say from within a work-queue routine -- provided they
380 retain an active reference to the interface (v 380 retain an active reference to the interface (via ``usb_get_intf`` and
381 ``usb_put_intf``). 381 ``usb_put_intf``).
382 382
383 Drivers using the async routines are responsib 383 Drivers using the async routines are responsible for their own
384 synchronization and mutual exclusion. 384 synchronization and mutual exclusion.
385 385
386 :c:func:`usb_autopm_get_interface` inc 386 :c:func:`usb_autopm_get_interface` increments the usage counter and
387 does an autoresume if the device is su 387 does an autoresume if the device is suspended. If the
388 autoresume fails, the counter is decre 388 autoresume fails, the counter is decremented back.
389 389
390 :c:func:`usb_autopm_put_interface` dec 390 :c:func:`usb_autopm_put_interface` decrements the usage counter and
391 attempts an autosuspend if the new val 391 attempts an autosuspend if the new value is = 0.
392 392
393 :c:func:`usb_autopm_get_interface_asyn 393 :c:func:`usb_autopm_get_interface_async` and
394 :c:func:`usb_autopm_put_interface_asyn 394 :c:func:`usb_autopm_put_interface_async` do almost the same things as
395 their non-async counterparts. The big 395 their non-async counterparts. The big difference is that they
396 use a workqueue to do the resume or su 396 use a workqueue to do the resume or suspend part of their
397 jobs. As a result they can be called 397 jobs. As a result they can be called in an atomic context,
398 such as an URB's completion handler, b 398 such as an URB's completion handler, but when they return the
399 device will generally not yet be in th 399 device will generally not yet be in the desired state.
400 400
401 :c:func:`usb_autopm_get_interface_no_r 401 :c:func:`usb_autopm_get_interface_no_resume` and
402 :c:func:`usb_autopm_put_interface_no_s 402 :c:func:`usb_autopm_put_interface_no_suspend` merely increment or
403 decrement the usage counter; they do n 403 decrement the usage counter; they do not attempt to carry out
404 an autoresume or an autosuspend. Henc 404 an autoresume or an autosuspend. Hence they can be called in
405 an atomic context. 405 an atomic context.
406 406
407 The simplest usage pattern is that a driver ca 407 The simplest usage pattern is that a driver calls
408 :c:func:`usb_autopm_get_interface` in its open 408 :c:func:`usb_autopm_get_interface` in its open routine and
409 :c:func:`usb_autopm_put_interface` in its clos 409 :c:func:`usb_autopm_put_interface` in its close or release routine. But other
410 patterns are possible. 410 patterns are possible.
411 411
412 The autosuspend attempts mentioned above will 412 The autosuspend attempts mentioned above will often fail for one
413 reason or another. For example, the ``power/c 413 reason or another. For example, the ``power/control`` attribute might be
414 set to ``on``, or another interface in the sam 414 set to ``on``, or another interface in the same device might not be
415 idle. This is perfectly normal. If the reaso 415 idle. This is perfectly normal. If the reason for failure was that
416 the device hasn't been idle for long enough, a 416 the device hasn't been idle for long enough, a timer is scheduled to
417 carry out the operation automatically when the 417 carry out the operation automatically when the autosuspend idle-delay
418 has expired. 418 has expired.
419 419
420 Autoresume attempts also can fail, although fa 420 Autoresume attempts also can fail, although failure would mean that
421 the device is no longer present or operating p 421 the device is no longer present or operating properly. Unlike
422 autosuspend, there's no idle-delay for an auto 422 autosuspend, there's no idle-delay for an autoresume.
423 423
424 424
425 Other parts of the driver interface 425 Other parts of the driver interface
426 ----------------------------------- 426 -----------------------------------
427 427
428 Drivers can enable autosuspend for their devic 428 Drivers can enable autosuspend for their devices by calling::
429 429
430 usb_enable_autosuspend(struct usb_devi 430 usb_enable_autosuspend(struct usb_device *udev);
431 431
432 in their :c:func:`probe` routine, if they know 432 in their :c:func:`probe` routine, if they know that the device is capable of
433 suspending and resuming correctly. This is ex 433 suspending and resuming correctly. This is exactly equivalent to
434 writing ``auto`` to the device's ``power/contr 434 writing ``auto`` to the device's ``power/control`` attribute. Likewise,
435 drivers can disable autosuspend by calling:: 435 drivers can disable autosuspend by calling::
436 436
437 usb_disable_autosuspend(struct usb_dev 437 usb_disable_autosuspend(struct usb_device *udev);
438 438
439 This is exactly the same as writing ``on`` to 439 This is exactly the same as writing ``on`` to the ``power/control`` attribute.
440 440
441 Sometimes a driver needs to make sure that rem 441 Sometimes a driver needs to make sure that remote wakeup is enabled
442 during autosuspend. For example, there's not 442 during autosuspend. For example, there's not much point
443 autosuspending a keyboard if the user can't ca 443 autosuspending a keyboard if the user can't cause the keyboard to do a
444 remote wakeup by typing on it. If the driver 444 remote wakeup by typing on it. If the driver sets
445 ``intf->needs_remote_wakeup`` to 1, the kernel 445 ``intf->needs_remote_wakeup`` to 1, the kernel won't autosuspend the
446 device if remote wakeup isn't available. (If 446 device if remote wakeup isn't available. (If the device is already
447 autosuspended, though, setting this flag won't 447 autosuspended, though, setting this flag won't cause the kernel to
448 autoresume it. Normally a driver would set th 448 autoresume it. Normally a driver would set this flag in its ``probe``
449 method, at which time the device is guaranteed 449 method, at which time the device is guaranteed not to be
450 autosuspended.) 450 autosuspended.)
451 451
452 If a driver does its I/O asynchronously in int 452 If a driver does its I/O asynchronously in interrupt context, it
453 should call :c:func:`usb_autopm_get_interface_ 453 should call :c:func:`usb_autopm_get_interface_async` before starting output and
454 :c:func:`usb_autopm_put_interface_async` when 454 :c:func:`usb_autopm_put_interface_async` when the output queue drains. When
455 it receives an input event, it should call:: 455 it receives an input event, it should call::
456 456
457 usb_mark_last_busy(struct usb_device * 457 usb_mark_last_busy(struct usb_device *udev);
458 458
459 in the event handler. This tells the PM core 459 in the event handler. This tells the PM core that the device was just
460 busy and therefore the next autosuspend idle-d 460 busy and therefore the next autosuspend idle-delay expiration should
461 be pushed back. Many of the usb_autopm_* rout 461 be pushed back. Many of the usb_autopm_* routines also make this call,
462 so drivers need to worry only when interrupt-d 462 so drivers need to worry only when interrupt-driven input arrives.
463 463
464 Asynchronous operation is always subject to ra 464 Asynchronous operation is always subject to races. For example, a
465 driver may call the :c:func:`usb_autopm_get_in 465 driver may call the :c:func:`usb_autopm_get_interface_async` routine at a time
466 when the core has just finished deciding the d 466 when the core has just finished deciding the device has been idle for
467 long enough but not yet gotten around to calli 467 long enough but not yet gotten around to calling the driver's ``suspend``
468 method. The ``suspend`` method must be respon 468 method. The ``suspend`` method must be responsible for synchronizing with
469 the I/O request routine and the URB completion 469 the I/O request routine and the URB completion handler; it should
470 cause autosuspends to fail with -EBUSY if the 470 cause autosuspends to fail with -EBUSY if the driver needs to use the
471 device. 471 device.
472 472
473 External suspend calls should never be allowed 473 External suspend calls should never be allowed to fail in this way,
474 only autosuspend calls. The driver can tell t 474 only autosuspend calls. The driver can tell them apart by applying
475 the :c:func:`PMSG_IS_AUTO` macro to the messag 475 the :c:func:`PMSG_IS_AUTO` macro to the message argument to the ``suspend``
476 method; it will return True for internal PM ev 476 method; it will return True for internal PM events (autosuspend) and
477 False for external PM events. 477 False for external PM events.
478 478
479 479
480 Mutual exclusion 480 Mutual exclusion
481 ---------------- 481 ----------------
482 482
483 For external events -- but not necessarily for 483 For external events -- but not necessarily for autosuspend or
484 autoresume -- the device semaphore (udev->dev. 484 autoresume -- the device semaphore (udev->dev.sem) will be held when a
485 ``suspend`` or ``resume`` method is called. T 485 ``suspend`` or ``resume`` method is called. This implies that external
486 suspend/resume events are mutually exclusive w 486 suspend/resume events are mutually exclusive with calls to ``probe``,
487 ``disconnect``, ``pre_reset``, and ``post_rese 487 ``disconnect``, ``pre_reset``, and ``post_reset``; the USB core guarantees that
488 this is true of autosuspend/autoresume events 488 this is true of autosuspend/autoresume events as well.
489 489
490 If a driver wants to block all suspend/resume 490 If a driver wants to block all suspend/resume calls during some
491 critical section, the best way is to lock the 491 critical section, the best way is to lock the device and call
492 :c:func:`usb_autopm_get_interface` (and do the 492 :c:func:`usb_autopm_get_interface` (and do the reverse at the end of the
493 critical section). Holding the device semapho 493 critical section). Holding the device semaphore will block all
494 external PM calls, and the :c:func:`usb_autopm 494 external PM calls, and the :c:func:`usb_autopm_get_interface` will prevent any
495 internal PM calls, even if it fails. (Exercis 495 internal PM calls, even if it fails. (Exercise: Why?)
496 496
497 497
498 Interaction between dynamic PM and system PM 498 Interaction between dynamic PM and system PM
499 -------------------------------------------- 499 --------------------------------------------
500 500
501 Dynamic power management and system power mana 501 Dynamic power management and system power management can interact in
502 a couple of ways. 502 a couple of ways.
503 503
504 Firstly, a device may already be autosuspended 504 Firstly, a device may already be autosuspended when a system suspend
505 occurs. Since system suspends are supposed to 505 occurs. Since system suspends are supposed to be as transparent as
506 possible, the device should remain suspended f 506 possible, the device should remain suspended following the system
507 resume. But this theory may not work out well 507 resume. But this theory may not work out well in practice; over time
508 the kernel's behavior in this regard has chang 508 the kernel's behavior in this regard has changed. As of 2.6.37 the
509 policy is to resume all devices during a syste 509 policy is to resume all devices during a system resume and let them
510 handle their own runtime suspends afterward. 510 handle their own runtime suspends afterward.
511 511
512 Secondly, a dynamic power-management event may 512 Secondly, a dynamic power-management event may occur as a system
513 suspend is underway. The window for this is s 513 suspend is underway. The window for this is short, since system
514 suspends don't take long (a few seconds usuall 514 suspends don't take long (a few seconds usually), but it can happen.
515 For example, a suspended device may send a rem 515 For example, a suspended device may send a remote-wakeup signal while
516 the system is suspending. The remote wakeup m 516 the system is suspending. The remote wakeup may succeed, which would
517 cause the system suspend to abort. If the rem 517 cause the system suspend to abort. If the remote wakeup doesn't
518 succeed, it may still remain active and thus c 518 succeed, it may still remain active and thus cause the system to
519 resume as soon as the system suspend is comple 519 resume as soon as the system suspend is complete. Or the remote
520 wakeup may fail and get lost. Which outcome o 520 wakeup may fail and get lost. Which outcome occurs depends on timing
521 and on the hardware and firmware design. 521 and on the hardware and firmware design.
522 522
523 523
524 xHCI hardware link PM 524 xHCI hardware link PM
525 --------------------- 525 ---------------------
526 526
527 xHCI host controller provides hardware link po 527 xHCI host controller provides hardware link power management to usb2.0
528 (xHCI 1.0 feature) and usb3.0 devices which su 528 (xHCI 1.0 feature) and usb3.0 devices which support link PM. By
529 enabling hardware LPM, the host can automatica 529 enabling hardware LPM, the host can automatically put the device into
530 lower power state(L1 for usb2.0 devices, or U1 530 lower power state(L1 for usb2.0 devices, or U1/U2 for usb3.0 devices),
531 which state device can enter and resume very q 531 which state device can enter and resume very quickly.
532 532
533 The user interface for controlling hardware LP 533 The user interface for controlling hardware LPM is located in the
534 ``power/`` subdirectory of each USB device's s 534 ``power/`` subdirectory of each USB device's sysfs directory, that is, in
535 ``/sys/bus/usb/devices/.../power/`` where "... 535 ``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
536 relevant attribute files are ``usb2_hardware_l 536 relevant attribute files are ``usb2_hardware_lpm`` and ``usb3_hardware_lpm``.
537 537
538 ``power/usb2_hardware_lpm`` 538 ``power/usb2_hardware_lpm``
539 539
540 When a USB2 device which suppo 540 When a USB2 device which support LPM is plugged to a
541 xHCI host root hub which suppo 541 xHCI host root hub which support software LPM, the
542 host will run a software LPM t 542 host will run a software LPM test for it; if the device
543 enters L1 state and resume suc 543 enters L1 state and resume successfully and the host
544 supports USB2 hardware LPM, th 544 supports USB2 hardware LPM, this file will show up and
545 driver will enable hardware LP 545 driver will enable hardware LPM for the device. You
546 can write y/Y/1 or n/N/0 to th 546 can write y/Y/1 or n/N/0 to the file to enable/disable
547 USB2 hardware LPM manually. Th 547 USB2 hardware LPM manually. This is for test purpose mainly.
548 548
549 ``power/usb3_hardware_lpm_u1`` 549 ``power/usb3_hardware_lpm_u1``
550 ``power/usb3_hardware_lpm_u2`` 550 ``power/usb3_hardware_lpm_u2``
551 551
552 When a USB 3.0 lpm-capable dev 552 When a USB 3.0 lpm-capable device is plugged in to a
553 xHCI host which supports link 553 xHCI host which supports link PM, it will check if U1
554 and U2 exit latencies have bee 554 and U2 exit latencies have been set in the BOS
555 descriptor; if the check is pa 555 descriptor; if the check is passed and the host
556 supports USB3 hardware LPM, US 556 supports USB3 hardware LPM, USB3 hardware LPM will be
557 enabled for the device and the 557 enabled for the device and these files will be created.
558 The files hold a string value 558 The files hold a string value (enable or disable)
559 indicating whether or not USB3 559 indicating whether or not USB3 hardware LPM U1 or U2
560 is enabled for the device. 560 is enabled for the device.
561 561
562 USB Port Power Control 562 USB Port Power Control
563 ---------------------- 563 ----------------------
564 564
565 In addition to suspending endpoint devices and 565 In addition to suspending endpoint devices and enabling hardware
566 controlled link power management, the USB subs 566 controlled link power management, the USB subsystem also has the
567 capability to disable power to ports under som 567 capability to disable power to ports under some conditions. Power is
568 controlled through ``Set/ClearPortFeature(PORT 568 controlled through ``Set/ClearPortFeature(PORT_POWER)`` requests to a hub.
569 In the case of a root or platform-internal hub 569 In the case of a root or platform-internal hub the host controller
570 driver translates ``PORT_POWER`` requests into 570 driver translates ``PORT_POWER`` requests into platform firmware (ACPI)
571 method calls to set the port power state. For 571 method calls to set the port power state. For more background see the
572 Linux Plumbers Conference 2012 slides [#f1]_ a 572 Linux Plumbers Conference 2012 slides [#f1]_ and video [#f2]_:
573 573
574 Upon receiving a ``ClearPortFeature(PORT_POWER 574 Upon receiving a ``ClearPortFeature(PORT_POWER)`` request a USB port is
575 logically off, and may trigger the actual loss 575 logically off, and may trigger the actual loss of VBUS to the port [#f3]_.
576 VBUS may be maintained in the case where a hub 576 VBUS may be maintained in the case where a hub gangs multiple ports into
577 a shared power well causing power to remain un 577 a shared power well causing power to remain until all ports in the gang
578 are turned off. VBUS may also be maintained b 578 are turned off. VBUS may also be maintained by hub ports configured for
579 a charging application. In any event a logica 579 a charging application. In any event a logically off port will lose
580 connection with its device, not respond to hot 580 connection with its device, not respond to hotplug events, and not
581 respond to remote wakeup events. 581 respond to remote wakeup events.
582 582
583 .. warning:: 583 .. warning::
584 584
585 turning off a port may result in the inabil 585 turning off a port may result in the inability to hot add a device.
586 Please see "User Interface for Port Power C 586 Please see "User Interface for Port Power Control" for details.
587 587
588 As far as the effect on the device itself it i 588 As far as the effect on the device itself it is similar to what a device
589 goes through during system suspend, i.e. the p 589 goes through during system suspend, i.e. the power session is lost. Any
590 USB device or driver that misbehaves with syst 590 USB device or driver that misbehaves with system suspend will be
591 similarly affected by a port power cycle event 591 similarly affected by a port power cycle event. For this reason the
592 implementation shares the same device recovery 592 implementation shares the same device recovery path (and honors the same
593 quirks) as the system resume path for the hub. 593 quirks) as the system resume path for the hub.
594 594
595 .. [#f1] 595 .. [#f1]
596 596
597 http://dl.dropbox.com/u/96820575/sarah-sharp 597 http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf
598 598
599 .. [#f2] 599 .. [#f2]
600 600
601 http://linuxplumbers.ubicast.tv/videos/usb-p 601 http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/
602 602
603 .. [#f3] 603 .. [#f3]
604 604
605 USB 3.1 Section 10.12 605 USB 3.1 Section 10.12
606 606
607 wakeup note: if a device is configured to se 607 wakeup note: if a device is configured to send wakeup events the port
608 power control implementation will block powe 608 power control implementation will block poweroff attempts on that
609 port. 609 port.
610 610
611 611
612 User Interface for Port Power Control 612 User Interface for Port Power Control
613 ------------------------------------- 613 -------------------------------------
614 614
615 The port power control mechanism uses the PM r 615 The port power control mechanism uses the PM runtime system. Poweroff is
616 requested by clearing the ``power/pm_qos_no_po 616 requested by clearing the ``power/pm_qos_no_power_off`` flag of the port device
617 (defaults to 1). If the port is disconnected 617 (defaults to 1). If the port is disconnected it will immediately receive a
618 ``ClearPortFeature(PORT_POWER)`` request. Oth 618 ``ClearPortFeature(PORT_POWER)`` request. Otherwise, it will honor the pm
619 runtime rules and require the attached child d 619 runtime rules and require the attached child device and all descendants to be
620 suspended. This mechanism is dependent on the 620 suspended. This mechanism is dependent on the hub advertising port power
621 switching in its hub descriptor (wHubCharacter 621 switching in its hub descriptor (wHubCharacteristics logical power switching
622 mode field). 622 mode field).
623 623
624 Note, some interface devices/drivers do not su 624 Note, some interface devices/drivers do not support autosuspend. Userspace may
625 need to unbind the interface drivers before th 625 need to unbind the interface drivers before the :c:type:`usb_device` will
626 suspend. An unbound interface device is suspe 626 suspend. An unbound interface device is suspended by default. When unbinding,
627 be careful to unbind interface drivers, not th 627 be careful to unbind interface drivers, not the driver of the parent usb
628 device. Also, leave hub interface drivers bou 628 device. Also, leave hub interface drivers bound. If the driver for the usb
629 device (not interface) is unbound the kernel i 629 device (not interface) is unbound the kernel is no longer able to resume the
630 device. If a hub interface driver is unbound, 630 device. If a hub interface driver is unbound, control of its child ports is
631 lost and all attached child-devices will disco 631 lost and all attached child-devices will disconnect. A good rule of thumb is
632 that if the 'driver/module' link for a device 632 that if the 'driver/module' link for a device points to
633 ``/sys/module/usbcore`` then unbinding it will 633 ``/sys/module/usbcore`` then unbinding it will interfere with port power
634 control. 634 control.
635 635
636 Example of the relevant files for port power c 636 Example of the relevant files for port power control. Note, in this example
637 these files are relative to a usb hub device ( 637 these files are relative to a usb hub device (prefix)::
638 638
639 prefix=/sys/devices/pci0000:00/0000:00:14 639 prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1
640 640
641 attached child device + 641 attached child device +
642 hub port device + | 642 hub port device + |
643 hub interface device + | | 643 hub interface device + | |
644 v v v 644 v v v
645 $prefix/3-1:1.0/3-1-port1/de 645 $prefix/3-1:1.0/3-1-port1/device
646 646
647 $prefix/3-1:1.0/3-1-port1/power/pm_qos_no 647 $prefix/3-1:1.0/3-1-port1/power/pm_qos_no_power_off
648 $prefix/3-1:1.0/3-1-port1/device/power/co 648 $prefix/3-1:1.0/3-1-port1/device/power/control
649 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<i 649 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf0>/driver/unbind
650 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<i 650 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf1>/driver/unbind
651 ... 651 ...
652 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<i 652 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intfN>/driver/unbind
653 653
654 In addition to these files some ports may have 654 In addition to these files some ports may have a 'peer' link to a port on
655 another hub. The expectation is that all supe 655 another hub. The expectation is that all superspeed ports have a
656 hi-speed peer:: 656 hi-speed peer::
657 657
658 $prefix/3-1:1.0/3-1-port1/peer -> ../../../. 658 $prefix/3-1:1.0/3-1-port1/peer -> ../../../../usb2/2-1/2-1:1.0/2-1-port1
659 ../../../../usb2/2-1/2-1:1.0/2-1-port1/peer 659 ../../../../usb2/2-1/2-1:1.0/2-1-port1/peer -> ../../../../usb3/3-1/3-1:1.0/3-1-port1
660 660
661 Distinct from 'companion ports', or 'ehci/xhci 661 Distinct from 'companion ports', or 'ehci/xhci shared switchover ports'
662 peer ports are simply the hi-speed and supersp 662 peer ports are simply the hi-speed and superspeed interface pins that
663 are combined into a single usb3 connector. Pe 663 are combined into a single usb3 connector. Peer ports share the same
664 ancestor XHCI device. 664 ancestor XHCI device.
665 665
666 While a superspeed port is powered off a devic 666 While a superspeed port is powered off a device may downgrade its
667 connection and attempt to connect to the hi-sp 667 connection and attempt to connect to the hi-speed pins. The
668 implementation takes steps to prevent this: 668 implementation takes steps to prevent this:
669 669
670 1. Port suspend is sequenced to guarantee that 670 1. Port suspend is sequenced to guarantee that hi-speed ports are powered-off
671 before their superspeed peer is permitted t 671 before their superspeed peer is permitted to power-off. The implication is
672 that the setting ``pm_qos_no_power_off`` to 672 that the setting ``pm_qos_no_power_off`` to zero on a superspeed port may
673 not cause the port to power-off until its h 673 not cause the port to power-off until its highspeed peer has gone to its
674 runtime suspend state. Userspace must take 674 runtime suspend state. Userspace must take care to order the suspensions
675 if it wants to guarantee that a superspeed 675 if it wants to guarantee that a superspeed port will power-off.
676 676
677 2. Port resume is sequenced to force a supersp 677 2. Port resume is sequenced to force a superspeed port to power-on prior to its
678 highspeed peer. 678 highspeed peer.
679 679
680 3. Port resume always triggers an attached chi 680 3. Port resume always triggers an attached child device to resume. After a
681 power session is lost the device may have b 681 power session is lost the device may have been removed, or need reset.
682 Resuming the child device when the parent p 682 Resuming the child device when the parent port regains power resolves those
683 states and clamps the maximum port power cy 683 states and clamps the maximum port power cycle frequency at the rate the
684 child device can suspend (autosuspend-delay 684 child device can suspend (autosuspend-delay) and resume (reset-resume
685 latency). 685 latency).
686 686
687 Sysfs files relevant for port power control: 687 Sysfs files relevant for port power control:
688 688
689 ``<hubdev-portX>/power/pm_qos_no_power 689 ``<hubdev-portX>/power/pm_qos_no_power_off``:
690 This writable flag controls th 690 This writable flag controls the state of an idle port.
691 Once all children and descenda 691 Once all children and descendants have suspended the
692 port may suspend/poweroff prov 692 port may suspend/poweroff provided that
693 pm_qos_no_power_off is '0'. I 693 pm_qos_no_power_off is '0'. If pm_qos_no_power_off is
694 '1' the port will remain activ 694 '1' the port will remain active/powered regardless of
695 the stats of descendants. Def 695 the stats of descendants. Defaults to 1.
696 696
697 ``<hubdev-portX>/power/runtime_status` 697 ``<hubdev-portX>/power/runtime_status``:
698 This file reflects whether the 698 This file reflects whether the port is 'active' (power is on)
699 or 'suspended' (logically off) 699 or 'suspended' (logically off). There is no indication to
700 userspace whether VBUS is stil 700 userspace whether VBUS is still supplied.
701 701
702 ``<hubdev-portX>/connect_type``: 702 ``<hubdev-portX>/connect_type``:
703 An advisory read-only flag to 703 An advisory read-only flag to userspace indicating the
704 location and connection type o 704 location and connection type of the port. It returns
705 one of four values 'hotplug', 705 one of four values 'hotplug', 'hardwired', 'not used',
706 and 'unknown'. All values, be 706 and 'unknown'. All values, besides unknown, are set by
707 platform firmware. 707 platform firmware.
708 708
709 ``hotplug`` indicates an exter 709 ``hotplug`` indicates an externally connectable/visible
710 port on the platform. Typical 710 port on the platform. Typically userspace would choose
711 to keep such a port powered to 711 to keep such a port powered to handle new device
712 connection events. 712 connection events.
713 713
714 ``hardwired`` refers to a port 714 ``hardwired`` refers to a port that is not visible but
715 connectable. Examples are inte 715 connectable. Examples are internal ports for USB
716 bluetooth that can be disconne 716 bluetooth that can be disconnected via an external
717 switch or a port with a hardwi 717 switch or a port with a hardwired USB camera. It is
718 expected to be safe to allow t 718 expected to be safe to allow these ports to suspend
719 provided pm_qos_no_power_off i 719 provided pm_qos_no_power_off is coordinated with any
720 switch that gates connections. 720 switch that gates connections. Userspace must arrange
721 for the device to be connected 721 for the device to be connected prior to the port
722 powering off, or to activate t 722 powering off, or to activate the port prior to enabling
723 connection via a switch. 723 connection via a switch.
724 724
725 ``not used`` refers to an inte 725 ``not used`` refers to an internal port that is expected
726 to never have a device connect 726 to never have a device connected to it. These may be
727 empty internal ports, or ports 727 empty internal ports, or ports that are not physically
728 exposed on a platform. Consid 728 exposed on a platform. Considered safe to be
729 powered-off at all times. 729 powered-off at all times.
730 730
731 ``unknown`` means platform fir 731 ``unknown`` means platform firmware does not provide
732 information for this port. Mo 732 information for this port. Most commonly refers to
733 external hub ports which shoul 733 external hub ports which should be considered 'hotplug'
734 for policy decisions. 734 for policy decisions.
735 735
736 .. note:: 736 .. note::
737 737
738 - since we are relying 738 - since we are relying on the BIOS to get this ACPI
739 information correct, 739 information correct, the USB port descriptions may
740 be missing or wrong. 740 be missing or wrong.
741 741
742 - Take care in clearin 742 - Take care in clearing ``pm_qos_no_power_off``. Once
743 power is off this po 743 power is off this port will
744 not respond to new c 744 not respond to new connect events.
745 745
746 Once a child device is attached additi 746 Once a child device is attached additional constraints are
747 applied before the port is allowed to 747 applied before the port is allowed to poweroff.
748 748
749 ``<child>/power/control``: 749 ``<child>/power/control``:
750 Must be ``auto``, and the port 750 Must be ``auto``, and the port will not
751 power down until ``<child>/pow 751 power down until ``<child>/power/runtime_status``
752 reflects the 'suspended' state 752 reflects the 'suspended' state. Default
753 value is controlled by child d 753 value is controlled by child device driver.
754 754
755 ``<child>/power/persist``: 755 ``<child>/power/persist``:
756 This defaults to ``1`` for mos 756 This defaults to ``1`` for most devices and indicates if
757 kernel can persist the device' 757 kernel can persist the device's configuration across a
758 power session loss (suspend / 758 power session loss (suspend / port-power event). When
759 this value is ``0`` (quirky de 759 this value is ``0`` (quirky devices), port poweroff is
760 disabled. 760 disabled.
761 761
762 ``<child>/driver/unbind``: 762 ``<child>/driver/unbind``:
763 Wakeup capable devices will bl 763 Wakeup capable devices will block port poweroff. At
764 this time the only mechanism t 764 this time the only mechanism to clear the usb-internal
765 wakeup-capability for an inter 765 wakeup-capability for an interface device is to unbind
766 its driver. 766 its driver.
767 767
768 Summary of poweroff pre-requisite settings rel 768 Summary of poweroff pre-requisite settings relative to a port device::
769 769
770 echo 0 > power/pm_qos_no_power_off 770 echo 0 > power/pm_qos_no_power_off
771 echo 0 > peer/power/pm_qos_no_power_of 771 echo 0 > peer/power/pm_qos_no_power_off # if it exists
772 echo auto > power/control # this is th 772 echo auto > power/control # this is the default value
773 echo auto > <child>/power/control 773 echo auto > <child>/power/control
774 echo 1 > <child>/power/persist # this 774 echo 1 > <child>/power/persist # this is the default value
775 775
776 Suggested Userspace Port Power Policy 776 Suggested Userspace Port Power Policy
777 ------------------------------------- 777 -------------------------------------
778 778
779 As noted above userspace needs to be careful a 779 As noted above userspace needs to be careful and deliberate about what
780 ports are enabled for poweroff. 780 ports are enabled for poweroff.
781 781
782 The default configuration is that all ports st 782 The default configuration is that all ports start with
783 ``power/pm_qos_no_power_off`` set to ``1`` cau 783 ``power/pm_qos_no_power_off`` set to ``1`` causing ports to always remain
784 active. 784 active.
785 785
786 Given confidence in the platform firmware's de 786 Given confidence in the platform firmware's description of the ports
787 (ACPI _PLD record for a port populates 'connec 787 (ACPI _PLD record for a port populates 'connect_type') userspace can
788 clear pm_qos_no_power_off for all 'not used' p 788 clear pm_qos_no_power_off for all 'not used' ports. The same can be
789 done for 'hardwired' ports provided poweroff i 789 done for 'hardwired' ports provided poweroff is coordinated with any
790 connection switch for the port. 790 connection switch for the port.
791 791
792 A more aggressive userspace policy is to enabl 792 A more aggressive userspace policy is to enable USB port power off for
793 all ports (set ``<hubdev-portX>/power/pm_qos_n 793 all ports (set ``<hubdev-portX>/power/pm_qos_no_power_off`` to ``0``) when
794 some external factor indicates the user has st 794 some external factor indicates the user has stopped interacting with the
795 system. For example, a distro may want to ena 795 system. For example, a distro may want to enable power off all USB
796 ports when the screen blanks, and re-power the 796 ports when the screen blanks, and re-power them when the screen becomes
797 active. Smart phones and tablets may want to 797 active. Smart phones and tablets may want to power off USB ports when
798 the user pushes the power button. 798 the user pushes the power button.
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