Power Management Components

A device is power manageable if the power consumption of the device can be reduced when it is idle. Conceptually, a power manageable device consists of a number of power-manageable hardware units called components.

The device driver notifies the system of the existence of device components and the power levels that they support by creating a pm-components(9P) property in its attach(9E) entry point as part of driver initialization.

Most devices that are power manageable implement only a single component. An example of a single-component, power-manageable device is a disk whose spindle motor can be stopped to save power when the disk is idle.

If a device has multiple power-manageable units that are separately controllable, it should implement multiple components.

An example of a two-component, power-manageable device is a frame buffer card with a monitor connected to it. Frame buffer electronics is the first component [component 0]. Its power consumption can be reduced when not in use. The monitor is the second component [component 1], which can also enter a lower power mode when not in use. The combination of frame buffer electronics and monitor is considered by the system as one device with two components.

Multiple Power Management Components

To the power management framework, all components are considered equal and completely independent of each other. If this is not true for a particular device, the device driver must ensure that undesirable state combinations do not occur. For example, with a frame buffer/monitor card with a monitor attached to it, for each possible power state of the monitor (On, Standby, Suspend, Off) there are states of the frame buffer electronics (D0, D1, D2, D3) that are not allowed if the device is to work properly. If the monitor is On, then the frame buffer must be at D0 (full on), so if the frame buffer driver gets a request to power up the monitor to On while the frame buffer is D3, it must ask the system to bring the frame buffer back up (by calling pm_raise_power(9F)) before setting the monitor On. If the frame buffer driver gets a request from the system to lower the power of the frame buffer while the monitor is On, it must fail that request.