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Using Single Transfer DMA

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By default, WDF sometimes splits a single DMA transaction into multiple DMA transfers. However, some devices cannot handle a fragmented transaction and must instead receive all data in a single DMA operation. For example, some PCI network controllers require one network packet at a time because they do not have the hardware to cache and reassemble partial data.

Starting in KMDF version 1.19, a KMDF driver using DMA v3 can specify that it requires single transfer DMA transactions. The driver can specify single transfer for a single DMA transaction only, or it can specify single transfer for all DMA transactions created using a specified DMA enabler.

Setting single transfer for a specific DMA transaction

To set single transfer for a single transaction, use the following sequence:

  1. Call WdfDmaTransactionCreate or WdfDmaTransactionRelease.
  2. Call WdfDmaTransactionSetSingleTransferRequirement.
  3. Call WdfDmaTransactionInitialize.
    If initialization fails due to transaction fragmentation, a driver can fail the I/O request or it can rearrange the transaction's memory buffers and reinitialize the transaction.
  4. Call WdfDmaTransactionExecute.

When debugging your driver, you can use the !wdfkd.wdfdmatransaction extension to determine if single transfer is set for a given transaction object.

Setting the single-transfer requirement for all DMA transactions created with a particular DMA enabler

To set single transfer for all transactions created with a given enabler, specify the WDF_DMA_ENABLER_CONFIG_REQUIRE_SINGLE_TRANSFER flag in WDF_DMA_ENABLER_CONFIG_FLAGS when calling WdfDmaEnablerCreate.

A driver that uses this flag does not need to call WdfDmaTransactionSetSingleTransferRequirement each time it creates or reuses a transaction object.

This setting also persists if the driver reuses the transaction object.

When debugging, use the !wdfkd.wdfdmaenabler extension to determine if single transfer is set for a given DMA enabler object.

For information about the order in which WDF calls your driver's DMA event callback functions, see Handling I/O Requests in a KMDF Driver for a Bus-Master DMA Device.