Programming DMA Hardware

[Applies to KMDF only]

This topic describes the functionality that a KMDF driver for a bus-master DMA device typically provides in its EvtProgramDma event callback function. If your driver uses the framework's DMA support, the driver must provide this callback. This information also applies to a KMDF driver for a system-mode DMA device that has a hardware interrupt.

The EvtProgramDma callback function, which is called at IRQL = DISPATCH_LEVEL, programs the device to start a DMA transfer. The input parameters for this callback function supply the transfer's direction (input or output) and a scatter/gather list. If the transfer consists of a single packet, the scatter/gather list contains a single element.

The EvtProgramDma callback function programs the device by using the hardware resources that the driver's EvtDevicePrepareHardware callback function received. If the EvtProgramDma callback function successfully programs the hardware, it returns TRUE.

After the hardware has completed the DMA transfer, typically the hardware issues an interrupt and the system calls the driver's EvtInterruptIsr callback function. The driver's EvtInterruptIsr callback function usually:

  • Clears the hardware interrupt.

  • Saves the interrupt's context information if it is needed. This information might be lost after the callback function returns and the system lowers the IRQL (because lowering the IRQL allows additional interrupts to occur).

  • Calls WdfInterruptQueueDpcForIsr to schedule an EvtInterruptDpc callback function.

The EvtInterruptDpc callback function completes the DMA transfer by using context information that the EvtInterruptIsr callback function saved.

If the EvtProgramDma callback function detects an error, the driver can stop the transaction.

To stop a transaction when the driver detects an error, the EvtProgramDma callback function must:

  1. Call WdfDmaTransactionDmaCompletedFinal.

  2. Call WdfObjectDelete to delete the DMA transaction object, or call WdfDmaTransactionRelease to release and reuse the DMA transaction object.

  3. Requeue the I/O request or complete the I/O request, if the transaction is associated with a framework request object. To retrieve a handle to the request, the driver can call WdfDmaTransactionGetRequest.

  4. Return FALSE.

Steps 1 and 4 are illustrated in the following code example, taken from the PLX9x5x sample’s EvtProgramDma callback function for read requests in the Read.c file.

    // If errors occur in the EvtProgramDma callback,
    // release the DMA transaction object and complete the request.

    if (errors) {
        NTSTATUS status;

        // Must abort the transaction before deleting.
        (VOID) WdfDmaTransactionDmaCompletedFinal(Transaction, 0, &status);

        PLxReadRequestComplete( Transaction, STATUS_INVALID_DEVICE_STATE );
        TraceEvents(TRACE_LEVEL_ERROR, DBG_READ,
                    "<-- PLxEvtProgramReadDma: errors ****");
        return FALSE;

The example calls the PLxReadRequestComplete function to perform steps 2 and 3:

    IN NTSTATUS           Status

Routine Description:


Return Value:

    WDFREQUEST         request;
    size_t             bytesTransferred;

    // Get the associated request from the transaction.
    request = WdfDmaTransactionGetRequest(DmaTransaction);


    // Get the final bytes transferred count.
    bytesTransferred =  WdfDmaTransactionGetBytesTransferred( DmaTransaction );

                "PLxReadRequestComplete:  Request %p, Status %!STATUS!, "
                "bytes transferred %d\n",
                 request, Status, (int) bytesTransferred );


    // Complete this Request.
    WdfRequestCompleteWithInformation( request, Status, bytesTransferred);