kernel-ark/Documentation/DMA-attributes.txt
Arthur Kepner a75b0a2f68 dma: document dma_*map*_attrs() interfaces
Document the new dma_*map*_attrs() functions.

[markn@au1.ibm.com: fix up for dma-add-dma_map_attrs-interfaces and update docs]
Signed-off-by: Arthur Kepner <akepner@sgi.com>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Jes Sorensen <jes@sgi.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Roland Dreier <rdreier@cisco.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Grant Grundler <grundler@parisc-linux.org>
Cc: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-04-29 08:06:11 -07:00

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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
----------------------
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.