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- DMA attributes
- ==============
-This document describes the semantics of the DMA attributes that are
-defined in linux/dma-attrs.h.
-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 idiosyncracies 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 specifies that reads and writes to the mapping
-may be weakly ordered, that is that reads and writes may pass each other.
-Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
-those that do not will simply ignore the attribute and exhibit default
-DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
-buffered to improve performance.
-Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
-those that do not will simply ignore the attribute and exhibit default
-DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
-consistent or non-consistent memory as it sees fit. By using this API,
-you are guaranteeing to the platform that you have all the correct and
-necessary sync points for this memory in the driver.