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Date: Thu, 3 Nov 2011 12:02:11 +0100 (CET)
From: Robert Jarzmik <robert.jarzmik@free.fr>
To: Ivan Djelic <ivan.djelic@parrot.com>
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Subject: Re: [PATCH v2] mtd: nand: Add driver for M-sys / Sandisk diskonchip G4
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Cc: Mike Dunn <mikedunn@newsguy.com>, linux-mtd@lists.infradead.org
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----- Mail Original -----
De: "Ivan Djelic" <ivan.djelic@parrot.com>
=C3=80: "Mike Dunn" <mikedunn@newsguy.com>
Cc: linux-mtd@lists.infradead.org
Envoy=C3=A9: Jeudi 3 Novembre 2011 09h58:24 GMT +01:00 Amsterdam / Berlin /=
 Berne / Rome / Stockholm / Vienne
Objet: Re: [PATCH v2] mtd: nand: Add driver for M-sys / Sandisk diskonchip =
G4

> Using this blank ecc value to detect blank pages after reading them is no=
t that
> reliable, because if a bitflip appears in an erased page, the HW ecc gene=
rator
> will generate a completely different sequence of bytes. Your driver will =
think
> the page is not blank, it will try to correct errors and fail. And UBIFS =
will
> not appreciate that.
> I can see two cleaner alternatives to solve this issue:
>
> 1. When you program a page, before writing hwecc to oob, adjust it like t=
his:
> 2. Use unprotected spare oob byte 15 as a programmming marker: remove it =
from
> ...zip...

I would see a third one:
3. After reading the page, check bit DOC_ECCCONF1_PAGE_IS_WRITTEN in DOC_EC=
CCONF1 register.
   I think this bit is calculated from the Hamming Code (not sure), but I'm=
 pretty sure that
   this bit is 0 when the page is blank, and 1 if the page was written befo=
re.

--
Robert