Starting a DMA Transaction

[Applies to KMDF only]

After your driver has created and initialized a DMA transaction, the driver can call the WdfDmaTransactionExecute method to start the transaction. This method builds a scatter/gather list for the first DMA transfer that is associated with the transaction. Next, the method calls the EvtProgramDma callback function that the driver registered for the transaction. The callback function programs the DMA hardware to start the transfer.

Before your driver calls WdfDmaTransactionExecute, the driver must store the DMA transaction handle so that it can be retrieved later when the driver completes each DMA transfer that is associated with the transaction. A good place to store the transaction handle is in the context memory of a framework object, typically the device's framework device object. For more information about using object context memory, see Framework Object Context Space.

The following code example from the PLX9x5x sample shows how to initialize and then execute a DMA transaction. This code appears in the Read.c file.

VOID PLxEvtIoRead(
    IN WDFQUEUE         Queue,
    IN WDFREQUEST       Request,
    IN size_t           Length
    NTSTATUS            status = STATUS_UNSUCCESSFUL;
    // Get the DevExt from the queue handle
    devExt = PLxGetDeviceContext(WdfIoQueueGetDevice(Queue));
    do {
        // Validate the Length parameter.
        if (Length > PCI9656_SRAM_SIZE)  {
            status = STATUS_INVALID_BUFFER_SIZE;
        // Initialize the DmaTransaction.
        status = 
        if(!NT_SUCCESS(status)) {
            . . . //Error-handling code omitted
        // Execute this DmaTransaction.
        status = WdfDmaTransactionExecute( devExt->ReadDmaTransaction, 
        if(!NT_SUCCESS(status)) {
            . . . //Error-handling code omitted
        // Indicate that the DMA transaction started successfully.
        // The DPC routine will complete the request when the DMA
        // transaction is complete.
        status = STATUS_SUCCESS;
    } while (0);
    // If there are errors, clean up and complete the request.
    if (!NT_SUCCESS(status )) {
        WdfRequestComplete(Request, status);