linux-kernel - Re: Scatter-gather list constraints

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Message-Id: <20080626110615U.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 26 Jun 2008 11:06:03 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	stern@...land.harvard.edu
Cc:	fujita.tomonori@....ntt.co.jp, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, antonio.lin@...ormicro.com,
	david.vrabel@....com
Subject: Re: Scatter-gather list constraints

On Wed, 25 Jun 2008 10:23:00 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> On Wed, 25 Jun 2008, FUJITA Tomonori wrote:
> 
> > > For example, suppose an I/O request starts out with two S-G elements
> > > of 1536 bytes and 2048 bytes respectively, and the DMA requirement is
> > > that all elements except the last must have length divisible by 1024.  
> > > Then the request could be broken up into three requests of 1024, 512,
> > > and 2048 bytes.
> > 
> > I can't say that it's easy to implement a clean mechanism to break up
> > a request into multiple requests until I see a patch.
> 
> And I can't write a patch without learning a lot more about how the
> block core works.
> 
> > What I said is that you think that this is about extending something
> > in the block layer but it's about adding a new concept to the block
> > layer.
> 
> Is it?  What does the block layer do when it receives an I/O request
> that don't satisfy the other constraints (max_sectors or
> dma_alignment_mask, for example)?

As I explained, you need something new.

I don't think that max_sectors works as you expect.

dma_alignment_mask is not used in the FS path. And I think that
dma_alignment_mask doens't solve your problems.


> > > Is it reasonable to have 120-KB bounce buffers?
> > 
> > The block layer does. Why do you think that USB can't?
> 
> Why do you think I think that USB can't?  I didn't ask whether it was
> _possible_; I asked whether it was _reasonable_.

What the block layer does is reasonable with regard to this, I think.
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