question about write/read sub-page with BCH algorithm or disable sub-page operation when using ubifs

[曹荣荣]

Hi Matthieu,
I'm working for a controller without software intervention unfortunately.

Define chip->ecc.steps == 1 is a good idea, I will try it.
Thank you very much for your suggestion.

2012/7/31 Matthieu CASTET <matthieu.castet at parrot.com>:
>
> Hi,
>> Hello,
>> I have a question about sub-page when using ubifs, so ask help from here.
>>
>> Let me take a 2048-size page with 4 sub-page Nand as example.
>>
>> The documentation said, "To write a sub-page, the driver may actually
>> write whole NAND page, but put 0xFF bytes to the sub-pages which are
>> not relevant to this operation."
>> It's OK for some ECC algorithms such as Hamming algorithm, because the
>> ECC codes generated by Hamming algorithm for the 0xFF data are still
>> 0xFF'ed. However, if we use BCH algorithm, the generated ECC codes for
>> 0xFF data will be not 0xFF'ed, and they will overwrite the previous
>> OOB data, so error will be happened when reading next time.
> It depends of your ecc algorithm. The software BCH algo in kernel take care of
> generate code such as 0xFF data give 0xFF ecc.
>
> This can be done by XORing the ecc before writing/after reading.
>
> This may not possible on some controller that compute and send ecc without
> software intervention.
>
>
>>
>> I try to resolve this issue by disabling sub-page operation, so I add
>> NAND_NO_SUBPAGE_WRITE into chip->options, but chip->options will be
>> flushed in nand_get_flash_type() funtion (chip->options &=
>> ~NAND_CHIPOPTIONS_MSK).
>
> You may better define chip->ecc.steps == 1
>
> Matthieu