Using Packet-Based System DMA

Drivers of subordinate devices that use packet-based DMA call the following general sequence of support routines as they process an IRP requesting a DMA transfer:

• KeFlushIoBuffers just before attempting to allocate the system DMA controller (for more information, see Maintaining Cache Coherency)

• AllocateAdapterChannel when the driver is ready to program its device for DMA and needs the system DMA controller

  AllocateAdapterChannel, in turn, calls the driver's AdapterControl routine.

• MmGetMdlVirtualAddress to get an index into the MDL, required as an parameter in the initial call to MapTransfer

• MapTransfer to program the system DMA controller for the transfer operation

  A driver might need to call MapTransfer more than once to transfer all the requested data, as explained in Splitting Transfer Requests.

• FlushAdapterBuffers just after each DMA transfer operation to/from the subordinate device

  If a driver must call MapTransfer more than once to transfer all the requested data, it must call FlushAdapterBuffers as many times as it calls MapTransfer.

• FreeAdapterChannel either as soon as all the requested data has been transferred or if the driver fails the IRP because of a device I/O error

The adapter object pointer returned by IoGetDmaAdapter is a required parameter to each of these routines except KeFlushIoBuffers and MmGetMdlVirtualAddress, which require the pointer to the MDL passed at Irp->MdlAddress.

Individual drivers call this sequence of support routines at different points, depending on how each driver is implemented to service its device. For example, one driver's StartIo routine might make the call to AllocateAdapterChannel, another driver might make this call from a routine that removes IRPs from a driver-created interlocked queue, and still another driver might make this call when its subordinate DMA device indicates it is ready to transfer data.