linux-kernel - Re: [PATCH 4/4] mtd: nand: omap2: Add data correction support

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Date:	Thu, 11 Oct 2012 10:21:49 +0200
From:	Ivan Djelic <ivan.djelic@...rot.com>
To:	"Philip, Avinash" <avinashphilip@...com>, <tony@...mide.com>,
	<afzal@...com>
CC:	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"artem.bityutskiy@...ux.intel.com" <artem.bityutskiy@...ux.intel.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 4/4] mtd: nand: omap2: Add data correction support

On Thu, Oct 11, 2012 at 06:27:13AM +0100, Philip, Avinash wrote:
(...)
> > Another simple strategy could use the fact that you add a 14th zero byte to
> > the 13 BCH bytes for RBL compatibility:
> 
> RBL compatibility (14th byte) is applicable only for BCH8 ecc scheme.
> 
> So I am planning adding an extra byte (0) for BCH4 ecc scheme. So with this
> we can go for same approaches in BCH4 & BCH8 ecc scheme.
> 
> If I understood correctly, software BCH ecc scheme is modifying calculated
> ecc data to handle bit flips in erased pages.
> 
> If that is the only reason, whether same logic can go for same ECC calculation
> (remove modification of calculated ecc in case of software ecc correction)
> by adding an extra byte (0) in spare area to handle erased pages.
> 
> So can you share if I am missing something?

Yes, the only reason why a constant polynomial is added to hw-generated ECC bytes is to transparently handle bitflips in erased pages.
Handling erased pages this way has several benefits over the zero byte hack:
- cleaner code, no checking of the zero byte
- no expensive scan of data+spare area when reading an erased block: this step can significantly slow down the initial UBI scan (lots of erased pages to read)
- no need to worry about the (very unlikely) possibility of having more than 4 bitflips in the zero byte

OTOH, having the same ECC codes for both ELM and non-ELM chips with RBL compatibility sounds nice and would also simplify things.
Note: on platforms where we use SW BCH correction, we also use the MLC OMAP boot mode, which is more robust and not compatible with 8-bit/4-bit BCH layouts.

I don't know which way is better for the OMAP community:
1. Unifying ECC modes = loosing the constant polynomial benefits, but gaining RBL compat and simplifying code
2. Keeping separate ECC modes = code bloat

Tony, do you have an opinion on this ?

BTW, Afzal is submitting a series of patches [1] which are not compatible with your series; is there any plan to merge your patches ?

BR,
--
Ivan

[1] http://lists.infradead.org/pipermail/linux-mtd/2012-October/044374.html
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