Types of Host Platform DMA
The platform on which the device operates provides either 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, the transfer of a
DMA object can actually consist of a number of physically discontiguous transfers. An
example is 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, memory management units (MMU) provided by
the underlying platform translate device accesses to these virtual addresses into the
proper physical addresses. The device transfers to and from a contiguous virtual image
that can be mapped to discontiguous physical pages. Devices that operate in these
platforms do not need scatter-gather DMA capability. Typically, SPARC platforms provide virtual addresses for
direct memory transfers.