linux-kernel - Re: [PATCH 0/5] sg

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Message-Id: <200712272252.27273.rusty@rustcorp.com.au>
Date:	Thu, 27 Dec 2007 22:52:26 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	davem@...emloft.net, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
	dougg@...que.net
Subject: Re: [PATCH 0/5] sg_ring for scsi

On Thursday 27 December 2007 13:09:27 FUJITA Tomonori wrote:
> On Wed, 26 Dec 2007 11:27:40 +1100
>
> Rusty Russell <rusty@...tcorp.com.au> wrote:
> > There are many signs through the code that it needs a great deal of
> > work: what is the purpose of sg_break? Why does the code check if
> > kmap_atomic fails?
>
> sg_break is a workaround for the hardware restrictions, I think.

Well, scb->breakup does that, can't see what scb->sg_break is for...  May have 
to wade through git history to understand it.

> > ===
> > Convert the ips SCSI driver to sg_ring.
> >
> > Slightly non-trivial conversion, will need testing.
>
> As I said, I don't think that converting SCSI drivers to sg_ring (with
> lots of non-trivial work) provides any benefits. Surely, sg_ring
> enables us to modify sg lists but SCSI drivers don't need the
> feature. What SCSI drivers needs is just a efficient way to get the
> next sg entry (they use 'sg++' in the past and sg_next now).

Sure, iteration over a two-level structure is a little more tricky, but it's 
pretty trivial for most drivers.

Indeed, I think that it's probably better to have an explicitly different 
interface for sg_ring, and keep the current interface for small sg arrays.  
That would provide a nicer changeover.

I'll prototype something and see what it's like...
Rusty.
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