Types of Host Platform DMA

The platform that the device operates on may provide one of two types of memory access: direct memory access (DMA) or direct virtual memory access (DVMA).

On platforms that support DMA, the system provides the device with a physical address in order to perform transfers. In this case, one logical transfer may actually consist of a number of physically discontiguous transfers. An example of this occurs when an application transfers a buffer that spans several contiguous virtual pages that map to physically discontiguous pages. To deal with the discontiguous memory, devices for these platforms usually have some kind of scatter-gather DMA capability. Typically, x86 systems provide physical addresses for direct memory transfers.

On platforms that support DVMA, the system provides the device with a virtual address to perform transfers. In this case, the underlying platform provides some form of memory management unit (MMU) that translates device accesses to these virtual addresses into the proper physical addresses. The device transfers to and from a contiguous virtual image that may be mapped to discontiguous physical pages. Devices that operate in these platforms don't need scatter-gather DMA capability. Typically, the system that supports SPARC platforms provides virtual addresses for direct memory transfers.