893 lines
42 KiB
Plaintext
893 lines
42 KiB
Plaintext
Runtime Power Management Framework for I/O Devices
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(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
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(C) 2010 Alan Stern <stern@rowland.harvard.edu>
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1. Introduction
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Support for runtime power management (runtime 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 runtime 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 runtime 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 runtime PM operations with one another.
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* Three device runtime 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 runtime 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 runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
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fields of 'struct dev_pm_info' and the core helper functions provided for
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runtime PM are described below.
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2. Device Runtime PM Callbacks
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There are three device runtime 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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int (*runtime_idle)(struct device *dev);
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...
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};
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The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks
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are executed by the PM core for the device's subsystem that may be either of
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the following:
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1. PM domain of the device, if the device's PM domain object, dev->pm_domain,
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is present.
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2. Device type of the device, if both dev->type and dev->type->pm are present.
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3. Device class of the device, if both dev->class and dev->class->pm are
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present.
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4. Bus type of the device, if both dev->bus and dev->bus->pm are present.
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If the subsystem chosen by applying the above rules doesn't provide the relevant
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callback, the PM core will invoke the corresponding driver callback stored in
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dev->driver->pm directly (if present).
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The PM core always checks which callback to use in the order given above, so the
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priority order of callbacks from high to low is: PM domain, device type, class
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and bus type. Moreover, the high-priority one will always take precedence over
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a low-priority one. The PM domain, bus type, device type and class callbacks
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are referred to as subsystem-level callbacks in what follows.
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By default, the callbacks are always invoked in process context with interrupts
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enabled. However, the pm_runtime_irq_safe() helper function can be used to tell
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the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
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and ->runtime_idle() callbacks for the given device in atomic context with
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interrupts disabled. This implies that the callback routines in question must
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not block or sleep, but it also means that the synchronous helper functions
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listed at the end of Section 4 may be used for that device within an interrupt
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handler or generally in an atomic context.
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The subsystem-level suspend callback, if present, is _entirely_ _responsible_
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for handling the suspend of 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 subsystem-level suspend callback
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knows what to do to handle the device).
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* Once the subsystem-level suspend callback (or the driver suspend callback,
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if invoked directly) has completed successfully for the given device, the PM
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core regards the device as suspended, which need not mean that it has been
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put into a low power state. It is supposed to mean, however, that the
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device will not process data and will not communicate with the CPU(s) and
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RAM until the appropriate resume callback is executed for it. The runtime
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PM status of a device after successful execution of the suspend callback is
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'suspended'.
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* If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM
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status remains 'active', which means that the device _must_ be fully
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operational afterwards.
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* If the suspend callback returns an error code different from -EBUSY and
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-EAGAIN, the PM core regards this as a fatal error and will refuse to run
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the helper functions described in Section 4 for the device until its status
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is directly set to either'active', or 'suspended' (the PM core provides
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special helper functions for this purpose).
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In particular, if the driver requires remote wakeup capability (i.e. hardware
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mechanism allowing the device to request a change of its power state, such as
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PCI PME) for proper functioning and device_run_wake() returns 'false' for the
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device, then ->runtime_suspend() should return -EBUSY. On the other hand, if
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device_run_wake() returns 'true' for the device and the device is put into a
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low-power state during the execution of the suspend callback, it is expected
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that remote wakeup will be enabled for the device. Generally, remote wakeup
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should be enabled for all input devices put into low-power states at run time.
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The subsystem-level resume callback, if present, is _entirely_ _responsible_ for
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handling the resume of 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 subsystem-level resume callback knows
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what to do to handle the device).
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* Once the subsystem-level resume callback (or the driver resume callback, if
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invoked directly) has completed successfully, the PM core regards the device
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as fully operational, which means that the device _must_ be able to complete
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I/O operations as needed. The runtime PM status of the device is then
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'active'.
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* If the resume callback returns an error code, the PM core regards this as a
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fatal error and will refuse to run the helper functions described in Section
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4 for the device, until its status is directly set to either 'active', or
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'suspended' (by means of special helper functions provided by the PM core
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for this purpose).
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The idle callback (a subsystem-level one, if present, or the driver one) is
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executed by the PM core whenever the device appears to be idle, which is
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indicated to the PM core by two counters, the device's usage counter and the
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counter of 'active' 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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idle callback with the device as its argument.
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The action performed by the idle callback is totally dependent on the subsystem
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(or driver) in question, but the expected and recommended action is to check
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if the device can be suspended (i.e. if all of the conditions necessary for
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suspending the device are satisfied) and to queue up a suspend request for the
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device in that case. If there is no idle callback, or if the callback returns
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0, then the PM core will attempt to carry out a runtime suspend of the device,
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also respecting devices configured for autosuspend. In essence this means a
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call to pm_runtime_autosuspend() (do note that drivers needs to update the
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device last busy mark, pm_runtime_mark_last_busy(), to control the delay under
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this circumstance). To prevent this (for example, if the callback routine has
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started a delayed suspend), the routine must return a non-zero value. Negative
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error return codes are ignored by the PM core.
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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 runtime PM callbacks for
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one device:
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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 runtime 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 runtime
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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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except for scheduled autosuspends.
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3. Runtime PM Device Fields
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The following device runtime 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 and autosuspend requests
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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. runtime 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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unsigned int run_wake;
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- set if the device is capable of generating runtime wake-up events
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enum rpm_status runtime_status;
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- the runtime 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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unsigned int runtime_auto;
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- if set, indicates that the user space has allowed the device driver to
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power manage the device at run time via the /sys/devices/.../power/control
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interface; it may only be modified with the help of the pm_runtime_allow()
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and pm_runtime_forbid() helper functions
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unsigned int no_callbacks;
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- indicates that the device does not use the runtime PM callbacks (see
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Section 8); it may be modified only by the pm_runtime_no_callbacks()
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helper function
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unsigned int irq_safe;
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- indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
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will be invoked with the spinlock held and interrupts disabled
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unsigned int use_autosuspend;
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- indicates that the device's driver supports delayed autosuspend (see
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Section 9); it may be modified only by the
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pm_runtime{_dont}_use_autosuspend() helper functions
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unsigned int timer_autosuspends;
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- indicates that the PM core should attempt to carry out an autosuspend
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when the timer expires rather than a normal suspend
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int autosuspend_delay;
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- the delay time (in milliseconds) to be used for autosuspend
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unsigned long last_busy;
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- the time (in jiffies) when the pm_runtime_mark_last_busy() helper
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function was last called for this device; used in calculating inactivity
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periods for autosuspend
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All of the above fields are members of the 'power' member of 'struct device'.
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4. Runtime PM Device Helper Functions
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The following runtime 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 runtime 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 runtime 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 the subsystem-level idle callback for the device; returns an
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error code on failure, where -EINPROGRESS means that ->runtime_idle() is
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already being executed; if there is no callback or the callback returns 0
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then run pm_runtime_autosuspend(dev) and return its result
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int pm_runtime_suspend(struct device *dev);
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- execute the subsystem-level suspend callback for the device; returns 0 on
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success, 1 if the device's runtime 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 and -EACCES means that
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'power.disable_depth' is different from 0
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int pm_runtime_autosuspend(struct device *dev);
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- same as pm_runtime_suspend() except that the autosuspend delay is taken
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into account; if pm_runtime_autosuspend_expiration() says the delay has
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not yet expired then an autosuspend is scheduled for the appropriate time
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and 0 is returned
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int pm_runtime_resume(struct device *dev);
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- execute the subsystem-level resume callback for the device; returns 0 on
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success, 1 if the device's runtime 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, and -EACCES means that 'power.disable_depth' is
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different from 0
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int pm_request_idle(struct device *dev);
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- submit a request to execute the subsystem-level idle callback for the
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device (the request is represented by a work item in pm_wq); returns 0 on
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success or error code if the request has not been queued up
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int pm_request_autosuspend(struct device *dev);
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- schedule the execution of the subsystem-level suspend callback for the
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device when the autosuspend delay has expired; if the delay has already
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expired then the work item is queued up immediately
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int pm_schedule_suspend(struct device *dev, unsigned int delay);
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- schedule the execution of the subsystem-level suspend callback for the
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device in future, where 'delay' is the time to wait before queuing up a
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suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
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item is queued up immediately); returns 0 on success, 1 if the device's PM
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runtime 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 the subsystem-level resume callback for the
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device (the request is represented by a work item in pm_wq); returns 0 on
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success, 1 if the device's runtime 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; if the result is 0 then run
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pm_request_idle(dev) and return its result
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int pm_runtime_put_autosuspend(struct device *dev);
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- decrement the device's usage counter; if the result is 0 then run
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pm_request_autosuspend(dev) and return its result
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int pm_runtime_put_sync(struct device *dev);
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- decrement the device's usage counter; if the result is 0 then run
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pm_runtime_idle(dev) and return its result
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int pm_runtime_put_sync_suspend(struct device *dev);
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- decrement the device's usage counter; if the result is 0 then run
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pm_runtime_suspend(dev) and return its result
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int pm_runtime_put_sync_autosuspend(struct device *dev);
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- decrement the device's usage counter; if the result is 0 then run
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pm_runtime_autosuspend(dev) and return its result
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void pm_runtime_enable(struct device *dev);
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- decrement the device's 'power.disable_depth' field; if that field is equal
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to zero, the runtime PM helper functions can execute subsystem-level
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callbacks described in Section 2 for the device
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int pm_runtime_disable(struct device *dev);
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- increment the device's 'power.disable_depth' field (if the value of that
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field was previously zero, this prevents subsystem-level runtime PM
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callbacks from being run for the device), make sure that all of the pending
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runtime PM operations on the device are either completed or canceled;
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returns 1 if there was a resume request pending and it was necessary to
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execute the subsystem-level resume callback for the device to satisfy that
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request, otherwise 0 is returned
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int pm_runtime_barrier(struct device *dev);
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- check if there's a resume request pending for the device and resume it
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(synchronously) in that case, cancel any other pending runtime PM requests
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regarding it and wait for all runtime PM operations on it in progress to
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complete; returns 1 if there was a resume request pending and it was
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necessary to execute the subsystem-level resume callback for the device to
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satisfy that request, 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 runtime
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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 runtime
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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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bool pm_runtime_active(struct device *dev);
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- return true if the device's runtime PM status is 'active' or its
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'power.disable_depth' field is not equal to zero, or false otherwise
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bool pm_runtime_suspended(struct device *dev);
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- 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
|
|
callbacks to be invoked with interrupts off
|
|
|
|
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_idle()
|
|
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().
|
|
|
|
It may be desirable to suspend the device once ->probe() has finished.
|
|
Therefore the driver core uses the asyncronous pm_request_idle() to submit a
|
|
request to execute the subsystem-level idle callback for the device at that
|
|
time. A driver that makes use of the runtime autosuspend feature, may want to
|
|
update the last busy mark before returning from ->probe().
|
|
|
|
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() with 'false' as the second argument for every device
|
|
right before executing the subsystem-level .suspend_late() callback for it.
|
|
|
|
* During system resume it calls pm_runtime_enable() and pm_runtime_put()
|
|
for every device right after executing the subsystem-level .resume_early()
|
|
callback 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_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.
|
|
|
|
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.
|