| Runtime 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 runtime power management (runtime 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 runtime 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 runtime 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 runtime PM operations with one another. |
| |
| * Three device runtime 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 runtime 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 runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM |
| fields of 'struct dev_pm_info' and the core helper functions provided for |
| runtime PM are described below. |
| |
| 2. Device Runtime PM Callbacks |
| |
| There are three device runtime 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. |
| 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 runtime 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 runtime 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 runtime 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 runtime |
| 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 runtime 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 runtime |
| 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. Runtime PM Device Fields |
| |
| The following device runtime 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. runtime 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 runtime wake-up events |
| |
| enum rpm_status runtime_status; |
| - the runtime 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 runtime 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. Runtime PM Device Helper Functions |
| |
| The following runtime 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 runtime PM fields in 'struct dev_pm_info' |
| |
| void pm_runtime_remove(struct device *dev); |
| - make sure that the runtime 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 runtime 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 and -EACCES means that |
| 'power.disable_depth' is different from 0 |
| |
| 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 runtime 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, and -EACCES means that 'power.disable_depth' is |
| different from 0 |
| |
| 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 |
| runtime 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 runtime 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); |
| - decrement the device's 'power.disable_depth' field; if that field is equal |
| to zero, the runtime PM helper functions can execute subsystem-level |
| callbacks described in Section 2 for the device |
| |
| int pm_runtime_disable(struct device *dev); |
| - increment the device's 'power.disable_depth' field (if the value of that |
| field was previously zero, this prevents subsystem-level runtime PM |
| callbacks from being run for the device), make sure that all of the pending |
| runtime 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 |
| |
| int pm_runtime_barrier(struct device *dev); |
| - check if there's a resume request pending for the device and resume it |
| (synchronously) in that case, cancel any other pending runtime PM requests |
| regarding it and wait for all runtime PM operations on it in progress to |
| complete; 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 runtime |
| 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 runtime |
| 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 |
| |
| bool pm_runtime_status_suspended(struct device *dev); |
| - return true if the device's runtime PM status is 'suspended' |
| |
| 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 runtime |
| 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 runtime 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() |
| pm_runtime_put_sync_suspend() |
| pm_runtime_put_sync_autosuspend() |
| |
| 5. Runtime PM Initialization, Device Probing and Removal |
| |
| Initially, the runtime PM is disabled for all devices, which means that the |
| majority of the runtime 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 runtime 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 |
| runtime 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 runtime 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 |
| runtime 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 runtime PM |
| status should be changed back to 'suspended' with the help of |
| pm_runtime_set_suspended(). |
| |
| If the default initial runtime 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 runtime 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 |
| runtime power management of the device until the user space turns it on. |
| Namely, during the initialization the driver can make sure that the runtime 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. Runtime PM and System Sleep |
| |
| Runtime 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 runtime PM and system sleep. |
| For example, remote wake-up may be enabled for runtime 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 runtime suspend and system sleep. |
| |
| During system resume, the simplest approach is to bring all devices back to full |
| power, even if they had been suspended before the system suspend 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 runtime resume in the near future anyway. |
| |
| If the device had been suspended before the system suspend began and it's |
| brought back to full power during resume, then its runtime 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); |
| |
| The PM core always increments the runtime usage counter before calling the |
| ->suspend() callback and decrements it after calling the ->resume() callback. |
| Hence disabling runtime PM temporarily like this will not cause any runtime |
| suspend attempts to be permanently lost. If the usage count goes to zero |
| following the return of the ->resume() callback, the ->runtime_idle() callback |
| will be invoked as usual. |
| |
| On some systems, however, system sleep is not entered through a global firmware |
| or hardware operation. Instead, all hardware components are put into low-power |
| states directly by the kernel in a coordinated way. Then, the system sleep |
| state effectively follows from the states the hardware components end up in |
| and the system is woken up from that state by a hardware interrupt or a similar |
| mechanism entirely under the kernel's control. As a result, the kernel never |
| gives control away and the states of all devices during resume are precisely |
| known to it. If that is the case and none of the situations listed above takes |
| place (in particular, if the system is not waking up from hibernation), it may |
| be more efficient to leave the devices that had been suspended before the system |
| suspend began in the suspended state. |
| |
| The PM core does its best to reduce the probability of race conditions between |
| the runtime PM and system suspend/resume (and hibernation) callbacks by carrying |
| out the following operations: |
| |
| * During system suspend it calls pm_runtime_get_noresume() and |
| pm_runtime_barrier() for every device right before executing the |
| subsystem-level .suspend() callback for it. In addition to that it calls |
| pm_runtime_disable() for every device right after executing the |
| subsystem-level .suspend() callback for it. |
| |
| * During system resume it calls pm_runtime_enable() and pm_runtime_put_sync() |
| for every device right before and right after executing the subsystem-level |
| .resume() callback for it, respectively. |
| |
| 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_suspend_noirq(struct device *dev); |
| - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq() |
| callback provided by the device's 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_resume_noirq(struct device *dev); |
| - invoke the ->resume_noirq() callback provided by the driver of this device |
| |
| 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_freeze_noirq(struct device *dev); |
| - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq() |
| callback provided by the device's 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_thaw_noirq(struct device *dev); |
| - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq() |
| callback provided by the device's 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_poweroff_noirq(struct device *dev); |
| - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq() |
| callback provided by the device's 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' |
| |
| int pm_generic_restore_noirq(struct device *dev); |
| - invoke the ->restore_noirq() callback provided by the device's driver |
| |
| These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(), |
| ->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(), |
| ->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(), |
| ->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() 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 runtime suspend callback, and similarly for system resume, thaw, |
| restore, and runtime 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 runtime 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 runtime 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 runtime 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 runtime 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 runtime 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. |
| |
| 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. |