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>

Documentation/power/runtime_pm.txt

@@ -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.