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Completing a DMA Transfer

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[Applies to KMDF only]

Typically, your driver's EvtInterruptDpc callback function completes the processing of each DMA transfer.

First, because multiple DMA transactions can be in progress concurrently, the EvtInterruptDpc callback function must determine which DMA transaction the completed transfer is associated with. The callback function can do this by retrieving the transaction handle that the driver stored when it started the DMA transaction. To retrieve the device extension, the PLX9x5x sample defines a function called PLxGetDeviceContext in its Private.h header file:

WDF_DECLARE_CONTEXT_TYPE_WITH_NAME(DEVICE_EXTENSION, PLxGetDeviceContext)

Then, in the driver's EvtInterruptDpc callback, it does the following:

WDFDMATRANSACTION   dmaTransaction;
PDEVICE_EXTENSION   devExt;
...
devExt  = PLxGetDeviceContext(WdfInterruptGetDevice(Interrupt));
...
dmaTransaction = devExt->WriteDmaTransaction;

Next, the EvtInterruptDpc callback function must inform the framework that a transfer is complete, by calling one of the following transfer completion methods:

  • WdfDmaTransactionDmaCompleted, if the transfer completed successfully and the hardware does not report a count of transferred bytes.

  • WdfDmaTransactionDmaCompletedWithLength, if the transfer completed successfully and the hardware reports a count of transferred bytes (or a count of bytes not transferred), or if the driver detected an error and specifies a transfer count of zero to retry the transfer. If the driver specifies a transfer count of zero, the framework subtracts zero from the number of bytes that remain and thus sends the same transfer to the EvtProgramDma callback function.

  • WdfDmaTransactionDmaCompletedFinal, if the hardware reports an underrun or failure condition.

Your driver can call WdfDmaTransactionGetCurrentDmaTransferLength to obtain the original length of the completed transfer. This call is useful if your device reports a count of bytes that were not transferred, because the driver can subtract the number of non-transferred bytes from the original transfer length and then call WdfDmaTransactionGetCurrentDmaTransferLength to report the actual transfer size.

Each of the preceding transfer completion methods informs the framework that a single DMA transfer (not the entire DMA transaction) is complete. After your driver calls one of these methods, the driver checks the method's return value to see if the transaction requires more transfers:

  • If the completion method's return value is FALSE, the framework has determined that additional DMA transfers are required to finish processing the DMA transaction.

    Typically, the driver's EvtInterruptDpc callback function just returns. The framework calls the driver's EvtProgramDma callback function again, and the callback function can program the hardware for the next transfer.

    The transfer completion methods provide a status value, which is always STATUS_MORE_PROCESSING_REQUIRED in this case.

  • If the return value is TRUE, no more transfers will occur for the DMA transaction.

    The transfer completion methods provide a status value. If the status value is STATUS_SUCCESS, all transfers for the DMA transaction are complete and the driver must complete the DMA transaction. If the status value is anything else, an error occurred and the DMA transaction might not have been completed.

If the EvtInterruptDpc callback function detects an error, typically due to a timer expiring or a hardware interrupt signaling a transfer error, the driver can restart the transaction's current transfer.

To restart the transaction's current transfer, the driver's EvtInterruptDpc callback function can call WdfDmaTransactionDmaCompletedWithLength with the TransferredLength parameter set to zero.