State of read-only filesystems in NAND / MTD bad blocks handling when reading

[Thilo Fromm]

Hello Matthieu,

>> [ MTD read might return bad blocks ]
>>> I principally agree, however, jffs2 is doing just that, managing bad blocks.
>>> Whether that is a good idea or not is debatable of course. Technically it
>>> mixes up file system and device properties. Also, you might not plan to
>>> change read-only FS's very often so the effort might be quite minimal in
>>> practice.
>>
>> It still feels like the wrong place to do this. Every FS would just
>> skip this block, and be done with it.
>
> There also bit-flip handling that ubi manage, but other fs don't handle.
> And with newer nand, bit-flip happen sooner.

You are right - during the conversation in this thread I learned that
bit flip can become another issue we will have to take care about. I
do not think bit flip / NAND scrubbing is an unsolvable issue. So I
postpone a solution to a later time. However, the problem we're trying
to fix right now is reading back what was written to an MTD. Which
does not work for us at all.

>> Apart from the fact that my NAND SW stack will then be two layers
>> higher than it actually needs to be (and the added complexity this
>> brings to boot loader and kernel) -  as far as I know UBI does not
>> provide the functionality we require: being able to store a read-only
>> file system image, and mount it at boot time. At least we don't get
>> this without another boot step (initrd, see above).
>>
> gluebi + mtdblock_ro doesn't work ?

Can you paint the abstraction layer call flow involved in this for a
single read transaction and _not_ burst out laughing ;) ? We're on an
embedded system, man :)

Regards,
Thilo

-- 
Dipl.-Ing (FH) Thilo Fromm, MSc., Embedded Systems Architect
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