Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 EST.
1 Run-time Power Management Framework for I/O Devices 2 3 (C) 2009 Rafael J. Wysocki <rjw[AT]sisk.pl>, Novell Inc[DOT] 4 5 1. Introduction 6 7 Support for run-time power management (run-time PM) of I/O devices is provided 8 at the power management core (PM core) level by means of: 9 10 * The power management workqueue pm_wq in which bus types and device drivers can 11 put their PM-related work items. It is strongly recommended that pm_wq be 12 used for queuing all work items related to run-time PM, because this allows 13 them to be synchronized with system-wide power transitions (suspend to RAM, 14 hibernation and resume from system sleep states). pm_wq is declared in 15 include/linux/pm_runtime.h and defined in kernel/power/main.c. 16 17 * A number of run-time PM fields in the 'power' member of 'struct device' (which 18 is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can 19 be used for synchronizing run-time PM operations with one another. 20 21 * Three device run-time PM callbacks in 'struct dev_pm_ops' (defined in 22 include/linux/pm.h). 23 24 * A set of helper functions defined in drivers/base/power/runtime.c that can be 25 used for carrying out run-time PM operations in such a way that the 26 synchronization between them is taken care of by the PM core. Bus types and 27 device drivers are encouraged to use these functions. 28 29 The run-time PM callbacks present in 'struct dev_pm_ops', the device run-time PM 30 fields of 'struct dev_pm_info' and the core helper functions provided for 31 run-time PM are described below. 32 33 2. Device Run-time PM Callbacks 34 35 There are three device run-time PM callbacks defined in 'struct dev_pm_ops': 36 37 struct dev_pm_ops { 38 ... 39 int (*runtime_suspend)(struct device *dev); 40 int (*runtime_resume)(struct device *dev); 41 int (*runtime_idle)(struct device *dev); 42 ... 43 }; 44 45 The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks are 46 executed by the PM core for either the bus type, or device type (if the bus 47 type's callback is not defined), or device class (if the bus type's and device 48 type's callbacks are not defined) of given device. The bus type, device type 49 and device class callbacks are referred to as subsystem-level callbacks in what 50 follows. 51 52 The subsystem-level suspend callback is _entirely_ _responsible_ for handling 53 the suspend of the device as appropriate, which may, but need not include 54 executing the device driver's own ->runtime_suspend() callback (from the 55 PM core's point of view it is not necessary to implement a ->runtime_suspend() 56 callback in a device driver as long as the subsystem-level suspend callback 57 knows what to do to handle the device). 58 59 * Once the subsystem-level suspend callback has completed successfully 60 for given device, the PM core regards the device as suspended, which need 61 not mean that the device has been put into a low power state. It is 62 supposed to mean, however, that the device will not process data and will 63 not communicate with the CPU(s) and RAM until the subsystem-level resume 64 callback is executed for it. The run-time PM status of a device after 65 successful execution of the subsystem-level suspend callback is 'suspended'. 66 67 * If the subsystem-level suspend callback returns -EBUSY or -EAGAIN, 68 the device's run-time PM status is 'active', which means that the device 69 _must_ be fully operational afterwards. 70 71 * If the subsystem-level suspend callback returns an error code different 72 from -EBUSY or -EAGAIN, the PM core regards this as a fatal error and will 73 refuse to run the helper functions described in Section 4 for the device, 74 until the status of it is directly set either to 'active', or to 'suspended' 75 (the PM core provides special helper functions for this purpose). 76 77 In particular, if the driver requires remote wake-up capability (i.e. hardware 78 mechanism allowing the device to request a change of its power state, such as 79 PCI PME) for proper functioning and device_run_wake() returns 'false' for the 80 device, then ->runtime_suspend() should return -EBUSY. On the other hand, if 81 device_run_wake() returns 'true' for the device and the device is put into a low 82 power state during the execution of the subsystem-level suspend callback, it is 83 expected that remote wake-up will be enabled for the device. Generally, remote 84 wake-up should be enabled for all input devices put into a low power state at 85 run time. 86 87 The subsystem-level resume callback is _entirely_ _responsible_ for handling the 88 resume of the device as appropriate, which may, but need not include executing 89 the device driver's own ->runtime_resume() callback (from the PM core's point of 90 view it is not necessary to implement a ->runtime_resume() callback in a device 91 driver as long as the subsystem-level resume callback knows what to do to handle 92 the device). 93 94 * Once the subsystem-level resume callback has completed successfully, the PM 95 core regards the device as fully operational, which means that the device 96 _must_ be able to complete I/O operations as needed. The run-time PM status 97 of the device is then 'active'. 98 99 * If the subsystem-level resume callback returns an error code, the PM core 100 regards this as a fatal error and will refuse to run the helper functions 101 described in Section 4 for the device, until its status is directly set 102 either to 'active' or to 'suspended' (the PM core provides special helper 103 functions for this purpose). 104 105 The subsystem-level idle callback is executed by the PM core whenever the device 106 appears to be idle, which is indicated to the PM core by two counters, the 107 device's usage counter and the counter of 'active' children of the device. 108 109 * If any of these counters is decreased using a helper function provided by 110 the PM core and it turns out to be equal to zero, the other counter is 111 checked. If that counter also is equal to zero, the PM core executes the 112 subsystem-level idle callback with the device as an argument. 113 114 The action performed by a subsystem-level idle callback is totally dependent on 115 the subsystem in question, but the expected and recommended action is to check 116 if the device can be suspended (i.e. if all of the conditions necessary for 117 suspending the device are satisfied) and to queue up a suspend request for the 118 device in that case. The value returned by this callback is ignored by the PM 119 core. 120 121 The helper functions provided by the PM core, described in Section 4, guarantee 122 that the following constraints are met with respect to the bus type's run-time 123 PM callbacks: 124 125 (1) The callbacks are mutually exclusive (e.g. it is forbidden to execute 126 ->runtime_suspend() in parallel with ->runtime_resume() or with another 127 instance of ->runtime_suspend() for the same device) with the exception that 128 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with 129 ->runtime_idle() (although ->runtime_idle() will not be started while any 130 of the other callbacks is being executed for the same device). 131 132 (2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active' 133 devices (i.e. the PM core will only execute ->runtime_idle() or 134 ->runtime_suspend() for the devices the run-time PM status of which is 135 'active'). 136 137 (3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device 138 the usage counter of which is equal to zero _and_ either the counter of 139 'active' children of which is equal to zero, or the 'power.ignore_children' 140 flag of which is set. 141 142 (4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the 143 PM core will only execute ->runtime_resume() for the devices the run-time 144 PM status of which is 'suspended'). 145 146 Additionally, the helper functions provided by the PM core obey the following 147 rules: 148 149 * If ->runtime_suspend() is about to be executed or there's a pending request 150 to execute it, ->runtime_idle() will not be executed for the same device. 151 152 * A request to execute or to schedule the execution of ->runtime_suspend() 153 will cancel any pending requests to execute ->runtime_idle() for the same 154 device. 155 156 * If ->runtime_resume() is about to be executed or there's a pending request 157 to execute it, the other callbacks will not be executed for the same device. 158 159 * A request to execute ->runtime_resume() will cancel any pending or 160 scheduled requests to execute the other callbacks for the same device. 161 162 3. Run-time PM Device Fields 163 164 The following device run-time PM fields are present in 'struct dev_pm_info', as 165 defined in include/linux/pm.h: 166 167 struct timer_list suspend_timer; 168 - timer used for scheduling (delayed) suspend request 169 170 unsigned long timer_expires; 171 - timer expiration time, in jiffies (if this is different from zero, the 172 timer is running and will expire at that time, otherwise the timer is not 173 running) 174 175 struct work_struct work; 176 - work structure used for queuing up requests (i.e. work items in pm_wq) 177 178 wait_queue_head_t wait_queue; 179 - wait queue used if any of the helper functions needs to wait for another 180 one to complete 181 182 spinlock_t lock; 183 - lock used for synchronisation 184 185 atomic_t usage_count; 186 - the usage counter of the device 187 188 atomic_t child_count; 189 - the count of 'active' children of the device 190 191 unsigned int ignore_children; 192 - if set, the value of child_count is ignored (but still updated) 193 194 unsigned int disable_depth; 195 - used for disabling the helper funcions (they work normally if this is 196 equal to zero); the initial value of it is 1 (i.e. run-time PM is 197 initially disabled for all devices) 198 199 unsigned int runtime_error; 200 - if set, there was a fatal error (one of the callbacks returned error code 201 as described in Section 2), so the helper funtions will not work until 202 this flag is cleared; this is the error code returned by the failing 203 callback 204 205 unsigned int idle_notification; 206 - if set, ->runtime_idle() is being executed 207 208 unsigned int request_pending; 209 - if set, there's a pending request (i.e. a work item queued up into pm_wq) 210 211 enum rpm_request request; 212 - type of request that's pending (valid if request_pending is set) 213 214 unsigned int deferred_resume; 215 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is 216 being executed for that device and it is not practical to wait for the 217 suspend to complete; means "start a resume as soon as you've suspended" 218 219 unsigned int run_wake; 220 - set if the device is capable of generating run-time wake-up events 221 222 enum rpm_status runtime_status; 223 - the run-time PM status of the device; this field's initial value is 224 RPM_SUSPENDED, which means that each device is initially regarded by the 225 PM core as 'suspended', regardless of its real hardware status 226 227 All of the above fields are members of the 'power' member of 'struct device'. 228 229 4. Run-time PM Device Helper Functions 230 231 The following run-time PM helper functions are defined in 232 drivers/base/power/runtime.c and include/linux/pm_runtime.h: 233 234 void pm_runtime_init(struct device *dev); 235 - initialize the device run-time PM fields in 'struct dev_pm_info' 236 237 void pm_runtime_remove(struct device *dev); 238 - make sure that the run-time PM of the device will be disabled after 239 removing the device from device hierarchy 240 241 int pm_runtime_idle(struct device *dev); 242 - execute the subsystem-level idle callback for the device; returns 0 on 243 success or error code on failure, where -EINPROGRESS means that 244 ->runtime_idle() is already being executed 245 246 int pm_runtime_suspend(struct device *dev); 247 - execute the subsystem-level suspend callback for the device; returns 0 on 248 success, 1 if the device's run-time PM status was already 'suspended', or 249 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt 250 to suspend the device again in future 251 252 int pm_runtime_resume(struct device *dev); 253 - execute the subsystem-leve resume callback for the device; returns 0 on 254 success, 1 if the device's run-time PM status was already 'active' or 255 error code on failure, where -EAGAIN means it may be safe to attempt to 256 resume the device again in future, but 'power.runtime_error' should be 257 checked additionally 258 259 int pm_request_idle(struct device *dev); 260 - submit a request to execute the subsystem-level idle callback for the 261 device (the request is represented by a work item in pm_wq); returns 0 on 262 success or error code if the request has not been queued up 263 264 int pm_schedule_suspend(struct device *dev, unsigned int delay); 265 - schedule the execution of the subsystem-level suspend callback for the 266 device in future, where 'delay' is the time to wait before queuing up a 267 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work 268 item is queued up immediately); returns 0 on success, 1 if the device's PM 269 run-time status was already 'suspended', or error code if the request 270 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of 271 ->runtime_suspend() is already scheduled and not yet expired, the new 272 value of 'delay' will be used as the time to wait 273 274 int pm_request_resume(struct device *dev); 275 - submit a request to execute the subsystem-level resume callback for the 276 device (the request is represented by a work item in pm_wq); returns 0 on 277 success, 1 if the device's run-time PM status was already 'active', or 278 error code if the request hasn't been queued up 279 280 void pm_runtime_get_noresume(struct device *dev); 281 - increment the device's usage counter 282 283 int pm_runtime_get(struct device *dev); 284 - increment the device's usage counter, run pm_request_resume(dev) and 285 return its result 286 287 int pm_runtime_get_sync(struct device *dev); 288 - increment the device's usage counter, run pm_runtime_resume(dev) and 289 return its result 290 291 void pm_runtime_put_noidle(struct device *dev); 292 - decrement the device's usage counter 293 294 int pm_runtime_put(struct device *dev); 295 - decrement the device's usage counter, run pm_request_idle(dev) and return 296 its result 297 298 int pm_runtime_put_sync(struct device *dev); 299 - decrement the device's usage counter, run pm_runtime_idle(dev) and return 300 its result 301 302 void pm_runtime_enable(struct device *dev); 303 - enable the run-time PM helper functions to run the device bus type's 304 run-time PM callbacks described in Section 2 305 306 int pm_runtime_disable(struct device *dev); 307 - prevent the run-time PM helper functions from running subsystem-level 308 run-time PM callbacks for the device, make sure that all of the pending 309 run-time PM operations on the device are either completed or canceled; 310 returns 1 if there was a resume request pending and it was necessary to 311 execute the subsystem-level resume callback for the device to satisfy that 312 request, otherwise 0 is returned 313 314 void pm_suspend_ignore_children(struct device *dev, bool enable); 315 - set/unset the power.ignore_children flag of the device 316 317 int pm_runtime_set_active(struct device *dev); 318 - clear the device's 'power.runtime_error' flag, set the device's run-time 319 PM status to 'active' and update its parent's counter of 'active' 320 children as appropriate (it is only valid to use this function if 321 'power.runtime_error' is set or 'power.disable_depth' is greater than 322 zero); it will fail and return error code if the device has a parent 323 which is not active and the 'power.ignore_children' flag of which is unset 324 325 void pm_runtime_set_suspended(struct device *dev); 326 - clear the device's 'power.runtime_error' flag, set the device's run-time 327 PM status to 'suspended' and update its parent's counter of 'active' 328 children as appropriate (it is only valid to use this function if 329 'power.runtime_error' is set or 'power.disable_depth' is greater than 330 zero) 331 332 It is safe to execute the following helper functions from interrupt context: 333 334 pm_request_idle() 335 pm_schedule_suspend() 336 pm_request_resume() 337 pm_runtime_get_noresume() 338 pm_runtime_get() 339 pm_runtime_put_noidle() 340 pm_runtime_put() 341 pm_suspend_ignore_children() 342 pm_runtime_set_active() 343 pm_runtime_set_suspended() 344 pm_runtime_enable() 345 346 5. Run-time PM Initialization, Device Probing and Removal 347 348 Initially, the run-time PM is disabled for all devices, which means that the 349 majority of the run-time PM helper funtions described in Section 4 will return 350 -EAGAIN until pm_runtime_enable() is called for the device. 351 352 In addition to that, the initial run-time PM status of all devices is 353 'suspended', but it need not reflect the actual physical state of the device. 354 Thus, if the device is initially active (i.e. it is able to process I/O), its 355 run-time PM status must be changed to 'active', with the help of 356 pm_runtime_set_active(), before pm_runtime_enable() is called for the device. 357 358 However, if the device has a parent and the parent's run-time PM is enabled, 359 calling pm_runtime_set_active() for the device will affect the parent, unless 360 the parent's 'power.ignore_children' flag is set. Namely, in that case the 361 parent won't be able to suspend at run time, using the PM core's helper 362 functions, as long as the child's status is 'active', even if the child's 363 run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for 364 the child yet or pm_runtime_disable() has been called for it). For this reason, 365 once pm_runtime_set_active() has been called for the device, pm_runtime_enable() 366 should be called for it too as soon as reasonably possible or its run-time PM 367 status should be changed back to 'suspended' with the help of 368 pm_runtime_set_suspended(). 369 370 If the default initial run-time PM status of the device (i.e. 'suspended') 371 reflects the actual state of the device, its bus type's or its driver's 372 ->probe() callback will likely need to wake it up using one of the PM core's 373 helper functions described in Section 4. In that case, pm_runtime_resume() 374 should be used. Of course, for this purpose the device's run-time PM has to be 375 enabled earlier by calling pm_runtime_enable(). 376 377 If the device bus type's or driver's ->probe() or ->remove() callback runs 378 pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts, 379 they will fail returning -EAGAIN, because the device's usage counter is 380 incremented by the core before executing ->probe() and ->remove(). Still, it 381 may be desirable to suspend the device as soon as ->probe() or ->remove() has 382 finished, so the PM core uses pm_runtime_idle_sync() to invoke the 383 subsystem-level idle callback for the device at that time. 384 385 6. Run-time PM and System Sleep 386 387 Run-time PM and system sleep (i.e., system suspend and hibernation, also known 388 as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of 389 ways. If a device is active when a system sleep starts, everything is 390 straightforward. But what should happen if the device is already suspended? 391 392 The device may have different wake-up settings for run-time PM and system sleep. 393 For example, remote wake-up may be enabled for run-time suspend but disallowed 394 for system sleep (device_may_wakeup(dev) returns 'false'). When this happens, 395 the subsystem-level system suspend callback is responsible for changing the 396 device's wake-up setting (it may leave that to the device driver's system 397 suspend routine). It may be necessary to resume the device and suspend it again 398 in order to do so. The same is true if the driver uses different power levels 399 or other settings for run-time suspend and system sleep. 400 401 During system resume, devices generally should be brought back to full power, 402 even if they were suspended before the system sleep began. There are several 403 reasons for this, including: 404 405 * The device might need to switch power levels, wake-up settings, etc. 406 407 * Remote wake-up events might have been lost by the firmware. 408 409 * The device's children may need the device to be at full power in order 410 to resume themselves. 411 412 * The driver's idea of the device state may not agree with the device's 413 physical state. This can happen during resume from hibernation. 414 415 * The device might need to be reset. 416 417 * Even though the device was suspended, if its usage counter was > 0 then most 418 likely it would need a run-time resume in the near future anyway. 419 420 * Always going back to full power is simplest. 421 422 If the device was suspended before the sleep began, then its run-time PM status 423 will have to be updated to reflect the actual post-system sleep status. The way 424 to do this is: 425 426 pm_runtime_disable(dev); 427 pm_runtime_set_active(dev); 428 pm_runtime_enable(dev); 429 430 The PM core always increments the run-time usage counter before calling the 431 ->prepare() callback and decrements it after calling the ->complete() callback. 432 Hence disabling run-time PM temporarily like this will not cause any run-time 433 suspend callbacks to be lost.