On 2006-12-15 11:34 +0100, Stoltz Douchet, Olivier wrote:
> Hello Wookey,
> 
> I do not know if you remember me, but we met some time ago during a
> Linux Expo in London.

Hi, yes. Best to have this discussion on the YAFFS list, where other
experts can weigh in.

> Anyway, I have a few question regarding YAFFS2 and NAND flash spare
> bytes usage. 
> 
> Correct me if I am wrong:
> 
> - it appears that YAFFS2 is currently the only FFS that extensively uses
> the spare bytes of NAND flash devices, other FFS such as JFFS2 on Linux
> leave this spare byte area almost only to ECC (at the noticeable
> exception of their clean marker). Is that correct or do you know any
> other FFS in any OS that extensively uses the spare bytes?

That is correct. (this has given us a certain amount of hassle with
changes in MTD and OOB usage, as they only affect us).

> - you noted that "YAFFS applies the ECC itself, so the MTD should not do
> this", how would you then make use of hardware ECC calculation, which is
> now often found in SoCs such as our OMAP chips? 

That is out of date. yaffs can do its own ECC (for simple systems
where there is no MTD or hardware assistance), or it can let some
other hard or software system do it - then you just have to make sure
everyone agrees who is dealing with which bytes.

> Besides, our hardware
> ECC calculation hardware generates 13 bytes per 512 bytes of data thus
> leaving only 6 bytes (32-13*2) of unused spare bytes in case of 1 kB
> chunks or 12 bytes (64 - 13*4) of unused spare bytes in case of 2 kB
> chunks;

Hmm, that's not much. yaffs conventially needs 8 bytes for tags, and
one for block status, but that can be pared down
somwehat at the expense of reducing maximum limits (e.g. file size),
or making things a bit less efficient.

> - I also want to draw your attention to the issue that arises between
> your statement "Each page only gets written to twice (once when written
> and once when discarded)" and the latest MLC parts that only guarantee
> (and allow) a single write to a page. I do not know if you have already
> considered these issues.

We are aware of trends in NAND away from exposing the OOB and reducing
page writes. YAFFS will adapt accordingly - there is already a plan to
let it not use the oob at all for chips where it is not available. Not
sure about the single-page-write. Charles?

Wookey
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