linux-kernel - Re: [PATCH 1/3] mtd: nand: omap: Revert to using software ECC by default

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Date:	Wed, 6 Aug 2014 11:02:42 +0300
From:	Roger Quadros <rogerq@...com>
To:	Grazvydas Ignotas <notasas@...il.com>
CC:	Brian Norris <computersforpeace@...il.com>,
	Tony Lindgren <tony@...mide.com>, Felipe Balbi <balbi@...com>,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	<pekon.gupta@...il.com>, <artem.bityutskiy@...ux.intel.com>,
	<dwmw2@...radead.org>, <jg1.han@...sung.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] mtd: nand: omap: Revert to using software ECC by
 default

Hi Gražvydas,

On 08/05/2014 07:15 PM, Grazvydas Ignotas wrote:
> On Tue, Aug 5, 2014 at 1:11 PM, Roger Quadros <rogerq@...com> wrote:
>> For v3.12 and prior, 1-bit Hamming code ECC via software was the
>> default choice. Commit c66d039197e4 in v3.13 changed the behaviour
>> to use 1-bit Hamming code via Hardware using a different ECC layout
>> i.e. (ROM code layout) than what is used by software ECC.
>>
>> This ECC layout change causes NAND filesystems created in v3.12
>> and prior to be unusable in v3.13 and later. So revert back to
>> using software ECC by default if an ECC scheme is not explicitely
>> specified.
>>
>> This defect can be observed on the following boards during legacy boot
>>
>> -omap3beagle
>> -omap3touchbook
>> -overo
>> -am3517crane
>> -devkit8000
>> -ldp
>> -3430sdp
> 
> omap3pandora is also using sw ecc, with ubifs. Some time ago I tried
> booting mainline (I think it was 3.14) with rootfs on NAND, and while
> it did boot and reached a shell, there were lots of ubifs errors, fs
> got corrupted and I lost all my data. I used to be able to boot
> mainline this way fine sometime ~3.8 release. It's interesting that
> 3.14 was able to read the data, even with wrong ecc setup.

This is due to another bug introduced in 3.7 by commit 65b97cf6b8deca3ad7a3e00e8316bb89617190fb.
Because of that bug (i.e. inverted CS_MASK in omap_calculate_ecc), omap_calculate_ecc() always fails with -EINVAL and calculated ECC bytes are always 0. I'll be sending a patch to fix that as well. But that will only affect the cases where OMAP_ECC_HAM1_CODE_HW is used which happened for pandora from 3.13 onwards.

> 
> Do you think it's safe again to boot ubifs created on 3.2 after
> applying this series?
> 

Yes. If you boot pandora using legacy boot (non DT method), it passes 0 for .ecc_opt in pandora_nand_data. This used to mean OMAP_ECC_HAMMING_CODE_DEFAULT which is software ecc. i.e. NAND_ECC_SOFT with default ECC layout. Until the above mentioned commits changed the meaning. We now call that option OMAP_ECC_HAM1_CODE_SW.

Please let me know if it works for you. Thanks.

cheers,
-roger
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