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diff --git a/Documentation/power/runtime_pm.txt b/Documentation/power/runtime_pm.txt new file mode 100644 index 00000000..a6b34307 --- /dev/null +++ b/Documentation/power/runtime_pm.txt @@ -0,0 +1,793 @@ +Run-time Power Management Framework for I/O Devices + +(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. +(C) 2010 Alan Stern <stern@rowland.harvard.edu> + +1. Introduction + +Support for run-time power management (run-time PM) of I/O devices is provided +at the power management core (PM core) level by means of: + +* The power management workqueue pm_wq in which bus types and device drivers can + put their PM-related work items. It is strongly recommended that pm_wq be + used for queuing all work items related to run-time PM, because this allows + them to be synchronized with system-wide power transitions (suspend to RAM, + hibernation and resume from system sleep states). pm_wq is declared in + include/linux/pm_runtime.h and defined in kernel/power/main.c. + +* A number of run-time PM fields in the 'power' member of 'struct device' (which + is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can + be used for synchronizing run-time PM operations with one another. + +* Three device run-time PM callbacks in 'struct dev_pm_ops' (defined in + include/linux/pm.h). + +* A set of helper functions defined in drivers/base/power/runtime.c that can be + used for carrying out run-time PM operations in such a way that the + synchronization between them is taken care of by the PM core. Bus types and + device drivers are encouraged to use these functions. + +The run-time PM callbacks present in 'struct dev_pm_ops', the device run-time PM +fields of 'struct dev_pm_info' and the core helper functions provided for +run-time PM are described below. + +2. Device Run-time PM Callbacks + +There are three device run-time PM callbacks defined in 'struct dev_pm_ops': + +struct dev_pm_ops { + ... + int (*runtime_suspend)(struct device *dev); + int (*runtime_resume)(struct device *dev); + int (*runtime_idle)(struct device *dev); + ... +}; + +The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks are +executed by the PM core for either the device type, or the class (if the device +type's struct dev_pm_ops object does not exist), or the bus type (if the +device type's and class' struct dev_pm_ops objects do not exist) of the given +device (this allows device types to override callbacks provided by bus types or +classes if necessary). The bus type, device type and class callbacks are +referred to as subsystem-level callbacks in what follows. + +By default, the callbacks are always invoked in process context with interrupts +enabled. However, subsystems can use the pm_runtime_irq_safe() helper function +to tell the PM core that a device's ->runtime_suspend() and ->runtime_resume() +callbacks should be invoked in atomic context with interrupts disabled +(->runtime_idle() is still invoked the default way). This implies that these +callback routines must not block or sleep, but it also means that the +synchronous helper functions listed at the end of Section 4 can be used within +an interrupt handler or in an atomic context. + +The subsystem-level suspend callback is _entirely_ _responsible_ for handling +the suspend of the device as appropriate, which may, but need not include +executing the device driver's own ->runtime_suspend() callback (from the +PM core's point of view it is not necessary to implement a ->runtime_suspend() +callback in a device driver as long as the subsystem-level suspend callback +knows what to do to handle the device). + + * Once the subsystem-level suspend callback has completed successfully + for given device, the PM core regards the device as suspended, which need + not mean that the device has been put into a low power state. It is + supposed to mean, however, that the device will not process data and will + not communicate with the CPU(s) and RAM until the subsystem-level resume + callback is executed for it. The run-time PM status of a device after + successful execution of the subsystem-level suspend callback is 'suspended'. + + * If the subsystem-level suspend callback returns -EBUSY or -EAGAIN, + the device's run-time PM status is 'active', which means that the device + _must_ be fully operational afterwards. + + * If the subsystem-level suspend callback returns an error code different + from -EBUSY or -EAGAIN, the PM core regards this as a fatal error and will + refuse to run the helper functions described in Section 4 for the device, + until the status of it is directly set either to 'active', or to 'suspended' + (the PM core provides special helper functions for this purpose). + +In particular, if the driver requires remote wake-up capability (i.e. hardware +mechanism allowing the device to request a change of its power state, such as +PCI PME) for proper functioning and device_run_wake() returns 'false' for the +device, then ->runtime_suspend() should return -EBUSY. On the other hand, if +device_run_wake() returns 'true' for the device and the device is put into a low +power state during the execution of the subsystem-level suspend callback, it is +expected that remote wake-up will be enabled for the device. Generally, remote +wake-up should be enabled for all input devices put into a low power state at +run time. + +The subsystem-level resume callback is _entirely_ _responsible_ for handling the +resume of the device as appropriate, which may, but need not include executing +the device driver's own ->runtime_resume() callback (from the PM core's point of +view it is not necessary to implement a ->runtime_resume() callback in a device +driver as long as the subsystem-level resume callback knows what to do to handle +the device). + + * Once the subsystem-level resume callback has completed successfully, the PM + core regards the device as fully operational, which means that the device + _must_ be able to complete I/O operations as needed. The run-time PM status + of the device is then 'active'. + + * If the subsystem-level resume callback returns an error code, the PM core + regards this as a fatal error and will refuse to run the helper functions + described in Section 4 for the device, until its status is directly set + either to 'active' or to 'suspended' (the PM core provides special helper + functions for this purpose). + +The subsystem-level idle callback is executed by the PM core whenever the device +appears to be idle, which is indicated to the PM core by two counters, the +device's usage counter and the counter of 'active' children of the device. + + * If any of these counters is decreased using a helper function provided by + the PM core and it turns out to be equal to zero, the other counter is + checked. If that counter also is equal to zero, the PM core executes the + subsystem-level idle callback with the device as an argument. + +The action performed by a subsystem-level idle callback is totally dependent on +the subsystem in question, but the expected and recommended action is to check +if the device can be suspended (i.e. if all of the conditions necessary for +suspending the device are satisfied) and to queue up a suspend request for the +device in that case. The value returned by this callback is ignored by the PM +core. + +The helper functions provided by the PM core, described in Section 4, guarantee +that the following constraints are met with respect to the bus type's run-time +PM callbacks: + +(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute + ->runtime_suspend() in parallel with ->runtime_resume() or with another + instance of ->runtime_suspend() for the same device) with the exception that + ->runtime_suspend() or ->runtime_resume() can be executed in parallel with + ->runtime_idle() (although ->runtime_idle() will not be started while any + of the other callbacks is being executed for the same device). + +(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active' + devices (i.e. the PM core will only execute ->runtime_idle() or + ->runtime_suspend() for the devices the run-time PM status of which is + 'active'). + +(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device + the usage counter of which is equal to zero _and_ either the counter of + 'active' children of which is equal to zero, or the 'power.ignore_children' + flag of which is set. + +(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the + PM core will only execute ->runtime_resume() for the devices the run-time + PM status of which is 'suspended'). + +Additionally, the helper functions provided by the PM core obey the following +rules: + + * If ->runtime_suspend() is about to be executed or there's a pending request + to execute it, ->runtime_idle() will not be executed for the same device. + + * A request to execute or to schedule the execution of ->runtime_suspend() + will cancel any pending requests to execute ->runtime_idle() for the same + device. + + * If ->runtime_resume() is about to be executed or there's a pending request + to execute it, the other callbacks will not be executed for the same device. + + * A request to execute ->runtime_resume() will cancel any pending or + scheduled requests to execute the other callbacks for the same device, + except for scheduled autosuspends. + +3. Run-time PM Device Fields + +The following device run-time PM fields are present in 'struct dev_pm_info', as +defined in include/linux/pm.h: + + struct timer_list suspend_timer; + - timer used for scheduling (delayed) suspend and autosuspend requests + + unsigned long timer_expires; + - timer expiration time, in jiffies (if this is different from zero, the + timer is running and will expire at that time, otherwise the timer is not + running) + + struct work_struct work; + - work structure used for queuing up requests (i.e. work items in pm_wq) + + wait_queue_head_t wait_queue; + - wait queue used if any of the helper functions needs to wait for another + one to complete + + spinlock_t lock; + - lock used for synchronisation + + atomic_t usage_count; + - the usage counter of the device + + atomic_t child_count; + - the count of 'active' children of the device + + unsigned int ignore_children; + - if set, the value of child_count is ignored (but still updated) + + unsigned int disable_depth; + - used for disabling the helper funcions (they work normally if this is + equal to zero); the initial value of it is 1 (i.e. run-time PM is + initially disabled for all devices) + + unsigned int runtime_error; + - if set, there was a fatal error (one of the callbacks returned error code + as described in Section 2), so the helper funtions will not work until + this flag is cleared; this is the error code returned by the failing + callback + + unsigned int idle_notification; + - if set, ->runtime_idle() is being executed + + unsigned int request_pending; + - if set, there's a pending request (i.e. a work item queued up into pm_wq) + + enum rpm_request request; + - type of request that's pending (valid if request_pending is set) + + unsigned int deferred_resume; + - set if ->runtime_resume() is about to be run while ->runtime_suspend() is + being executed for that device and it is not practical to wait for the + suspend to complete; means "start a resume as soon as you've suspended" + + unsigned int run_wake; + - set if the device is capable of generating run-time wake-up events + + enum rpm_status runtime_status; + - the run-time PM status of the device; this field's initial value is + RPM_SUSPENDED, which means that each device is initially regarded by the + PM core as 'suspended', regardless of its real hardware status + + unsigned int runtime_auto; + - if set, indicates that the user space has allowed the device driver to + power manage the device at run time via the /sys/devices/.../power/control + interface; it may only be modified with the help of the pm_runtime_allow() + and pm_runtime_forbid() helper functions + + unsigned int no_callbacks; + - indicates that the device does not use the run-time PM callbacks (see + Section 8); it may be modified only by the pm_runtime_no_callbacks() + helper function + + unsigned int irq_safe; + - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks + will be invoked with the spinlock held and interrupts disabled + + unsigned int use_autosuspend; + - indicates that the device's driver supports delayed autosuspend (see + Section 9); it may be modified only by the + pm_runtime{_dont}_use_autosuspend() helper functions + + unsigned int timer_autosuspends; + - indicates that the PM core should attempt to carry out an autosuspend + when the timer expires rather than a normal suspend + + int autosuspend_delay; + - the delay time (in milliseconds) to be used for autosuspend + + unsigned long last_busy; + - the time (in jiffies) when the pm_runtime_mark_last_busy() helper + function was last called for this device; used in calculating inactivity + periods for autosuspend + +All of the above fields are members of the 'power' member of 'struct device'. + +4. Run-time PM Device Helper Functions + +The following run-time PM helper functions are defined in +drivers/base/power/runtime.c and include/linux/pm_runtime.h: + + void pm_runtime_init(struct device *dev); + - initialize the device run-time PM fields in 'struct dev_pm_info' + + void pm_runtime_remove(struct device *dev); + - make sure that the run-time PM of the device will be disabled after + removing the device from device hierarchy + + int pm_runtime_idle(struct device *dev); + - execute the subsystem-level idle callback for the device; returns 0 on + success or error code on failure, where -EINPROGRESS means that + ->runtime_idle() is already being executed + + int pm_runtime_suspend(struct device *dev); + - execute the subsystem-level suspend callback for the device; returns 0 on + success, 1 if the device's run-time PM status was already 'suspended', or + error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt + to suspend the device again in future + + int pm_runtime_autosuspend(struct device *dev); + - same as pm_runtime_suspend() except that the autosuspend delay is taken + into account; if pm_runtime_autosuspend_expiration() says the delay has + not yet expired then an autosuspend is scheduled for the appropriate time + and 0 is returned + + int pm_runtime_resume(struct device *dev); + - execute the subsystem-level resume callback for the device; returns 0 on + success, 1 if the device's run-time PM status was already 'active' or + error code on failure, where -EAGAIN means it may be safe to attempt to + resume the device again in future, but 'power.runtime_error' should be + checked additionally + + int pm_request_idle(struct device *dev); + - submit a request to execute the subsystem-level idle callback for the + device (the request is represented by a work item in pm_wq); returns 0 on + success or error code if the request has not been queued up + + int pm_request_autosuspend(struct device *dev); + - schedule the execution of the subsystem-level suspend callback for the + device when the autosuspend delay has expired; if the delay has already + expired then the work item is queued up immediately + + int pm_schedule_suspend(struct device *dev, unsigned int delay); + - schedule the execution of the subsystem-level suspend callback for the + device in future, where 'delay' is the time to wait before queuing up a + suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work + item is queued up immediately); returns 0 on success, 1 if the device's PM + run-time status was already 'suspended', or error code if the request + hasn't been scheduled (or queued up if 'delay' is 0); if the execution of + ->runtime_suspend() is already scheduled and not yet expired, the new + value of 'delay' will be used as the time to wait + + int pm_request_resume(struct device *dev); + - submit a request to execute the subsystem-level resume callback for the + device (the request is represented by a work item in pm_wq); returns 0 on + success, 1 if the device's run-time PM status was already 'active', or + error code if the request hasn't been queued up + + void pm_runtime_get_noresume(struct device *dev); + - increment the device's usage counter + + int pm_runtime_get(struct device *dev); + - increment the device's usage counter, run pm_request_resume(dev) and + return its result + + int pm_runtime_get_sync(struct device *dev); + - increment the device's usage counter, run pm_runtime_resume(dev) and + return its result + + void pm_runtime_put_noidle(struct device *dev); + - decrement the device's usage counter + + int pm_runtime_put(struct device *dev); + - decrement the device's usage counter; if the result is 0 then run + pm_request_idle(dev) and return its result + + int pm_runtime_put_autosuspend(struct device *dev); + - decrement the device's usage counter; if the result is 0 then run + pm_request_autosuspend(dev) and return its result + + int pm_runtime_put_sync(struct device *dev); + - decrement the device's usage counter; if the result is 0 then run + pm_runtime_idle(dev) and return its result + + int pm_runtime_put_sync_suspend(struct device *dev); + - decrement the device's usage counter; if the result is 0 then run + pm_runtime_suspend(dev) and return its result + + int pm_runtime_put_sync_autosuspend(struct device *dev); + - decrement the device's usage counter; if the result is 0 then run + pm_runtime_autosuspend(dev) and return its result + + void pm_runtime_enable(struct device *dev); + - enable the run-time PM helper functions to run the device bus type's + run-time PM callbacks described in Section 2 + + int pm_runtime_disable(struct device *dev); + - prevent the run-time PM helper functions from running subsystem-level + run-time PM callbacks for the device, make sure that all of the pending + run-time PM operations on the device are either completed or canceled; + returns 1 if there was a resume request pending and it was necessary to + execute the subsystem-level resume callback for the device to satisfy that + request, otherwise 0 is returned + + void pm_suspend_ignore_children(struct device *dev, bool enable); + - set/unset the power.ignore_children flag of the device + + int pm_runtime_set_active(struct device *dev); + - clear the device's 'power.runtime_error' flag, set the device's run-time + PM status to 'active' and update its parent's counter of 'active' + children as appropriate (it is only valid to use this function if + 'power.runtime_error' is set or 'power.disable_depth' is greater than + zero); it will fail and return error code if the device has a parent + which is not active and the 'power.ignore_children' flag of which is unset + + void pm_runtime_set_suspended(struct device *dev); + - clear the device's 'power.runtime_error' flag, set the device's run-time + PM status to 'suspended' and update its parent's counter of 'active' + children as appropriate (it is only valid to use this function if + 'power.runtime_error' is set or 'power.disable_depth' is greater than + zero) + + bool pm_runtime_suspended(struct device *dev); + - return true if the device's runtime PM status is 'suspended' and its + 'power.disable_depth' field is equal to zero, or false otherwise + + void pm_runtime_allow(struct device *dev); + - set the power.runtime_auto flag for the device and decrease its usage + counter (used by the /sys/devices/.../power/control interface to + effectively allow the device to be power managed at run time) + + void pm_runtime_forbid(struct device *dev); + - unset the power.runtime_auto flag for the device and increase its usage + counter (used by the /sys/devices/.../power/control interface to + effectively prevent the device from being power managed at run time) + + void pm_runtime_no_callbacks(struct device *dev); + - set the power.no_callbacks flag for the device and remove the run-time + PM attributes from /sys/devices/.../power (or prevent them from being + added when the device is registered) + + void pm_runtime_irq_safe(struct device *dev); + - set the power.irq_safe flag for the device, causing the runtime-PM + suspend and resume callbacks (but not the idle callback) to be invoked + with interrupts disabled + + void pm_runtime_mark_last_busy(struct device *dev); + - set the power.last_busy field to the current time + + void pm_runtime_use_autosuspend(struct device *dev); + - set the power.use_autosuspend flag, enabling autosuspend delays + + void pm_runtime_dont_use_autosuspend(struct device *dev); + - clear the power.use_autosuspend flag, disabling autosuspend delays + + void pm_runtime_set_autosuspend_delay(struct device *dev, int delay); + - set the power.autosuspend_delay value to 'delay' (expressed in + milliseconds); if 'delay' is negative then run-time suspends are + prevented + + unsigned long pm_runtime_autosuspend_expiration(struct device *dev); + - calculate the time when the current autosuspend delay period will expire, + based on power.last_busy and power.autosuspend_delay; if the delay time + is 1000 ms or larger then the expiration time is rounded up to the + nearest second; returns 0 if the delay period has already expired or + power.use_autosuspend isn't set, otherwise returns the expiration time + in jiffies + +It is safe to execute the following helper functions from interrupt context: + +pm_request_idle() +pm_request_autosuspend() +pm_schedule_suspend() +pm_request_resume() +pm_runtime_get_noresume() +pm_runtime_get() +pm_runtime_put_noidle() +pm_runtime_put() +pm_runtime_put_autosuspend() +pm_runtime_enable() +pm_suspend_ignore_children() +pm_runtime_set_active() +pm_runtime_set_suspended() +pm_runtime_suspended() +pm_runtime_mark_last_busy() +pm_runtime_autosuspend_expiration() + +If pm_runtime_irq_safe() has been called for a device then the following helper +functions may also be used in interrupt context: + +pm_runtime_suspend() +pm_runtime_autosuspend() +pm_runtime_resume() +pm_runtime_get_sync() +pm_runtime_put_sync_suspend() +pm_runtime_put_sync_autosuspend() + +5. Run-time PM Initialization, Device Probing and Removal + +Initially, the run-time PM is disabled for all devices, which means that the +majority of the run-time PM helper funtions described in Section 4 will return +-EAGAIN until pm_runtime_enable() is called for the device. + +In addition to that, the initial run-time PM status of all devices is +'suspended', but it need not reflect the actual physical state of the device. +Thus, if the device is initially active (i.e. it is able to process I/O), its +run-time PM status must be changed to 'active', with the help of +pm_runtime_set_active(), before pm_runtime_enable() is called for the device. + +However, if the device has a parent and the parent's run-time PM is enabled, +calling pm_runtime_set_active() for the device will affect the parent, unless +the parent's 'power.ignore_children' flag is set. Namely, in that case the +parent won't be able to suspend at run time, using the PM core's helper +functions, as long as the child's status is 'active', even if the child's +run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for +the child yet or pm_runtime_disable() has been called for it). For this reason, +once pm_runtime_set_active() has been called for the device, pm_runtime_enable() +should be called for it too as soon as reasonably possible or its run-time PM +status should be changed back to 'suspended' with the help of +pm_runtime_set_suspended(). + +If the default initial run-time PM status of the device (i.e. 'suspended') +reflects the actual state of the device, its bus type's or its driver's +->probe() callback will likely need to wake it up using one of the PM core's +helper functions described in Section 4. In that case, pm_runtime_resume() +should be used. Of course, for this purpose the device's run-time PM has to be +enabled earlier by calling pm_runtime_enable(). + +If the device bus type's or driver's ->probe() callback runs +pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts, +they will fail returning -EAGAIN, because the device's usage counter is +incremented by the driver core before executing ->probe(). Still, it may be +desirable to suspend the device as soon as ->probe() has finished, so the driver +core uses pm_runtime_put_sync() to invoke the subsystem-level idle callback for +the device at that time. + +Moreover, the driver core prevents runtime PM callbacks from racing with the bus +notifier callback in __device_release_driver(), which is necessary, because the +notifier is used by some subsystems to carry out operations affecting the +runtime PM functionality. It does so by calling pm_runtime_get_sync() before +driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This +resumes the device if it's in the suspended state and prevents it from +being suspended again while those routines are being executed. + +To allow bus types and drivers to put devices into the suspended state by +calling pm_runtime_suspend() from their ->remove() routines, the driver core +executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER +notifications in __device_release_driver(). This requires bus types and +drivers to make their ->remove() callbacks avoid races with runtime PM directly, +but also it allows of more flexibility in the handling of devices during the +removal of their drivers. + +The user space can effectively disallow the driver of the device to power manage +it at run time by changing the value of its /sys/devices/.../power/control +attribute to "on", which causes pm_runtime_forbid() to be called. In principle, +this mechanism may also be used by the driver to effectively turn off the +run-time power management of the device until the user space turns it on. +Namely, during the initialization the driver can make sure that the run-time PM +status of the device is 'active' and call pm_runtime_forbid(). It should be +noted, however, that if the user space has already intentionally changed the +value of /sys/devices/.../power/control to "auto" to allow the driver to power +manage the device at run time, the driver may confuse it by using +pm_runtime_forbid() this way. + +6. Run-time PM and System Sleep + +Run-time PM and system sleep (i.e., system suspend and hibernation, also known +as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of +ways. If a device is active when a system sleep starts, everything is +straightforward. But what should happen if the device is already suspended? + +The device may have different wake-up settings for run-time PM and system sleep. +For example, remote wake-up may be enabled for run-time suspend but disallowed +for system sleep (device_may_wakeup(dev) returns 'false'). When this happens, +the subsystem-level system suspend callback is responsible for changing the +device's wake-up setting (it may leave that to the device driver's system +suspend routine). It may be necessary to resume the device and suspend it again +in order to do so. The same is true if the driver uses different power levels +or other settings for run-time suspend and system sleep. + +During system resume, devices generally should be brought back to full power, +even if they were suspended before the system sleep began. There are several +reasons for this, including: + + * The device might need to switch power levels, wake-up settings, etc. + + * Remote wake-up events might have been lost by the firmware. + + * The device's children may need the device to be at full power in order + to resume themselves. + + * The driver's idea of the device state may not agree with the device's + physical state. This can happen during resume from hibernation. + + * The device might need to be reset. + + * Even though the device was suspended, if its usage counter was > 0 then most + likely it would need a run-time resume in the near future anyway. + + * Always going back to full power is simplest. + +If the device was suspended before the sleep began, then its run-time PM status +will have to be updated to reflect the actual post-system sleep status. The way +to do this is: + + pm_runtime_disable(dev); + pm_runtime_set_active(dev); + pm_runtime_enable(dev); + +7. Generic subsystem callbacks + +Subsystems may wish to conserve code space by using the set of generic power +management callbacks provided by the PM core, defined in +driver/base/power/generic_ops.c: + + int pm_generic_runtime_idle(struct device *dev); + - invoke the ->runtime_idle() callback provided by the driver of this + device, if defined, and call pm_runtime_suspend() for this device if the + return value is 0 or the callback is not defined + + int pm_generic_runtime_suspend(struct device *dev); + - invoke the ->runtime_suspend() callback provided by the driver of this + device and return its result, or return -EINVAL if not defined + + int pm_generic_runtime_resume(struct device *dev); + - invoke the ->runtime_resume() callback provided by the driver of this + device and return its result, or return -EINVAL if not defined + + int pm_generic_suspend(struct device *dev); + - if the device has not been suspended at run time, invoke the ->suspend() + callback provided by its driver and return its result, or return 0 if not + defined + + int pm_generic_resume(struct device *dev); + - invoke the ->resume() callback provided by the driver of this device and, + if successful, change the device's runtime PM status to 'active' + + int pm_generic_freeze(struct device *dev); + - if the device has not been suspended at run time, invoke the ->freeze() + callback provided by its driver and return its result, or return 0 if not + defined + + int pm_generic_thaw(struct device *dev); + - if the device has not been suspended at run time, invoke the ->thaw() + callback provided by its driver and return its result, or return 0 if not + defined + + int pm_generic_poweroff(struct device *dev); + - if the device has not been suspended at run time, invoke the ->poweroff() + callback provided by its driver and return its result, or return 0 if not + defined + + int pm_generic_restore(struct device *dev); + - invoke the ->restore() callback provided by the driver of this device and, + if successful, change the device's runtime PM status to 'active' + +These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(), +->runtime_resume(), ->suspend(), ->resume(), ->freeze(), ->thaw(), ->poweroff(), +or ->restore() callback pointers in the subsystem-level dev_pm_ops structures. + +If a subsystem wishes to use all of them at the same time, it can simply assign +the GENERIC_SUBSYS_PM_OPS macro, defined in include/linux/pm.h, to its +dev_pm_ops structure pointer. + +Device drivers that wish to use the same function as a system suspend, freeze, +poweroff and run-time suspend callback, and similarly for system resume, thaw, +restore, and run-time resume, can achieve this with the help of the +UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its +last argument to NULL). + +8. "No-Callback" Devices + +Some "devices" are only logical sub-devices of their parent and cannot be +power-managed on their own. (The prototype example is a USB interface. Entire +USB devices can go into low-power mode or send wake-up requests, but neither is +possible for individual interfaces.) The drivers for these devices have no +need of run-time PM callbacks; if the callbacks did exist, ->runtime_suspend() +and ->runtime_resume() would always return 0 without doing anything else and +->runtime_idle() would always call pm_runtime_suspend(). + +Subsystems can tell the PM core about these devices by calling +pm_runtime_no_callbacks(). This should be done after the device structure is +initialized and before it is registered (although after device registration is +also okay). The routine will set the device's power.no_callbacks flag and +prevent the non-debugging run-time PM sysfs attributes from being created. + +When power.no_callbacks is set, the PM core will not invoke the +->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks. +Instead it will assume that suspends and resumes always succeed and that idle +devices should be suspended. + +As a consequence, the PM core will never directly inform the device's subsystem +or driver about run-time power changes. Instead, the driver for the device's +parent must take responsibility for telling the device's driver when the +parent's power state changes. + +9. Autosuspend, or automatically-delayed suspends + +Changing a device's power state isn't free; it requires both time and energy. +A device should be put in a low-power state only when there's some reason to +think it will remain in that state for a substantial time. A common heuristic +says that a device which hasn't been used for a while is liable to remain +unused; following this advice, drivers should not allow devices to be suspended +at run-time until they have been inactive for some minimum period. Even when +the heuristic ends up being non-optimal, it will still prevent devices from +"bouncing" too rapidly between low-power and full-power states. + +The term "autosuspend" is an historical remnant. It doesn't mean that the +device is automatically suspended (the subsystem or driver still has to call +the appropriate PM routines); rather it means that run-time suspends will +automatically be delayed until the desired period of inactivity has elapsed. + +Inactivity is determined based on the power.last_busy field. Drivers should +call pm_runtime_mark_last_busy() to update this field after carrying out I/O, +typically just before calling pm_runtime_put_autosuspend(). The desired length +of the inactivity period is a matter of policy. Subsystems can set this length +initially by calling pm_runtime_set_autosuspend_delay(), but after device +registration the length should be controlled by user space, using the +/sys/devices/.../power/autosuspend_delay_ms attribute. + +In order to use autosuspend, subsystems or drivers must call +pm_runtime_use_autosuspend() (preferably before registering the device), and +thereafter they should use the various *_autosuspend() helper functions instead +of the non-autosuspend counterparts: + + Instead of: pm_runtime_suspend use: pm_runtime_autosuspend; + Instead of: pm_schedule_suspend use: pm_request_autosuspend; + Instead of: pm_runtime_put use: pm_runtime_put_autosuspend; + Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend. + +Drivers may also continue to use the non-autosuspend helper functions; they +will behave normally, not taking the autosuspend delay into account. +Similarly, if the power.use_autosuspend field isn't set then the autosuspend +helper functions will behave just like the non-autosuspend counterparts. + +Under some circumstances a driver or subsystem may want to prevent a device +from autosuspending immediately, even though the usage counter is zero and the +autosuspend delay time has expired. If the ->runtime_suspend() callback +returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is +in the future (as it normally would be if the callback invoked +pm_runtime_mark_last_busy()), the PM core will automatically reschedule the +autosuspend. The ->runtime_suspend() callback can't do this rescheduling +itself because no suspend requests of any kind are accepted while the device is +suspending (i.e., while the callback is running). + +The implementation is well suited for asynchronous use in interrupt contexts. +However such use inevitably involves races, because the PM core can't +synchronize ->runtime_suspend() callbacks with the arrival of I/O requests. +This synchronization must be handled by the driver, using its private lock. +Here is a schematic pseudo-code example: + + foo_read_or_write(struct foo_priv *foo, void *data) + { + lock(&foo->private_lock); + add_request_to_io_queue(foo, data); + if (foo->num_pending_requests++ == 0) + pm_runtime_get(&foo->dev); + if (!foo->is_suspended) + foo_process_next_request(foo); + unlock(&foo->private_lock); + } + + foo_io_completion(struct foo_priv *foo, void *req) + { + lock(&foo->private_lock); + if (--foo->num_pending_requests == 0) { + pm_runtime_mark_last_busy(&foo->dev); + pm_runtime_put_autosuspend(&foo->dev); + } else { + foo_process_next_request(foo); + } + unlock(&foo->private_lock); + /* Send req result back to the user ... */ + } + + int foo_runtime_suspend(struct device *dev) + { + struct foo_priv foo = container_of(dev, ...); + int ret = 0; + + lock(&foo->private_lock); + if (foo->num_pending_requests > 0) { + ret = -EBUSY; + } else { + /* ... suspend the device ... */ + foo->is_suspended = 1; + } + unlock(&foo->private_lock); + return ret; + } + + int foo_runtime_resume(struct device *dev) + { + struct foo_priv foo = container_of(dev, ...); + + lock(&foo->private_lock); + /* ... resume the device ... */ + foo->is_suspended = 0; + pm_runtime_mark_last_busy(&foo->dev); + if (foo->num_pending_requests > 0) + foo_process_requests(foo); + unlock(&foo->private_lock); + return 0; + } + +The important point is that after foo_io_completion() asks for an autosuspend, +the foo_runtime_suspend() callback may race with foo_read_or_write(). +Therefore foo_runtime_suspend() has to check whether there are any pending I/O +requests (while holding the private lock) before allowing the suspend to +proceed. + +In addition, the power.autosuspend_delay field can be changed by user space at +any time. If a driver cares about this, it can call +pm_runtime_autosuspend_expiration() from within the ->runtime_suspend() +callback while holding its private lock. If the function returns a nonzero +value then the delay has not yet expired and the callback should return +-EAGAIN. |