linux-kernel - Re: [PATCH 1/4] scatterlist: new helper functions

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Message-Id: <20110316135528U.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 16 Mar 2011 13:55:32 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	oakad@...oo.com
Cc:	akpm@...ux-foundation.org, fujita.tomonori@....ntt.co.jp,
	maximlevitsky@...il.com, James.Bottomley@...senPartnership.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] scatterlist: new helper functions

On Tue, 15 Mar 2011 21:18:14 -0700 (PDT)
Alex Dubov <oakad@...oo.com> wrote:

> 
> > 
> > I don't think so.
> > 
> > Splitting (or merging) a request and playing with sg lists
> > inside a
> > driver is a bad idea. Such should be done in the block
> > layer.
> > 
> > I still don't see why the block layer can't do that for the
> > driver.
> > 
> > I believe that the helper functions for such should not be
> > added.
> > 
> 
> 
> In a particular case of flash-like devices, letting the block layer
> split requests into memory sequential blocks too often results in
> unnecessary fragmentation of writes/erases.

Why?

Why can't the block layer split a request in the way the driver wants
to do?

That is, why can't the driver tell the block layer how to split a
request?


> If only one sg entry is requested from the block layer, it will be (more
> often than not) only 1 or 2 pages in length, even if total size of
> prospective write request spans multiple erase blocks.

In this case, what does the driver do? Why can't the block layer do
the same?


> So there are really only two options for legacy memorystick driver:
> 1. Play with scatterlists explicitly.
> 2. Make it an MTD backend, rather then stand-alone block device.
> 
> The second option makes more sense, but it is not necessarily the optimal
> approach for implementation of this particular media format.
> 
> 
> 
>       
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