Configuring Dispatch Mode for an I/O Queue


UMDF 2 is the latest version of UMDF and supersedes UMDF 1. All new UMDF drivers should be written using UMDF 2. No new features are being added to UMDF 1 and there is limited support for UMDF 1 on newer versions of Windows 10. Universal Windows drivers must use UMDF 2.

For more info, see Getting Started with UMDF.

When I/O requests from applications arrive, the framework places each request in the appropriate I/O queue. How and when the requests are delivered to the driver depend on how the driver configures dispatching for the I/O queue and on how the driver specifies callback-function synchronization. The I/O queue also interacts with the PnP and power management subsystem of UMDF to hold I/O requests in the queue until the device reaches the proper state.

Note The dispatch mode for the I/O queue is not related to the synchronization mode. The I/O queue's dispatch configuration controls the number of requests that the driver can accept for processing at any given time, while synchronization controls the simultaneous execution of event callback functions that are presenting or canceling requests. However, several modes of operation are created by combining dispatch and synchronization modes.

The driver configures dispatching for an I/O queue when the driver calls the IWDFDevice::CreateIoQueue method to configure the default queue or to create a secondary queue. The driver can specify one of the values from the WDF_IO_QUEUE_DISPATCH_TYPE enumeration type in the DispatchType parameter of IWDFDevice::CreateIoQueue to identify the dispatch mode. An I/O queue object can support the following dispatch modes:

  • Sequential

    The sequential dispatch mode is specified using the WdfIoQueueDispatchSequential value. In this dispatch mode, a queue in the processing state raises events so that a driver only processes one request at a time. The queue defers any additional requests until the driver finishes processing its current request or calls the IWDFIoRequest::ForwardToIoQueue method to requeue the request. When the current request completes or is forwarded, the queue raises an event to provide the next request.

  • Parallel

    The parallel dispatch mode is specified using the WdfIoQueueDispatchParallel value. In this dispatch mode, a queue in the processing state raises events as soon as I/O requests are ready for the driver. When the driver receives an I/O request, the driver can process the I/O request in one of the following ways:

  • Manual

    The manual dispatch mode is specified using the WdfIoQueueDispatchManual value. In this dispatch mode, the I/O queue does not automatically notify the driver when requests arrive at the queue. The driver must call the IWDFIoQueue::RetrieveNextRequest method to retrieve requests manually from the queue. This is a polling model.

    In UMDF versions 1.9 and later, if your driver is using the manual dispatch mode, it can call IWDFIoRequest2::Requeue to return an I/O request to the head of the I/O queue from which the driver obtained it. After calling IWDFIoRequest2::Requeue, the driver's next call to IWDFIoQueue::RetrieveNextRequest retrieves the requeued request.

For all dispatch modes, the I/O queue object receives and tracks the request until the driver handles the request or the request is canceled.

If the driver configures the queue for serial or parallel dispatching, the framework notifies the driver of a request through the callback functions that are registered by the driver when the driver creates the queue or configures the default queue. For more information, see I/O Queue Event Callback Functions.