linux-kernel - Re: Scatter-gather list constraints

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Message-Id: <20080624194119H.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 24 Jun 2008 19:41:42 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	stern@...land.harvard.edu
Cc:	andi@...stfloor.org, linux-kernel@...r.kernel.org,
	antonio.lin@...ormicro.com, david.vrabel@....com
Subject: Re: Scatter-gather list constraints

On Sat, 21 Jun 2008 17:50:13 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> > >> I don't think the block layer knows about such kinds of restrictions.
> > > 
> > > Evidently not.  Is it feasible to add such knowledge to the block
> > > layer?
> > 
> > You would need to ask Jens, but I would assume he would ask:
> > - Is it common?
> 
> This is the only situation I know about.  But of course it will become 
> more and more common as wireless USB devices spread into use.
> 
> > - Is it performance critical?
> 
> For people using wireless USB drives, yes.
> 
> > and presumably the answer to both would be "no" ?
> 
> In theory this could be fixed at the host controller level, by making 
> packets span S-G elements.  But this would be a very large and 
> difficult change.  Altering the block layer should be a lot easier (he 
> said, secure in his blind ignorance).  For example, the DMA alignment 
> restriction could be made to apply to the _end_ of each S-G element as 
> well as the _beginning_, except for the last element in the list.

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.

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.
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