[Yaffs] cvs YAFFS + MTD cvs + 2.4.27-vrs1 problems

[Thomas Gleixner]

On Fri, 2004-12-10 at 13:32 +1300, Charles Manning wrote:
> I might argue the latter point here. I believe the ECC byte ordering 
> calculated by SJHill's nand_ecc.c is wrong.   yaffs_ecc.c can calculate this 
> ECC order or the SmartMedia "correct" order.  yaffs_ecc.c is also slightly 
> faster than nand_ecc.c ;-).

I have no problem to put a faster algorithm into nand_ecc.c. The byte
ordering is simple to fix by

-        eccpos: {8, 9, 10, 13, 14, 15}
+        eccpos: {9, 8, 10, 14, 13, 15}

or whatever byte ordering is the correct one.

> I don't think I have a gripe with the ECC stuff, it's the write verification 
> that gets me.

Why ? The nand driver provides write verify and tells you when it
failed.

> From my understanding this is only a configuration issue.
> 
> YAFFS can be build with CONFIG_YAFFS_USE_NAND_ECC which means yaffs won't do 
> ECC - it lets mtd do all ECC. This is, IMHO, the best way to do things with 
> Linux - particularly if there are other non-yaffs users of the NAND.
> 
> With non-Linux usage, I recommend using yaffs_ecc.c if software ECC is 
> required.

Sure, but I strongly recommend to seperate this from the filesystem. 

> Where things do break down a bit is with mkyaffs which does calculate ECC. 
> You need to be careful to use a consistent ECC strategy.

Yep, but I notice that most newer designs use hardware ecc, which
sometimes leads to strange page layouts and uses other algorithms e.g.
Reed-Solomon, so the generic mkyaffs - generate everything - will not
work anymore.

As we discussed before the only way to solve this is ECC autoplacement,
which is unfortunately not possible with YAFFS1 although it could be
tweaked to use it. YAFFS2 uses autoplacement ?

To provide images which can be programmed into the FLASH during
manufacturing you will need a different approach.

- mkyaffs provides the filesystem data

- an ecc generator which must be aware of the algortihm and the
placement builds the final image which can be programmed to the chip

- If you can programm it from the OS, then you can read the image back
from the chip and you do not need a software emulation of the ecc and
placement

tglx