linux-kernel - Re: [PATCH v2 07/16] mtd/docg3: add OOB layout to mtdinfo

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Date:	Sun, 13 Nov 2011 13:03:18 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	dedekind1@...il.com
Cc:	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Mike Dunn <mikedunn@...sguy.com>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/16] mtd/docg3: add OOB layout to mtdinfo

On Sun, 2011-11-13 at 14:53 +0200, Artem Bityutskiy wrote:
> On Sun, 2011-11-13 at 11:18 +0100, Robert Jarzmik wrote:
> > I personally think this should be provided by the MTD API. A function
> > is_page_blank(ofs) could tell if the page was written or not. Now if the
> > function is NULL, the upper layer (UBIFS, ...) could decide _by itself_ to
> > assign a free OOB byte to that meaning. But IMHO it's not the drivers duty to
> > take these decisions to restrict the OOB, let it be done at an upper level.
> 
> Probably it is a good idea to introduce an mtd_is_page_blank. But it
> should either work for all flashes or not introduced at all. I do not
> think upper layers should use OOB at all. And this interface should also
> work for NOR flash. Probably it should just fall-back to comparing the
> data with 0xFF if the driver cannot offer anything special. 

We *tried* comparing with 0xFF in JFFS2 and found that it wasn't good
enough (it could be *mostly* erased before a powerfail but not
completely, so as soon as you start to program it you get a lot of
bitflips). Hence the cleanmarkers.

The only way you can treat a page as erased is if you *know* it was
successfully erased. So I'd be reluctant to introduce a function that
encourages people to draw inferences they shouldn't.

-- 
dwmw2

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