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

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Message-Id: <20100318104619S.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 18 Mar 2010 10:46:37 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	stern@...land.harvard.edu
Cc:	fujita.tomonori@....ntt.co.jp,
	James.Bottomley@...senPartnership.com, linux-kernel@...r.kernel.org
Subject: Re: dma_sync_sg_for_cpu applied to a single scatterlist element

On Wed, 17 Mar 2010 11:22:01 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> Here's the situation.  The USB controller drivers don't all support
> scatter-gather operation.  So there's a library routine in the USB core
> which calls dma_map_sg() and then creates a separate I/O request for
> each scatterlist element.  The driver can process these requests one at
> a time, and when they are all finished the library routine calls
> dma_unmap_sg().
> 
> However...  For tracing purposes (usbmon -- like tcpdump but for USB),
> we may need to copy the data from each I/O request's transfer buffer.  
> Unfortunately, this copying is done as each request is submitted (for 
> output) or as it completes (for input), at which times the buffers are 
> all mapped for DMA.  That's the problem.
> 
> Would we be better off not using dma_map_sg() at all in this situation?
> We could map each scatterlist buffer individually with dma_map_single()
> as the request is submitted and then unmap the buffer when the request
> completes, just like with non-sg transfers; then the problem wouldn't
> arise.
> 
> Would there be any significant penalty for doing this?  I realize it
> would prevent adjacent buffers from getting coalesced, but that's
> probably okay.  Any other reason not to?

No reason. About merging adjacent buffers, there are few IOMMU
implementations that do. The recent IOMMU implementations such as VT-d
and AMD IOMMU don't.

If drivers don't support scatter-gather operation, they had better use
dma_map_page() instead of forging scatter-gather lists and playing
with dma_map_sg().
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