Supporting PnP and Power Management in Function Drivers

Function drivers control the operation of a device and therefore they access device hardware. These drivers must support PnP and power management operations and typically register several event callback functions when they create device objects.

Typically, a function driver's EvtDriverDeviceAdd event callback function calls WdfDeviceInitSetPnpPowerEventCallbacks to register the following callback functions:

  • EvtDevicePrepareHardware, which delivers the device's system-assigned resources to the driver. The driver can perform operations, such as mapping the device's bus-relative memory into the processor's virtual address space, that make the hardware accessible to the driver.

  • EvtDeviceD0Entry, which performs operations, such as loading firmware, that are needed each time that the driver's device enters its working (D0) state.

  • EvtDeviceD0Exit, which performs operations that are needed each time that the driver's device leaves its working (D0) state and enters a low-power state.

  • EvtDeviceReleaseHardware, which releases any system resources that EvtDevicePrepareHardware allocated.

Like all framework-defined callback functions, the ones in the preceding list are optional. You have to supply them only if your driver needs them.

Function drivers can call WdfDeviceSetPnpCapabilities and WdfDeviceSetPowerCapabilities to report a device's PnP and power management capabilities to the operating system.

Typically, you will use the framework's power-managed I/O queues for most I/O requests. If an I/O queue is power-managed, the framework delivers requests to the driver only if its device is in its working (D0) state. For more information about power-managed I/O queues, see Power Management for I/O Queues.

Typically, the device's function driver is the power policy owner for the driver stack. The power policy owner determines the appropriate device power state for a device and sends requests to the device's driver stack whenever the device's power state should change. For framework-based drivers, the framework handles this responsibility, so you do not have to provide code in your driver to request changes in a device's power state.

The power policy owner has two additional responsibilities: it controls a device's ability to enter a low-power state when it is idle and the system remains in its working (S0) state, and it controls the device's ability to generate a wake signal when it detects an external event from a low-power state. If your device has idle or wake capabilities, your function driver can provide additional callback functions. For more information about the responsibilities of the power policy owner, see Power Policy Ownership.