linux-kernel - Re: Scatter-gather list constraints

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Message-Id: <20080625091813Z.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 25 Jun 2008 09:18:15 +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 Tue, 24 Jun 2008 10:57:13 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> On Tue, 24 Jun 2008, FUJITA Tomonori wrote:
> 
> > I don't think that the block layer has the DMA alignment concept in FS
> > I/O path. And I think that you need kinda the DMA padding instead the
> > DMA alignment though again The block layer doesn't have the DMA
> > padding concept in FS I/O path. And the DMA padding applies to only
> > the last SG element.
> > 
> > I guess that it's pretty hard to implement such a strange restriction
> > in the block layer cleanly.
> 
> I don't see why there should be any problem.  It's simply a matter of 
> splitting a single request into multiple requests, at places where 
> the length restriction would be violated.
>
> 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.

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.


> > The iSER driver has a strange restriction too. I think that as iSER
> > does, bouncing is a better option, though adding some generic
> > mechanism to reserve buffer in the block layer might be nice, I
> > gueess.
> 
> Is it reasonable to have 120-KB bounce buffers?

The block layer does. Why do you think that USB can't?
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