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PM: Runtime PM documentation update 

This patch (as1318) updates the runtime PM documentation, adding a
section discussing the interaction between runtime PM and system sleep.

[rjw: Rebased and made it agree with the other updates better.]

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
This commit is contained in:
Alan Stern 2009-12-22 20:43:40 +01:00 committed by Rafael J. Wysocki
parent a6ab7aa9f4
commit f1212ae133
1 changed files with 50 additions and 0 deletions
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Documentation/power/runtime_pm.txt
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@ -381,3 +381,53 @@ 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
subsystem-level idle callback for the device at that time.
6. Run-time PM and System Sleep
Run-time PM and system sleep (i.e., system suspend and hibernation, also known
as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
ways. If a device is active when a system sleep starts, everything is
straightforward. But what should happen if the device is already suspended?
The device may have different wake-up settings for run-time PM and system sleep.
For example, remote wake-up may be enabled for run-time suspend but disallowed
for system sleep (device_may_wakeup(dev) returns 'false'). When this happens,
the subsystem-level system suspend callback is responsible for changing the
device's wake-up setting (it may leave that to the device driver's system
suspend routine). It may be necessary to resume the device and suspend it again
in order to do so. The same is true if the driver uses different power levels
or other settings for run-time suspend and system sleep.
During system resume, devices generally should be brought back to full power,
even if they were suspended before the system sleep began. There are several
reasons for this, including:
* The device might need to switch power levels, wake-up settings, etc.
* Remote wake-up events might have been lost by the firmware.
* The device's children may need the device to be at full power in order
to resume themselves.
* The driver's idea of the device state may not agree with the device's
physical state. This can happen during resume from hibernation.
* The device might need to be reset.
* Even though the device was suspended, if its usage counter was > 0 then most
likely it would need a run-time resume in the near future anyway.
* Always going back to full power is simplest.
If the device was suspended before the sleep began, then its run-time PM status
will have to be updated to reflect the actual post-system sleep status. The way
to do this is:
pm_runtime_disable(dev);
pm_runtime_set_active(dev);
pm_runtime_enable(dev);
The PM core always increments the run-time usage counter before calling the
->prepare() callback and decrements it after calling the ->complete() callback.
Hence disabling run-time PM temporarily like this will not cause any run-time
suspend callbacks to be lost.