linux-kernel - Re: Scatter-gather list constraints

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Date:	Wed, 25 Jun 2008 10:23:00 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
cc:	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, 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)?

> > 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_.

Alan Stern

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