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