Table 10.2 Device power states
| State | Description | Enum name |
|---|---|---|
| D0 | Fully working | PowerDeviceD0(1) |
| D1 | Sleeping | PowerDeviceD1(2) |
| D2 | Sleeping | PowerDeviceD2(3) |
| D3 | Off | PowerDeviceD3(4) |
Both the state values increase from 1 the sleepier they are.
A device need not support all the device states. DO for on and D3 for off are a basic minimum. A device is not limited in its state transitions, so a change from D1 to D3 is possible.
Later, I describe how each device (or its bus driver) provides a table for device states for each system state.
Power IRPs
A driver's Power Management routines revolve around the Power IRP, IRP_MJ_POWER, by both handling it and generating it when necessary. There are four minor function code variants of the Power IRP shown in Table 10.3.
Table 10.3 IRP_MN_POWER minor function codes
| Minor function code | Description |
|---|---|
IRP_MN_SET_POWER | Set system or device power state |
IRP_MN_QUERY_POWER | Ask if a system or device state change is OK |
IRP_MN_WAIT_WAKE | Wake computer in response to an external event |
IRP_MN_POWER_SEQUENCE | Send this IRP to determine whether your device actuallyentered a specific power state |
The Power Manager maintains a separate internal queue of Power IRPs. This ensures that there is only one Set System Power IRP being processed in the system. It also ensures that there is only one Set Device Power IRP running for each device. These rules ensure that power transitions are handled smoothly. As each device is powered up, it may require an inrush of energy. Handling Power IRPs one at a time ensures that the peak power required is kept as low as possible.
As the Power Manager has its own queue of IRPs, do not use the standard I/O Manager routines when handling IRPs. Use
You must tell the Power Manager when you have finished processing a power IRP so that it can start the next. If you are simply passing an IRP down the stack (with no completion routine) then you should call the
A Query Power IRP request is sent to see if a state change is acceptable to all the drivers in the device stack. A Set Power IRP is then issued to actually make the change.
You can send yourself a Set Device Power State IRP at any time. However, only the Power Manager can send Set System Power State IRPs. To process a Set System Power State IRP, you must send yourself a Set Device Power State IRP. That's right — you must ask for a Power IRP to be sent
Handling Device Power IRPs
Handling a Set Device Power State is relatively straightforward. You must power down your device before all the lower drivers. Conversely, power your device up after all the lower drivers have powered up. This means setting a completion routine and doing the power up there.
Handling System Power IRPs
Handling a Set System Power State is a different kettle of fish. If you receive a Set System Power IRP, you must first determine the equivalent device power state. The Wdm2 driver is put in the fully on device state D0 for the fully on system state S0. For all other system states, the Wdm2 device state is fully off, D3.
If the current device state is not the same as the required device state, you must act to change it. As before, powering down requires that you change the device state before you pass the Set System Power State down to the lower drivers. Power up after all the lower drivers have had their say.
You must change device power state by issuing a Set Device Power State IRP to yourself. You must then wait for this IRP to complete. You can then continue processing the Set system state IRP.
Same System Power
Suppose you receive a Set System Power State IRP. You must work out the corresponding device power state. If your device is already at this power level, then all you need to do is pass the IRP to the lower drivers.
System Powering Down
If you decide that your device needs to power down, then you must send yourself a Set Device Power IRP before you pass down the system IRP. Figure 10.2 illustrates this scenario. It assumes that the first Query Set System Power State IRP has completed OK, at (1).
In the figure, the Set System State IRP handler decides that it must power its device down at (2). It sends a Set Device State IRP to itself, and sets a completion routine so that it knows when this second IRP has been completed.
In due course, your driver will receive the Set Device State IRP (that you sent yourself) at (3). Your driver will decide again that it must power down. It does this at (4) and then sends the IRP down to the lower drivers at (5).
When the Set Device State IRP has been processed by all the lower drivers, the completion routine is called at (6). This signals to the original IRP handler that it can continue, at © again. All this has to do is pass the IRP down to the lower drivers at (7).
Figure 10.2 Power down system processing
