Some drivers like Wdm1 (and in fact, Wdm2) can happily ignore all Power IRPs. All they have to do is pass all Power IRPs down the stack. Wdm1's Wdm1 Power dispatch routine shown in Listing 10.1 does just this. Note the use of PoCallDriver rather than IoCallDriver. If you do not process Power IRPs, please include a default power handler like Wdm1 Power so that these IRPs reach lower drivers.

Drivers for removable devices should check to see if its media is present; if not, it should call PoStartNextPowerIrp and complete the Power IRP straightaway with a status of STATUS_DELETE_PENDING.

Listing 10.1 Passing on all Power IRPs

NTSTATUS Wdm1Power(IN PDEVICE_OBJECT fdo, IN PIRP Irp) {

 DebugPrint("Power %I", Irp);

 PWDM1_DEVICE_EXTENSION dx = (PWDM1_DEVICE_EXTENSION)fdo->DeviceExtension;

 // Just pass to lower driver

 PoStartNextPowerIrp(Irp);

 IoSkipCurrentIrpStackLocation(Irp);

 return PoCallDriver(dx->NextStackDevice, Irp);

}

Noting Device Power State Changes

Suppose a function driver does not handle Power IRPs, but its bus driver does. In this case, the function driver might want to know when its device is powered down and take some appropriate action. Unfortunately, there is no simple call to discover the current device power state. Instead, it must remember the last device power state from a Set Power IRP.

The Power IRP handler can easily be enhanced to note any changes of power state if the minor function code is IRP_MN_SET_POWER. The IRP stack Parameters.Power.Type field is either SystemPowerState or DevicePowerState. The Parameters.Power.State field gives the system or device power state.

The following code snippet shows how to check for a Set Device Power IRP and store the new device power state in a DEVICE_POWER_STATE PowerState field in the device extension.

PIO_STACK_LOCATION IrpStack = IoGetCurrentIrpStackLocation(Irp);

POWER_STATE_TYPE PowerType = IrpStack->Parameters.Power.Type;

POWER_STATE PowerState = IrpStack->Parameters.Power.State;

if (IrpStack->MinorFunction==IRP_MN_SET_POWER && PowerType==DevicePowerState) dx->PowerState = PowerState.DeviceState;