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Writing Device Drivers
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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.

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