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Arthur Kepnera75b0a22008-04-29 01:00:31 -07001 DMA attributes
2 ==============
3
4This document describes the semantics of the DMA attributes that are
5defined in linux/dma-attrs.h.
6
7DMA_ATTR_WRITE_BARRIER
8----------------------
9
10DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA. DMA
11to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces
12all pending DMA writes to complete, and thus provides a mechanism to
13strictly order DMA from a device across all intervening busses and
14bridges. This barrier is not specific to a particular type of
15interconnect, it applies to the system as a whole, and so its
16implementation must account for the idiosyncracies of the system all
17the way from the DMA device to memory.
18
19As an example of a situation where DMA_ATTR_WRITE_BARRIER would be
20useful, suppose that a device does a DMA write to indicate that data is
21ready and available in memory. The DMA of the "completion indication"
22could race with data DMA. Mapping the memory used for completion
23indications with DMA_ATTR_WRITE_BARRIER would prevent the race.
24