Power States :- A power state indicates the level of power consumption—and thus the extent of computing activity—by the system or by a single device. The power manager sets the power state of the system as a whole. Device drivers set the power state of their individual devices.
The ACPI specification defines two sets of discrete power states: system power states and device power states. System power states are named Sx, where x is a state number between 0 and 5. Device power states are named Dx, where x is a state number between 0 and 3. The state number is inversely related to power consumption: higher numbered states use less power. States S0 and D0 are the highest-powered, most functional, fully on states. States S5 and D3 are the lowest-powered states and have the longest wake-up latency.
A component, or subdevice, is a functional hardware unit in a device that can be turned on or switched to a low-power state independently of the other components in the same device. For example, an audio device might have separate components for playback and recording whose power states can be managed independently of each other.
Power management framework (PoFx) supports power and clock management at the component (or subdevice) level. A device driver registers with PoFx to independently manage power usage in the individual components in a device. PoFx provides the fine-grained control necessary to extend the time that a Windows portable computer, tablet, phone, or other mobile device can run on a battery charge.
A KMDF driver can define multiple logical components within a single device, each of which can be independently power managed. For example, a function driver might define a unique set of functional power states for each logical component of a device. Similar to device and system power states, F0 indicates that the component is fully on, while optional states F1, F2, and so on indicate progressively lower power states.
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