linux-kernel - Re: dma_sync_sg_for_cpu applied to a single scatterlist element

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Message-Id: <20100316081620R.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 16 Mar 2010 08:16:51 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	stern@...land.harvard.edu
Cc:	James.Bottomley@...senPartnership.com, linux-kernel@...r.kernel.org
Subject: Re: dma_sync_sg_for_cpu applied to a single scatterlist element

On Mon, 15 Mar 2010 17:30:19 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> This is addressed to James Bottomley as he is the author of
> Documentation/DMA-API.txt, but anyone else who can contribute is
> invited to do so.
> 
> Suppose a scatter-gather transfer with multiple scatterlist elements
> has been mapped via dma_map_sg().  Is it then valid to call 
> dma_sync_sg_for_cpu() with the "sg" argument pointing to one of the 
> mapped scatterlist elements (not necessarily the first one) and the 
> "nelems" argument set to 1?

As James said, probably it works. As long as passed scatterlist
elements points to mapped regions, it works.

However, I think that the latest DMA-API.txt makes it clear that only
dma_sync_single_for_{cpu|device} supports a partial sync. So it's not
recommended, I guess.
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