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Starting a DMA Transfer

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Note  The NetDMA interface is not supported in Windows 8 and later.

The NetDMA interface calls a NetDMA provider driver's ProviderStartDma function to start a dynamic memory access (DMA) transfer. The NetDMA interface can call ProviderStartDma at any time after a DMA channel is allocated. The NetDMA interface must call ProviderStartDma for the first DMA transfer after a channel reset or stop, or after the DMA channel is first allocated.

To perform the start operation, the NetDMA provider must disregard the remaining unprocessed DMA descriptor list, if any, after it completes the processing of the current descriptor. That is, the NetDMA provider must ignore the NextDescriptor member in the current NET_DMA_DESCRIPTOR structure and load the descriptor that ProviderStartDma specifies.

The NetDMA interface supplies a NET_DMA_DESCRIPTOR structure in the DescriptorVirtualAddress parameter of ProviderStartDma. The physical address of the descriptor is passed at the DescriptorPhysicalAddress parameter. The NET_DMA_DESCRIPTOR structure contains:

  • The size of the DMA transfer that is associated with the current descriptor.

  • The physical addresses of the source and destination locations for the DMA transfer.

  • The physical address of the next DMA descriptor.

  • Reserved fields that provide space for vendor-defined information.

  • User context members that are reserved for the NetDMA interface.

The source of the DMA transfer is a linked list of DMA descriptors. The NextDescriptor member of a NET_DMA_DESCRIPTOR structure contains the physical address of the next NET_DMA_DESCRIPTOR structure in the linked list. The DescriptorCount parameter of ProviderStartDma contains the number of DMA descriptors that are in the linked list. The DescriptorCount parameter is provided for additional information, and the DMA engine should use the linked list to access all of the DMA descriptors.

After the initial DMA transfer is started, the NetDMA interface can call the ProviderAppendDma function to append additional descriptors to the transfer. For more information about appending descriptors to a DMA transfer, see Appending DMA Descriptors to a DMA Channel.