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Using WinUSB in a WBDI Driver

Last Updated: 1/24/2017

Microsoft recommends that WBDI drivers use the USB I/O target that is built into User-Mode Driver Framework (UMDF).

Setting UmdfDispatcher

An INF file that installs a UMDF driver must contain a WDF-specific DDInstall section. If you use the USB I/O target in UMDF, you must set the UmdfDispatcher registry directive within this DDInstall section.

The following section from WudfBioUsbSample.inx in the WudfBioUsbSample sample shows how to set this directive:

KmdfService=WINUSB, WinUsb_Install
UmdfService=WudfBioUsbSample, WudfBioUsbSample_Install

For specific information about UmdfDispatcher, see Specifying the UmdfDispatcher INF Directive. For general information about WDF registry directives, see Specifying WDF Directives.

Pending Asynchronous Read Requests

WinUsb can handle multiple outstanding read requests. Devices that require minimal latency between read operations during a scan should keep some number of outstanding asynchronous read requests pending. If the driver makes asynchronous requests, WinUsb issues these requests before the transfer back to user mode for the completion routines of earlier read requests.

You can refer to the CBiometricDevice::InitiatePendingRead method in Device.cpp in WudfBioUsbSample to see a code example of how to pend a read request.

The code to pend a read request should be a loop of the following steps:

  1. Create a pre-allocated framework memory object by calling IWDFDriver::CreatePreallocatedWdfMemory.

  2. Provide callback code in an OnCompletion routine. See CBiometricDevice::OnCompletion in the sample.

  3. Acquire a pointer to the IRequestCallbackRequestCompletion interface of the owning object.

  4. Register callback function by calling IWDFIoRequest::SetCompletionCallback and passing in the pointer to IRequestCallbackRequestCompletion that was obtained in the previous step. The framework will now call the callback when an I/O request completes.

  5. Call IWDFIoRequest::Send to send the read request to the device.

  6. Process read request when callback completion occurs. Before the OnCompletion routine initiates a new pending read request, it should check the state of the I/O target. To do this, query IWDFUsbTargetPipe for a pointer to its IWDFIoTargetStateManagement interface. Then call IWDFIoTargetStateManagement::GetState:

    IWDFIoTarget * pTarget
    IWDFIoTargetStateManagement * pStateMgmt = NULL;
    HRESULT hrQI = pTarget->QueryInterface(IID_PPV_ARGS(&pStateMgmt));
    state = pStateMgmt->GetState();

When the scan is complete, cancel any pending read requests.

If you use the UMDF-USB target, you can allow read requests to remain pending across power-down and power-up.

If you do not use the UMDF-USB target, the driver should stop sending pending read requests at D0Exit and restart at D0Entry.

Selective Suspend

A WBDI driver should support USB Selective Suspend.

A device that supports system wake and device idle should enable the registry settings for selective suspend in WinUsb, as shown in this code example from WudfBioUsbSample.inx:


The operating system USB stack cannot guarantee the timing between system wake and when the driver can start reading from the device.

Ideally, the device should be left in a state ready to capture a scan when the system is suspended. If a scan occurs while the system is suspended, the device should cache the input data for an entire fingerprint scan. When the system wakes up, the driver then reads in the data from the device. By supporting this scenario, you can enable system wake and unlock/login scenarios.

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