This document describes the semantics of the DMA attributes that are
defined in linux/dma-mapping.h.
-DMA_ATTR_WRITE_BARRIER
-----------------------
-
-DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA. DMA
-to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces
-all pending DMA writes to complete, and thus provides a mechanism to
-strictly order DMA from a device across all intervening busses and
-bridges. This barrier is not specific to a particular type of
-interconnect, it applies to the system as a whole, and so its
-implementation must account for the idiosyncrasies of the system all
-the way from the DMA device to memory.
-
-As an example of a situation where DMA_ATTR_WRITE_BARRIER would be
-useful, suppose that a device does a DMA write to indicate that data is
-ready and available in memory. The DMA of the "completion indication"
-could race with data DMA. Mapping the memory used for completion
-indications with DMA_ATTR_WRITE_BARRIER would prevent the race.
-
DMA_ATTR_WEAK_ORDERING
----------------------