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DMA-API.txt: fix description of pci_map_sg/dma_map_sg scatterlists handling 

- pci_map_sg/dma_map_sg are used with a scatter gather list that doesn't
  come from the block layer (e.g.  some network drivers do).

- how IOMMUs merge adjacent elements of the scatter/gather list is
  independent of how the block layer determines sees elements.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
FUJITA Tomonori 2008-12-01 13:14:01 -08:00 committed by Linus Torvalds
parent 4280e3126f
commit 1d678f365d
1 changed files with 3 additions and 5 deletions
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8
Documentation/DMA-API.txt
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@ -316,12 +316,10 @@ reduce current DMA mapping usage or delay and try again later).
pci_map_sg(struct pci_dev *hwdev, struct scatterlist *sg,
int nents, int direction)
Maps a scatter gather list from the block layer.
Returns: the number of physical segments mapped (this may be shorter
than <nents> passed in if the block layer determines that some
elements of the scatter/gather list are physically adjacent and thus
may be mapped with a single entry).
than <nents> passed in if some elements of the scatter/gather list are
physically or virtually adjacent and an IOMMU maps them with a single
entry).
Please note that the sg cannot be mapped again if it has been mapped once.
The mapping process is allowed to destroy information in the sg.