A device power state describes the power state of a device in a computer, independently of the other devices in the computer. Device power states are named D0, D1, D2, and D3. D0 is the fully on state, and D1, D2, and D3 are low-power states. The state number is inversely related to power consumption: higher numbered states use less power. The D3 state is divided into two substates, D3hot and D3cold.
Device power states are characterized by the following attributes:
- Power consumption: How much power does the device use?
- Device context: How much of its operational context does the device retain in this state?
- Device driver behavior: What must the drivers for the device do to restore the device to the fully operational state?
- Restore time: How long does it take to restore the device to the fully operational state? Most types of devices have modest restore times that differ little from one device class to the next. Only a few types of devices, such as GPUs, have very large hardware contexts that take significantly longer to restore.
- Wake-up capability: Can the device request wake-up from this state? In general, if a device can request wake-up from a given power state (for example, D2), it can also request wake-up from any higher-powered state (D1).
The exact definitions of the power states are device-specific. Not all devices define all the states; many devices define only the D0 and D3 states. The power state of a device need not match the system power state. For example, some devices can be in the off (D3) state even though the system is in the system working state (S0).
Like the system, a device can transition from the working state (D0) to any low-power state (D1, D2, or D3) and from any low-power state to the working state. The following diagram is a state graph that shows the valid device power state transitions.
Device power state transition
Source : Device Power States