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

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Message-Id: <20100317072827F.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 17 Mar 2010 07:28:43 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	stern@...land.harvard.edu
Cc:	James.Bottomley@...senPartnership.com,
	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org
Subject: Re: dma_sync_sg_for_cpu applied to a single scatterlist element

On Tue, 16 Mar 2010 10:30:47 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> 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.

It's true to dma_sync_sg_for_*. You can see there:

With the sync_single API, you can use dma_handle and size parameters
that aren't identical to those passed into the single mapping API to
do a partial sync.

dma_sync_single_for_* can do a partial sync but dma_sync_sg_for_*
doesn't support a partial sync.


> 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),

You should not do (though it might work).

> and it isn't 
> clear that dma_sync_single_for_cpu() can be used on a mapping created 
> by dma_map_sg().

You should not do (though it might work).


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

Well, it's undocumented. It might work but might not.
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