Run-time Power Management Framework for I/O Devices
(C) 2009 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
+(C) 2010 Alan Stern <stern@rowland.harvard.edu>
1. Introduction
to execute it, the other callbacks will not be executed for the same device.
* A request to execute ->runtime_resume() will cancel any pending or
- scheduled requests to execute the other callbacks for the same device.
+ scheduled requests to execute the other callbacks for the same device,
+ except for scheduled autosuspends.
3. Run-time PM Device Fields
defined in include/linux/pm.h:
struct timer_list suspend_timer;
- - timer used for scheduling (delayed) suspend request
+ - timer used for scheduling (delayed) suspend and autosuspend requests
unsigned long timer_expires;
- timer expiration time, in jiffies (if this is different from zero, the
interface; it may only be modified with the help of the pm_runtime_allow()
and pm_runtime_forbid() helper functions
+ unsigned int no_callbacks;
+ - indicates that the device does not use the run-time PM callbacks (see
+ Section 8); it may be modified only by the pm_runtime_no_callbacks()
+ helper function
+
+ unsigned int use_autosuspend;
+ - indicates that the device's driver supports delayed autosuspend (see
+ Section 9); it may be modified only by the
+ pm_runtime{_dont}_use_autosuspend() helper functions
+
+ unsigned int timer_autosuspends;
+ - indicates that the PM core should attempt to carry out an autosuspend
+ when the timer expires rather than a normal suspend
+
+ int autosuspend_delay;
+ - the delay time (in milliseconds) to be used for autosuspend
+
+ unsigned long last_busy;
+ - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
+ function was last called for this device; used in calculating inactivity
+ periods for autosuspend
+
All of the above fields are members of the 'power' member of 'struct device'.
4. Run-time PM Device Helper Functions
error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
to suspend the device again in future
+ int pm_runtime_autosuspend(struct device *dev);
+ - same as pm_runtime_suspend() except that the autosuspend delay is taken
+ into account; if pm_runtime_autosuspend_expiration() says the delay has
+ not yet expired then an autosuspend is scheduled for the appropriate time
+ and 0 is returned
+
int pm_runtime_resume(struct device *dev);
- execute the subsystem-level resume callback for the device; returns 0 on
success, 1 if the device's run-time PM status was already 'active' or
device (the request is represented by a work item in pm_wq); returns 0 on
success or error code if the request has not been queued up
+ int pm_request_autosuspend(struct device *dev);
+ - schedule the execution of the subsystem-level suspend callback for the
+ device when the autosuspend delay has expired; if the delay has already
+ expired then the work item is queued up immediately
+
int pm_schedule_suspend(struct device *dev, unsigned int delay);
- schedule the execution of the subsystem-level suspend callback for the
device in future, where 'delay' is the time to wait before queuing up a
- decrement the device's usage counter
int pm_runtime_put(struct device *dev);
- - decrement the device's usage counter, run pm_request_idle(dev) and return
- its result
+ - decrement the device's usage counter; if the result is 0 then run
+ pm_request_idle(dev) and return its result
+
+ int pm_runtime_put_autosuspend(struct device *dev);
+ - decrement the device's usage counter; if the result is 0 then run
+ pm_request_autosuspend(dev) and return its result
int pm_runtime_put_sync(struct device *dev);
- - decrement the device's usage counter, run pm_runtime_idle(dev) and return
- its result
+ - decrement the device's usage counter; if the result is 0 then run
+ pm_runtime_idle(dev) and return its result
+
+ int pm_runtime_put_sync_autosuspend(struct device *dev);
+ - decrement the device's usage counter; if the result is 0 then run
+ pm_runtime_autosuspend(dev) and return its result
void pm_runtime_enable(struct device *dev);
- enable the run-time PM helper functions to run the device bus type's
counter (used by the /sys/devices/.../power/control interface to
effectively prevent the device from being power managed at run time)
+ void pm_runtime_no_callbacks(struct device *dev);
+ - set the power.no_callbacks flag for the device and remove the run-time
+ PM attributes from /sys/devices/.../power (or prevent them from being
+ added when the device is registered)
+
+ void pm_runtime_mark_last_busy(struct device *dev);
+ - set the power.last_busy field to the current time
+
+ void pm_runtime_use_autosuspend(struct device *dev);
+ - set the power.use_autosuspend flag, enabling autosuspend delays
+
+ void pm_runtime_dont_use_autosuspend(struct device *dev);
+ - clear the power.use_autosuspend flag, disabling autosuspend delays
+
+ void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
+ - set the power.autosuspend_delay value to 'delay' (expressed in
+ milliseconds); if 'delay' is negative then run-time suspends are
+ prevented
+
+ unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
+ - calculate the time when the current autosuspend delay period will expire,
+ based on power.last_busy and power.autosuspend_delay; if the delay time
+ is 1000 ms or larger then the expiration time is rounded up to the
+ nearest second; returns 0 if the delay period has already expired or
+ power.use_autosuspend isn't set, otherwise returns the expiration time
+ in jiffies
+
It is safe to execute the following helper functions from interrupt context:
pm_request_idle()
+pm_request_autosuspend()
pm_schedule_suspend()
pm_request_resume()
pm_runtime_get_noresume()
pm_runtime_get()
pm_runtime_put_noidle()
pm_runtime_put()
+pm_runtime_put_autosuspend()
+pm_runtime_enable()
pm_suspend_ignore_children()
pm_runtime_set_active()
pm_runtime_set_suspended()
-pm_runtime_enable()
+pm_runtime_suspended()
+pm_runtime_mark_last_busy()
+pm_runtime_autosuspend_expiration()
5. Run-time PM Initialization, Device Probing and Removal
restore, and run-time resume, can achieve this with the help of the
UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
last argument to NULL).
+
+8. "No-Callback" Devices
+
+Some "devices" are only logical sub-devices of their parent and cannot be
+power-managed on their own. (The prototype example is a USB interface. Entire
+USB devices can go into low-power mode or send wake-up requests, but neither is
+possible for individual interfaces.) The drivers for these devices have no
+need of run-time PM callbacks; if the callbacks did exist, ->runtime_suspend()
+and ->runtime_resume() would always return 0 without doing anything else and
+->runtime_idle() would always call pm_runtime_suspend().
+
+Subsystems can tell the PM core about these devices by calling
+pm_runtime_no_callbacks(). This should be done after the device structure is
+initialized and before it is registered (although after device registration is
+also okay). The routine will set the device's power.no_callbacks flag and
+prevent the non-debugging run-time PM sysfs attributes from being created.
+
+When power.no_callbacks is set, the PM core will not invoke the
+->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
+Instead it will assume that suspends and resumes always succeed and that idle
+devices should be suspended.
+
+As a consequence, the PM core will never directly inform the device's subsystem
+or driver about run-time power changes. Instead, the driver for the device's
+parent must take responsibility for telling the device's driver when the
+parent's power state changes.
+
+9. Autosuspend, or automatically-delayed suspends
+
+Changing a device's power state isn't free; it requires both time and energy.
+A device should be put in a low-power state only when there's some reason to
+think it will remain in that state for a substantial time. A common heuristic
+says that a device which hasn't been used for a while is liable to remain
+unused; following this advice, drivers should not allow devices to be suspended
+at run-time until they have been inactive for some minimum period. Even when
+the heuristic ends up being non-optimal, it will still prevent devices from
+"bouncing" too rapidly between low-power and full-power states.
+
+The term "autosuspend" is an historical remnant. It doesn't mean that the
+device is automatically suspended (the subsystem or driver still has to call
+the appropriate PM routines); rather it means that run-time suspends will
+automatically be delayed until the desired period of inactivity has elapsed.
+
+Inactivity is determined based on the power.last_busy field. Drivers should
+call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
+typically just before calling pm_runtime_put_autosuspend(). The desired length
+of the inactivity period is a matter of policy. Subsystems can set this length
+initially by calling pm_runtime_set_autosuspend_delay(), but after device
+registration the length should be controlled by user space, using the
+/sys/devices/.../power/autosuspend_delay_ms attribute.
+
+In order to use autosuspend, subsystems or drivers must call
+pm_runtime_use_autosuspend() (preferably before registering the device), and
+thereafter they should use the various *_autosuspend() helper functions instead
+of the non-autosuspend counterparts:
+
+ Instead of: pm_runtime_suspend use: pm_runtime_autosuspend;
+ Instead of: pm_schedule_suspend use: pm_request_autosuspend;
+ Instead of: pm_runtime_put use: pm_runtime_put_autosuspend;
+ Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend.
+
+Drivers may also continue to use the non-autosuspend helper functions; they
+will behave normally, not taking the autosuspend delay into account.
+Similarly, if the power.use_autosuspend field isn't set then the autosuspend
+helper functions will behave just like the non-autosuspend counterparts.
+
+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.
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