Based on kernel version 3.9. Page generated on 2013-05-02 23:15 EST.
1 Power Management for USB 2 3 Alan Stern <email@example.com> 4 5 October 28, 2010 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 (which depends on 33 CONFIG_PM_RUNTIME). System PM support is present only if the kernel 34 was built with CONFIG_SUSPEND or CONFIG_HIBERNATION 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 occur when the kernel decides to suspend an idle 75 device. This is called "autosuspend" for short. In general, a device 76 won't be autosuspended unless it has been idle for some minimum period 77 of time, the so-called idle-delay time. 78 79 Of course, nothing the kernel does on its own initiative should 80 prevent the computer or its devices from working properly. If a 81 device has been autosuspended and a program tries to use it, the 82 kernel will automatically resume the device (autoresume). For the 83 same reason, an autosuspended device will usually have remote wakeup 84 enabled, if the device supports remote wakeup. 85 86 It is worth mentioning that many USB drivers don't support 87 autosuspend. In fact, at the time of this writing (Linux 2.6.23) the 88 only drivers which do support it are the hub driver, kaweth, asix, 89 usblp, usblcd, and usb-skeleton (which doesn't count). If a 90 non-supporting driver is bound to a device, the device won't be 91 autosuspended. In effect, the kernel pretends the device is never 92 idle. 93 94 We can categorize power management events in two broad classes: 95 external and internal. External events are those triggered by some 96 agent outside the USB stack: system suspend/resume (triggered by 97 userspace), manual dynamic resume (also triggered by userspace), and 98 remote wakeup (triggered by the device). Internal events are those 99 triggered within the USB stack: autosuspend and autoresume. Note that 100 all dynamic suspend events are internal; external agents are not 101 allowed to issue dynamic suspends. 102 103 104 The user interface for dynamic PM 105 --------------------------------- 106 107 The user interface for controlling dynamic PM is located in the power/ 108 subdirectory of each USB device's sysfs directory, that is, in 109 /sys/bus/usb/devices/.../power/ where "..." is the device's ID. The 110 relevant attribute files are: wakeup, control, and 111 autosuspend_delay_ms. (There may also be a file named "level"; this 112 file was deprecated as of the 2.6.35 kernel and replaced by the 113 "control" file. In 2.6.38 the "autosuspend" file will be deprecated 114 and replaced by the "autosuspend_delay_ms" file. The only difference 115 is that the newer file expresses the delay in milliseconds whereas the 116 older file uses seconds. Confusingly, both files are present in 2.6.37 117 but only "autosuspend" works.) 118 119 power/wakeup 120 121 This file is empty if the device does not support 122 remote wakeup. Otherwise the file contains either the 123 word "enabled" or the word "disabled", and you can 124 write those words to the file. The setting determines 125 whether or not remote wakeup will be enabled when the 126 device is next suspended. (If the setting is changed 127 while the device is suspended, the change won't take 128 effect until the following suspend.) 129 130 power/control 131 132 This file contains one of two words: "on" or "auto". 133 You can write those words to the file to change the 134 device's setting. 135 136 "on" means that the device should be resumed and 137 autosuspend is not allowed. (Of course, system 138 suspends are still allowed.) 139 140 "auto" is the normal state in which the kernel is 141 allowed to autosuspend and autoresume the device. 142 143 (In kernels up to 2.6.32, you could also specify 144 "suspend", meaning that the device should remain 145 suspended and autoresume was not allowed. This 146 setting is no longer supported.) 147 148 power/autosuspend_delay_ms 149 150 This file contains an integer value, which is the 151 number of milliseconds the device should remain idle 152 before the kernel will autosuspend it (the idle-delay 153 time). The default is 2000. 0 means to autosuspend 154 as soon as the device becomes idle, and negative 155 values mean never to autosuspend. You can write a 156 number to the file to change the autosuspend 157 idle-delay time. 158 159 Writing "-1" to power/autosuspend_delay_ms and writing "on" to 160 power/control do essentially the same thing -- they both prevent the 161 device from being autosuspended. Yes, this is a redundancy in the 162 API. 163 164 (In 2.6.21 writing "0" to power/autosuspend would prevent the device 165 from being autosuspended; the behavior was changed in 2.6.22. The 166 power/autosuspend attribute did not exist prior to 2.6.21, and the 167 power/level attribute did not exist prior to 2.6.22. power/control 168 was added in 2.6.34, and power/autosuspend_delay_ms was added in 169 2.6.37 but did not become functional until 2.6.38.) 170 171 172 Changing the default idle-delay time 173 ------------------------------------ 174 175 The default autosuspend idle-delay time (in seconds) is controlled by 176 a module parameter in usbcore. You can specify the value when usbcore 177 is loaded. For example, to set it to 5 seconds instead of 2 you would 178 do: 179 180 modprobe usbcore autosuspend=5 181 182 Equivalently, you could add to a configuration file in /etc/modprobe.d 183 a line saying: 184 185 options usbcore autosuspend=5 186 187 Some distributions load the usbcore module very early during the boot 188 process, by means of a program or script running from an initramfs 189 image. To alter the parameter value you would have to rebuild that 190 image. 191 192 If usbcore is compiled into the kernel rather than built as a loadable 193 module, you can add 194 195 usbcore.autosuspend=5 196 197 to the kernel's boot command line. 198 199 Finally, the parameter value can be changed while the system is 200 running. If you do: 201 202 echo 5 >/sys/module/usbcore/parameters/autosuspend 203 204 then each new USB device will have its autosuspend idle-delay 205 initialized to 5. (The idle-delay values for already existing devices 206 will not be affected.) 207 208 Setting the initial default idle-delay to -1 will prevent any 209 autosuspend of any USB device. This is a simple alternative to 210 disabling CONFIG_USB_SUSPEND and rebuilding the kernel, and it has the 211 added benefit of allowing you to enable autosuspend for selected 212 devices. 213 214 215 Warnings 216 -------- 217 218 The USB specification states that all USB devices must support power 219 management. Nevertheless, the sad fact is that many devices do not 220 support it very well. You can suspend them all right, but when you 221 try to resume them they disconnect themselves from the USB bus or 222 they stop working entirely. This seems to be especially prevalent 223 among printers and scanners, but plenty of other types of device have 224 the same deficiency. 225 226 For this reason, by default the kernel disables autosuspend (the 227 power/control attribute is initialized to "on") for all devices other 228 than hubs. Hubs, at least, appear to be reasonably well-behaved in 229 this regard. 230 231 (In 2.6.21 and 2.6.22 this wasn't the case. Autosuspend was enabled 232 by default for almost all USB devices. A number of people experienced 233 problems as a result.) 234 235 This means that non-hub devices won't be autosuspended unless the user 236 or a program explicitly enables it. As of this writing there aren't 237 any widespread programs which will do this; we hope that in the near 238 future device managers such as HAL will take on this added 239 responsibility. In the meantime you can always carry out the 240 necessary operations by hand or add them to a udev script. You can 241 also change the idle-delay time; 2 seconds is not the best choice for 242 every device. 243 244 If a driver knows that its device has proper suspend/resume support, 245 it can enable autosuspend all by itself. For example, the video 246 driver for a laptop's webcam might do this (in recent kernels they 247 do), since these devices are rarely used and so should normally be 248 autosuspended. 249 250 Sometimes it turns out that even when a device does work okay with 251 autosuspend there are still problems. For example, the usbhid driver, 252 which manages keyboards and mice, has autosuspend support. Tests with 253 a number of keyboards show that typing on a suspended keyboard, while 254 causing the keyboard to do a remote wakeup all right, will nonetheless 255 frequently result in lost keystrokes. Tests with mice show that some 256 of them will issue a remote-wakeup request in response to button 257 presses but not to motion, and some in response to neither. 258 259 The kernel will not prevent you from enabling autosuspend on devices 260 that can't handle it. It is even possible in theory to damage a 261 device by suspending it at the wrong time. (Highly unlikely, but 262 possible.) Take care. 263 264 265 The driver interface for Power Management 266 ----------------------------------------- 267 268 The requirements for a USB driver to support external power management 269 are pretty modest; the driver need only define 270 271 .suspend 272 .resume 273 .reset_resume 274 275 methods in its usb_driver structure, and the reset_resume method is 276 optional. The methods' jobs are quite simple: 277 278 The suspend method is called to warn the driver that the 279 device is going to be suspended. If the driver returns a 280 negative error code, the suspend will be aborted. Normally 281 the driver will return 0, in which case it must cancel all 282 outstanding URBs (usb_kill_urb()) and not submit any more. 283 284 The resume method is called to tell the driver that the 285 device has been resumed and the driver can return to normal 286 operation. URBs may once more be submitted. 287 288 The reset_resume method is called to tell the driver that 289 the device has been resumed and it also has been reset. 290 The driver should redo any necessary device initialization, 291 since the device has probably lost most or all of its state 292 (although the interfaces will be in the same altsettings as 293 before the suspend). 294 295 If the device is disconnected or powered down while it is suspended, 296 the disconnect method will be called instead of the resume or 297 reset_resume method. This is also quite likely to happen when 298 waking up from hibernation, as many systems do not maintain suspend 299 current to the USB host controllers during hibernation. (It's 300 possible to work around the hibernation-forces-disconnect problem by 301 using the USB Persist facility.) 302 303 The reset_resume method is used by the USB Persist facility (see 304 Documentation/usb/persist.txt) and it can also be used under certain 305 circumstances when CONFIG_USB_PERSIST is not enabled. Currently, if a 306 device is reset during a resume and the driver does not have a 307 reset_resume method, the driver won't receive any notification about 308 the resume. Later kernels will call the driver's disconnect method; 309 2.6.23 doesn't do this. 310 311 USB drivers are bound to interfaces, so their suspend and resume 312 methods get called when the interfaces are suspended or resumed. In 313 principle one might want to suspend some interfaces on a device (i.e., 314 force the drivers for those interface to stop all activity) without 315 suspending the other interfaces. The USB core doesn't allow this; all 316 interfaces are suspended when the device itself is suspended and all 317 interfaces are resumed when the device is resumed. It isn't possible 318 to suspend or resume some but not all of a device's interfaces. The 319 closest you can come is to unbind the interfaces' drivers. 320 321 322 The driver interface for autosuspend and autoresume 323 --------------------------------------------------- 324 325 To support autosuspend and autoresume, a driver should implement all 326 three of the methods listed above. In addition, a driver indicates 327 that it supports autosuspend by setting the .supports_autosuspend flag 328 in its usb_driver structure. It is then responsible for informing the 329 USB core whenever one of its interfaces becomes busy or idle. The 330 driver does so by calling these six functions: 331 332 int usb_autopm_get_interface(struct usb_interface *intf); 333 void usb_autopm_put_interface(struct usb_interface *intf); 334 int usb_autopm_get_interface_async(struct usb_interface *intf); 335 void usb_autopm_put_interface_async(struct usb_interface *intf); 336 void usb_autopm_get_interface_no_resume(struct usb_interface *intf); 337 void usb_autopm_put_interface_no_suspend(struct usb_interface *intf); 338 339 The functions work by maintaining a usage counter in the 340 usb_interface's embedded device structure. When the counter is > 0 341 then the interface is deemed to be busy, and the kernel will not 342 autosuspend the interface's device. When the usage counter is = 0 343 then the interface is considered to be idle, and the kernel may 344 autosuspend the device. 345 346 Drivers need not be concerned about balancing changes to the usage 347 counter; the USB core will undo any remaining "get"s when a driver 348 is unbound from its interface. As a corollary, drivers must not call 349 any of the usb_autopm_* functions after their disconnect() routine has 350 returned. 351 352 Drivers using the async routines are responsible for their own 353 synchronization and mutual exclusion. 354 355 usb_autopm_get_interface() increments the usage counter and 356 does an autoresume if the device is suspended. If the 357 autoresume fails, the counter is decremented back. 358 359 usb_autopm_put_interface() decrements the usage counter and 360 attempts an autosuspend if the new value is = 0. 361 362 usb_autopm_get_interface_async() and 363 usb_autopm_put_interface_async() do almost the same things as 364 their non-async counterparts. The big difference is that they 365 use a workqueue to do the resume or suspend part of their 366 jobs. As a result they can be called in an atomic context, 367 such as an URB's completion handler, but when they return the 368 device will generally not yet be in the desired state. 369 370 usb_autopm_get_interface_no_resume() and 371 usb_autopm_put_interface_no_suspend() merely increment or 372 decrement the usage counter; they do not attempt to carry out 373 an autoresume or an autosuspend. Hence they can be called in 374 an atomic context. 375 376 The simplest usage pattern is that a driver calls 377 usb_autopm_get_interface() in its open routine and 378 usb_autopm_put_interface() in its close or release routine. But other 379 patterns are possible. 380 381 The autosuspend attempts mentioned above will often fail for one 382 reason or another. For example, the power/control attribute might be 383 set to "on", or another interface in the same device might not be 384 idle. This is perfectly normal. If the reason for failure was that 385 the device hasn't been idle for long enough, a timer is scheduled to 386 carry out the operation automatically when the autosuspend idle-delay 387 has expired. 388 389 Autoresume attempts also can fail, although failure would mean that 390 the device is no longer present or operating properly. Unlike 391 autosuspend, there's no idle-delay for an autoresume. 392 393 394 Other parts of the driver interface 395 ----------------------------------- 396 397 Drivers can enable autosuspend for their devices by calling 398 399 usb_enable_autosuspend(struct usb_device *udev); 400 401 in their probe() routine, if they know that the device is capable of 402 suspending and resuming correctly. This is exactly equivalent to 403 writing "auto" to the device's power/control attribute. Likewise, 404 drivers can disable autosuspend by calling 405 406 usb_disable_autosuspend(struct usb_device *udev); 407 408 This is exactly the same as writing "on" to the power/control attribute. 409 410 Sometimes a driver needs to make sure that remote wakeup is enabled 411 during autosuspend. For example, there's not much point 412 autosuspending a keyboard if the user can't cause the keyboard to do a 413 remote wakeup by typing on it. If the driver sets 414 intf->needs_remote_wakeup to 1, the kernel won't autosuspend the 415 device if remote wakeup isn't available. (If the device is already 416 autosuspended, though, setting this flag won't cause the kernel to 417 autoresume it. Normally a driver would set this flag in its probe 418 method, at which time the device is guaranteed not to be 419 autosuspended.) 420 421 If a driver does its I/O asynchronously in interrupt context, it 422 should call usb_autopm_get_interface_async() before starting output and 423 usb_autopm_put_interface_async() when the output queue drains. When 424 it receives an input event, it should call 425 426 usb_mark_last_busy(struct usb_device *udev); 427 428 in the event handler. This tells the PM core that the device was just 429 busy and therefore the next autosuspend idle-delay expiration should 430 be pushed back. Many of the usb_autopm_* routines also make this call, 431 so drivers need to worry only when interrupt-driven input arrives. 432 433 Asynchronous operation is always subject to races. For example, a 434 driver may call the usb_autopm_get_interface_async() routine at a time 435 when the core has just finished deciding the device has been idle for 436 long enough but not yet gotten around to calling the driver's suspend 437 method. The suspend method must be responsible for synchronizing with 438 the I/O request routine and the URB completion handler; it should 439 cause autosuspends to fail with -EBUSY if the driver needs to use the 440 device. 441 442 External suspend calls should never be allowed to fail in this way, 443 only autosuspend calls. The driver can tell them apart by applying 444 the PMSG_IS_AUTO() macro to the message argument to the suspend 445 method; it will return True for internal PM events (autosuspend) and 446 False for external PM events. 447 448 449 Mutual exclusion 450 ---------------- 451 452 For external events -- but not necessarily for autosuspend or 453 autoresume -- the device semaphore (udev->dev.sem) will be held when a 454 suspend or resume method is called. This implies that external 455 suspend/resume events are mutually exclusive with calls to probe, 456 disconnect, pre_reset, and post_reset; the USB core guarantees that 457 this is true of autosuspend/autoresume events as well. 458 459 If a driver wants to block all suspend/resume calls during some 460 critical section, the best way is to lock the device and call 461 usb_autopm_get_interface() (and do the reverse at the end of the 462 critical section). Holding the device semaphore will block all 463 external PM calls, and the usb_autopm_get_interface() will prevent any 464 internal PM calls, even if it fails. (Exercise: Why?) 465 466 467 Interaction between dynamic PM and system PM 468 -------------------------------------------- 469 470 Dynamic power management and system power management can interact in 471 a couple of ways. 472 473 Firstly, a device may already be autosuspended when a system suspend 474 occurs. Since system suspends are supposed to be as transparent as 475 possible, the device should remain suspended following the system 476 resume. But this theory may not work out well in practice; over time 477 the kernel's behavior in this regard has changed. As of 2.6.37 the 478 policy is to resume all devices during a system resume and let them 479 handle their own runtime suspends afterward. 480 481 Secondly, a dynamic power-management event may occur as a system 482 suspend is underway. The window for this is short, since system 483 suspends don't take long (a few seconds usually), but it can happen. 484 For example, a suspended device may send a remote-wakeup signal while 485 the system is suspending. The remote wakeup may succeed, which would 486 cause the system suspend to abort. If the remote wakeup doesn't 487 succeed, it may still remain active and thus cause the system to 488 resume as soon as the system suspend is complete. Or the remote 489 wakeup may fail and get lost. Which outcome occurs depends on timing 490 and on the hardware and firmware design. 491 492 493 xHCI hardware link PM 494 --------------------- 495 496 xHCI host controller provides hardware link power management to usb2.0 497 (xHCI 1.0 feature) and usb3.0 devices which support link PM. By 498 enabling hardware LPM, the host can automatically put the device into 499 lower power state(L1 for usb2.0 devices, or U1/U2 for usb3.0 devices), 500 which state device can enter and resume very quickly. 501 502 The user interface for controlling USB2 hardware LPM is located in the 503 power/ subdirectory of each USB device's sysfs directory, that is, in 504 /sys/bus/usb/devices/.../power/ where "..." is the device's ID. The 505 relevant attribute files is usb2_hardware_lpm. 506 507 power/usb2_hardware_lpm 508 509 When a USB2 device which support LPM is plugged to a 510 xHCI host root hub which support software LPM, the 511 host will run a software LPM test for it; if the device 512 enters L1 state and resume successfully and the host 513 supports USB2 hardware LPM, this file will show up and 514 driver will enable hardware LPM for the device. You 515 can write y/Y/1 or n/N/0 to the file to enable/disable 516 USB2 hardware LPM manually. This is for test purpose mainly.