path: root/Documentation/DMA-attributes.txt
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authorArthur Kepner <akepner@sgi.com>2008-04-29 08:00:31 (GMT)
committerLinus Torvalds <torvalds@linux-foundation.org>2008-04-29 15:06:11 (GMT)
commita75b0a2f68d3937f96ed39525e4750601483e3b4 (patch)
tree1a84f2ef16515880f00e2db9a931299f22a07071 /Documentation/DMA-attributes.txt
parent74bc7ceebfa1c84ddd3a843ebfb56df013bf7ef5 (diff)
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>
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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.

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