Observing Device Power State Changes
The I/O Kit allows a driver to observe the power state of any device in the system and to receive notifications when the device's driver changes its power state. This can be used, for example, by a driver that is not part of the power plane but needs to interface with drivers that are power managed. Alternatively, these notifications allow a driver that initiates a change to its own power state through a method such as changePowerStateToPriv() or activityTickle() to determine when the power change has completed.
To receive notifications when another driver's power state changes, your driver must register for interest in that driver's power state. The IOService superclass provides two methods for doing this: ...
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