379 lines
18 KiB
Plaintext
379 lines
18 KiB
Plaintext
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Run-time Power Management Framework for I/O Devices
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(C) 2009 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
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1. Introduction
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Support for run-time power management (run-time PM) of I/O devices is provided
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at the power management core (PM core) level by means of:
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* The power management workqueue pm_wq in which bus types and device drivers can
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put their PM-related work items. It is strongly recommended that pm_wq be
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used for queuing all work items related to run-time PM, because this allows
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them to be synchronized with system-wide power transitions (suspend to RAM,
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hibernation and resume from system sleep states). pm_wq is declared in
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include/linux/pm_runtime.h and defined in kernel/power/main.c.
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* A number of run-time PM fields in the 'power' member of 'struct device' (which
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is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
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be used for synchronizing run-time PM operations with one another.
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* Three device run-time PM callbacks in 'struct dev_pm_ops' (defined in
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include/linux/pm.h).
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* A set of helper functions defined in drivers/base/power/runtime.c that can be
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used for carrying out run-time PM operations in such a way that the
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synchronization between them is taken care of by the PM core. Bus types and
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device drivers are encouraged to use these functions.
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The run-time PM callbacks present in 'struct dev_pm_ops', the device run-time PM
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fields of 'struct dev_pm_info' and the core helper functions provided for
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run-time PM are described below.
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2. Device Run-time PM Callbacks
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There are three device run-time PM callbacks defined in 'struct dev_pm_ops':
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struct dev_pm_ops {
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...
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int (*runtime_suspend)(struct device *dev);
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int (*runtime_resume)(struct device *dev);
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void (*runtime_idle)(struct device *dev);
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...
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};
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The ->runtime_suspend() callback is executed by the PM core for the bus type of
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the device being suspended. The bus type's callback is then _entirely_
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_responsible_ for handling the device as appropriate, which may, but need not
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include executing the device driver's own ->runtime_suspend() callback (from the
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PM core's point of view it is not necessary to implement a ->runtime_suspend()
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callback in a device driver as long as the bus type's ->runtime_suspend() knows
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what to do to handle the device).
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* Once the bus type's ->runtime_suspend() callback has completed successfully
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for given device, the PM core regards the device as suspended, which need
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not mean that the device has been put into a low power state. It is
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supposed to mean, however, that the device will not process data and will
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not communicate with the CPU(s) and RAM until its bus type's
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->runtime_resume() callback is executed for it. The run-time PM status of
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a device after successful execution of its bus type's ->runtime_suspend()
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callback is 'suspended'.
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* If the bus type's ->runtime_suspend() callback returns -EBUSY or -EAGAIN,
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the device's run-time PM status is supposed to be 'active', which means that
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the device _must_ be fully operational afterwards.
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* If the bus type's ->runtime_suspend() callback returns an error code
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different from -EBUSY or -EAGAIN, the PM core regards this as a fatal
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error and will refuse to run the helper functions described in Section 4
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for the device, until the status of it is directly set either to 'active'
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or to 'suspended' (the PM core provides special helper functions for this
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purpose).
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In particular, if the driver requires remote wakeup capability for proper
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functioning and device_may_wakeup() returns 'false' for the device, then
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->runtime_suspend() should return -EBUSY. On the other hand, if
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device_may_wakeup() returns 'true' for the device and the device is put
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into a low power state during the execution of its bus type's
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->runtime_suspend(), it is expected that remote wake-up (i.e. hardware mechanism
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allowing the device to request a change of its power state, such as PCI PME)
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will be enabled for the device. Generally, remote wake-up should be enabled
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for all input devices put into a low power state at run time.
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The ->runtime_resume() callback is executed by the PM core for the bus type of
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the device being woken up. The bus type's callback is then _entirely_
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_responsible_ for handling the device as appropriate, which may, but need not
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include executing the device driver's own ->runtime_resume() callback (from the
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PM core's point of view it is not necessary to implement a ->runtime_resume()
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callback in a device driver as long as the bus type's ->runtime_resume() knows
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what to do to handle the device).
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* Once the bus type's ->runtime_resume() callback has completed successfully,
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the PM core regards the device as fully operational, which means that the
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device _must_ be able to complete I/O operations as needed. The run-time
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PM status of the device is then 'active'.
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* If the bus type's ->runtime_resume() callback returns an error code, the PM
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core regards this as a fatal error and will refuse to run the helper
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functions described in Section 4 for the device, until its status is
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directly set either to 'active' or to 'suspended' (the PM core provides
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special helper functions for this purpose).
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The ->runtime_idle() callback is executed by the PM core for the bus type of
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given device whenever the device appears to be idle, which is indicated to the
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PM core by two counters, the device's usage counter and the counter of 'active'
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children of the device.
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* If any of these counters is decreased using a helper function provided by
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the PM core and it turns out to be equal to zero, the other counter is
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checked. If that counter also is equal to zero, the PM core executes the
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device bus type's ->runtime_idle() callback (with the device as an
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argument).
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The action performed by a bus type's ->runtime_idle() callback is totally
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dependent on the bus type in question, but the expected and recommended action
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is to check if the device can be suspended (i.e. if all of the conditions
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necessary for suspending the device are satisfied) and to queue up a suspend
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request for the device in that case.
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The helper functions provided by the PM core, described in Section 4, guarantee
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that the following constraints are met with respect to the bus type's run-time
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PM callbacks:
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(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
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->runtime_suspend() in parallel with ->runtime_resume() or with another
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instance of ->runtime_suspend() for the same device) with the exception that
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->runtime_suspend() or ->runtime_resume() can be executed in parallel with
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->runtime_idle() (although ->runtime_idle() will not be started while any
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of the other callbacks is being executed for the same device).
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(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
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devices (i.e. the PM core will only execute ->runtime_idle() or
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->runtime_suspend() for the devices the run-time PM status of which is
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'active').
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(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
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the usage counter of which is equal to zero _and_ either the counter of
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'active' children of which is equal to zero, or the 'power.ignore_children'
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flag of which is set.
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(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
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PM core will only execute ->runtime_resume() for the devices the run-time
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PM status of which is 'suspended').
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Additionally, the helper functions provided by the PM core obey the following
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rules:
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* If ->runtime_suspend() is about to be executed or there's a pending request
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to execute it, ->runtime_idle() will not be executed for the same device.
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* A request to execute or to schedule the execution of ->runtime_suspend()
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will cancel any pending requests to execute ->runtime_idle() for the same
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device.
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* If ->runtime_resume() is about to be executed or there's a pending request
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to execute it, the other callbacks will not be executed for the same device.
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* A request to execute ->runtime_resume() will cancel any pending or
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scheduled requests to execute the other callbacks for the same device.
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3. Run-time PM Device Fields
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The following device run-time PM fields are present in 'struct dev_pm_info', as
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defined in include/linux/pm.h:
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struct timer_list suspend_timer;
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- timer used for scheduling (delayed) suspend request
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unsigned long timer_expires;
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- timer expiration time, in jiffies (if this is different from zero, the
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timer is running and will expire at that time, otherwise the timer is not
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running)
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struct work_struct work;
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- work structure used for queuing up requests (i.e. work items in pm_wq)
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wait_queue_head_t wait_queue;
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- wait queue used if any of the helper functions needs to wait for another
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one to complete
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spinlock_t lock;
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- lock used for synchronisation
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atomic_t usage_count;
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- the usage counter of the device
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atomic_t child_count;
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- the count of 'active' children of the device
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unsigned int ignore_children;
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- if set, the value of child_count is ignored (but still updated)
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unsigned int disable_depth;
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- used for disabling the helper funcions (they work normally if this is
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equal to zero); the initial value of it is 1 (i.e. run-time PM is
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initially disabled for all devices)
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unsigned int runtime_error;
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- if set, there was a fatal error (one of the callbacks returned error code
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as described in Section 2), so the helper funtions will not work until
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this flag is cleared; this is the error code returned by the failing
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callback
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unsigned int idle_notification;
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- if set, ->runtime_idle() is being executed
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unsigned int request_pending;
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- if set, there's a pending request (i.e. a work item queued up into pm_wq)
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enum rpm_request request;
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- type of request that's pending (valid if request_pending is set)
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unsigned int deferred_resume;
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- set if ->runtime_resume() is about to be run while ->runtime_suspend() is
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being executed for that device and it is not practical to wait for the
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suspend to complete; means "start a resume as soon as you've suspended"
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enum rpm_status runtime_status;
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- the run-time PM status of the device; this field's initial value is
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RPM_SUSPENDED, which means that each device is initially regarded by the
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PM core as 'suspended', regardless of its real hardware status
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All of the above fields are members of the 'power' member of 'struct device'.
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4. Run-time PM Device Helper Functions
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The following run-time PM helper functions are defined in
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drivers/base/power/runtime.c and include/linux/pm_runtime.h:
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void pm_runtime_init(struct device *dev);
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- initialize the device run-time PM fields in 'struct dev_pm_info'
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void pm_runtime_remove(struct device *dev);
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- make sure that the run-time PM of the device will be disabled after
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removing the device from device hierarchy
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int pm_runtime_idle(struct device *dev);
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- execute ->runtime_idle() for the device's bus type; returns 0 on success
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or error code on failure, where -EINPROGRESS means that ->runtime_idle()
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is already being executed
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int pm_runtime_suspend(struct device *dev);
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- execute ->runtime_suspend() for the device's bus type; returns 0 on
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success, 1 if the device's run-time PM status was already 'suspended', or
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error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
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to suspend the device again in future
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int pm_runtime_resume(struct device *dev);
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- execute ->runtime_resume() for the device's bus type; returns 0 on
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success, 1 if the device's run-time PM status was already 'active' or
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error code on failure, where -EAGAIN means it may be safe to attempt to
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resume the device again in future, but 'power.runtime_error' should be
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checked additionally
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int pm_request_idle(struct device *dev);
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- submit a request to execute ->runtime_idle() for the device's bus type
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(the request is represented by a work item in pm_wq); returns 0 on success
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or error code if the request has not been queued up
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int pm_schedule_suspend(struct device *dev, unsigned int delay);
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- schedule the execution of ->runtime_suspend() for the device's bus type
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in future, where 'delay' is the time to wait before queuing up a suspend
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work item in pm_wq, in milliseconds (if 'delay' is zero, the work item is
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queued up immediately); returns 0 on success, 1 if the device's PM
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run-time status was already 'suspended', or error code if the request
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hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
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->runtime_suspend() is already scheduled and not yet expired, the new
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value of 'delay' will be used as the time to wait
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int pm_request_resume(struct device *dev);
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- submit a request to execute ->runtime_resume() for the device's bus type
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(the request is represented by a work item in pm_wq); returns 0 on
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success, 1 if the device's run-time PM status was already 'active', or
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error code if the request hasn't been queued up
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void pm_runtime_get_noresume(struct device *dev);
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- increment the device's usage counter
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int pm_runtime_get(struct device *dev);
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- increment the device's usage counter, run pm_request_resume(dev) and
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return its result
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int pm_runtime_get_sync(struct device *dev);
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- increment the device's usage counter, run pm_runtime_resume(dev) and
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return its result
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void pm_runtime_put_noidle(struct device *dev);
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- decrement the device's usage counter
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int pm_runtime_put(struct device *dev);
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- decrement the device's usage counter, run pm_request_idle(dev) and return
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its result
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int pm_runtime_put_sync(struct device *dev);
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- decrement the device's usage counter, run pm_runtime_idle(dev) and return
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its result
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void pm_runtime_enable(struct device *dev);
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- enable the run-time PM helper functions to run the device bus type's
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run-time PM callbacks described in Section 2
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int pm_runtime_disable(struct device *dev);
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- prevent the run-time PM helper functions from running the device bus
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type's run-time PM callbacks, make sure that all of the pending run-time
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PM operations on the device are either completed or canceled; returns
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1 if there was a resume request pending and it was necessary to execute
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->runtime_resume() for the device's bus type to satisfy that request,
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otherwise 0 is returned
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void pm_suspend_ignore_children(struct device *dev, bool enable);
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- set/unset the power.ignore_children flag of the device
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int pm_runtime_set_active(struct device *dev);
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- clear the device's 'power.runtime_error' flag, set the device's run-time
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PM status to 'active' and update its parent's counter of 'active'
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children as appropriate (it is only valid to use this function if
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'power.runtime_error' is set or 'power.disable_depth' is greater than
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zero); it will fail and return error code if the device has a parent
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which is not active and the 'power.ignore_children' flag of which is unset
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void pm_runtime_set_suspended(struct device *dev);
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- clear the device's 'power.runtime_error' flag, set the device's run-time
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PM status to 'suspended' and update its parent's counter of 'active'
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children as appropriate (it is only valid to use this function if
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'power.runtime_error' is set or 'power.disable_depth' is greater than
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zero)
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It is safe to execute the following helper functions from interrupt context:
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pm_request_idle()
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pm_schedule_suspend()
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pm_request_resume()
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pm_runtime_get_noresume()
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pm_runtime_get()
|
||
|
|
pm_runtime_put_noidle()
|
||
|
|
pm_runtime_put()
|
||
|
|
pm_suspend_ignore_children()
|
||
|
|
pm_runtime_set_active()
|
||
|
|
pm_runtime_set_suspended()
|
||
|
|
pm_runtime_enable()
|
||
|
|
|
||
|
|
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() or ->remove() 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 core before executing ->probe() and ->remove(). Still, it
|
||
|
|
may be desirable to suspend the device as soon as ->probe() or ->remove() has
|
||
|
|
finished, so the PM core uses pm_runtime_idle_sync() to invoke the device bus
|
||
|
|
type's ->runtime_idle() callback at that time.
|