linux-kernel - Re: MMC layer regression with single-block controllers

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Date:	Wed, 25 Mar 2009 17:24:41 +0100
From:	Pierre Ossman <drzeus@...eus.cx>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Manuel Lauss <mano@...rinelk.homelinux.net>,
	linux-kernel@...r.kernel.org
Subject: Re: MMC layer regression with single-block controllers

On Wed, 25 Mar 2009 17:14:25 +0100
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Wed, Mar 25 2009, Pierre Ossman wrote:
> > 
> > The code was there previously, but it seemed a bit redundant to have
> > functionality like that in the block driver since we've already told
> > the block layer about the restrictions.
> 
> You never saw the warnings? It's pretty clear that it does not support <
> PAGE_CACHE_SIZE blocks. It has always been so, I don't know why the
> subject says regression. I guess that is referring to a mmc layer
> regression?
> 

Yes. The MMC code did all of this magic by itself previously and
assumed very little about the block layer.

> > The code was pretty simple. Basically it just cropped the sg list at
> > the correct place. Couldn't that be as easily done in the block layer?
> 
> No, because if you do it transparently, then you have to keep partial
> state in the bio for completions. So it makes everything a lot more
> complex, I don't want to do that for something like this.
> 

How is this different from the low level driver partially completing a
request, which is how it would have to be handled otherwise?

Rgds
-- 
     -- Pierre Ossman

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