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OS X and iOS Kernel Programming by Douglas Clarke, Ole Henry Halvorsen

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Handling Device Idle

A common reason for a driver to lower the power state of its device is to reduce its power consumption when the device hasn't been accessed for a certain period of time. This involves creating a timer for the idle period of the device. If the timer expires and the device hasn't been accessed during that period, the driver places the device into a lower power state. After the device has been placed into a lower power state, the next time that the driver needs to access the device, it will need to place the hardware back in a usable state. Because these operations are common for all drivers that perform an idle power saving mode, this functionality is built into the I/O Kit and provided to driver developers through the IOService ...

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