Writing Device Drivers

Device Power State Transitions

A device driver can call ddi_dev_is_needed(9F) to request that a component be set to a given power level. This is necessary before using a component that has been powered off. For example, a SCSI disk target driver's read(9E) or write(9E) routine might need to spin up the disk if it had been powered off before completing the read or write. ddi_dev_is_needed(9F) notifies the Power Management framework of device state transitions.