should nand driver create ECC for a page after ERASE cmd?

[Oleksij Rempel]

Am 21.12.2014 um 17:28 schrieb Boris Brezillon:
> Hi Oleksij,
> 
> On Sat, 20 Dec 2014 09:42:29 +0100
> Oleksij Rempel <linux at rempel-privat.de> wrote:
> 
>> Hello all,
>>
>> i'm trying to pass mtd/tests for my driver.. on pagetest i fail on this
>> place:
>> http://lxr.free-electrons.com/source/drivers/mtd/tests/pagetest.c#L311
>>
>> after ERASE cmd this test trying to read same page. Driver will report
>> ECC error on read, because erased page has no ECC data in OOB.
>>
>> What is correct way to handle it?
> 
> Well, actually it depends on your NAND/ECC controller capabilities (but
> given your discussion with Richard on IRC, I guess you already
> understood that ;-))
> 
> 1) Wiser controllers are generating 0xff ECC bytes for a data chunk
> (chunk == ECC step size) filled with 0xff. With BCH algorithms this is
> easily done by XORing the ECC bytes with the appropriate pattern (see
> soft BCH implementation)
> 2) Some controller just verify if the data chunk + ECC bytes are all
> 0xff before passing it to the ECC engine. If they are filled with 0xff
> the ECC correction is bypassed.
> This method has one drawback: it does not properly handle bitflips
> occurring in erased pages (if one bitflip occurs the NAND controller
> consider the chunk as not empty, and pass it to the BCH engine).
> 3) The controller does not handle erased pages at all, and in this case
> you'll have to manually test it (as Ezequiel suggested) when you
> encounter an ECC error.

Thanks a lot to all responses,
i recognised my mistake and learned from it.

Boris,
i think your explanation can be placed to mtd web page, some where here :)
http://www.linux-mtd.infradead.org/doc/general.html#L_mtd_tests
-- 
Regards,
Oleksij

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