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

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Date:	Tue, 16 Mar 2010 10:30:47 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	James Bottomley <James.Bottomley@...senPartnership.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: dma_sync_sg_for_cpu applied to a single scatterlist element

On Mon, 15 Mar 2010, James Bottomley wrote:

> On Mon, 2010-03-15 at 17:30 -0400, Alan Stern 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?
> 
> It's not the design of the API, but I'm guessing, given the way the API
> works on most arch's that it will work.  However, if you just want a
> single element sync'd, won't dma_sync_single_for_cpu do that
> transparently (as in just feed in the address and length from the sg
> list), without mucking with the sg API?

On Tue, 16 Mar 2010, FUJITA Tomonori wrote:

> 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.

What the documentation actually says about the dma_sync_* functions is:

	All the parameters must be the same as those passed into the 
	single mapping API.

So it isn't clear that dma_sync_sg_for_cpu(dev, sg, 1, dir) can be used
on a mapping created by dma_map_sg(dev, sg, n, dir), and it isn't 
clear that dma_sync_single_for_cpu() can be used on a mapping created 
by dma_map_sg().

But if you guys say it will work, I'll go ahead and use
dma_sync_single_for_cpu().

Thanks,

Alan Stern

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