[U-Boot] [PATCH 0/5] introduce nand write.ubi, and drop ffs for jffs2 too

[Artem Bityutskiy]

On Fri, 2011-04-29 at 13:58 +0200, Detlev Zundel wrote:
> Hi Ben,
> 
> > It was found that on da850evm, where the NAND ECC used does not map all 0xff
> > data to 0xff ECC, that flashing UBI and JFFS2 image from U-boot with nand
> > write[.e] command resulted in alot of ECC errors... for UBI the result was
> > an unmountable filesystem on second attach from linux. For JFFS2 the result was
> > a multitude of ECC errors printed on the cosole on the second mount in Linux --
> > the filesystem remains mountable for awhile but eventually collapses.
> 
> I am not sure that I can follow you here so I have to ask you to clarify
> the problem for me.
> 
> I understand that a page of 0xffs does _not_ have an ECC of all 0xffs.
> Actually I would be surprised if there was any ECC having this property,
> so I doubt that this is da850 specific, correct?
> 
> So I am wondering about two things:
> 
> - If I erase a page in NAND, will the ECC be updated by someone or will
>   it be 0xffs?  If the latter, then is it "normal" to have ECC errors on
>   freshly erased pages?
> 
> - If we (correctly) "write" 0xffs even to an erased page, a generated
>   ECC should match this content, so I do not see where ECC errors should
>   come from in this setting.
> 
> Summarily, I do not understand where the ECC errors came from in your
> setup and what the faulting party in that scenario actually is/was.
> 
> Can you please enlighten me?

I do not know why I'm in CC, but I see that the code to skip all 0xFFs
looks like it was copied from UBI. The reason why UBI and UBI user-space
tools skip NAND pages with all 0xFFs when writing is described here:

http://www.linux-mtd.infradead.org/doc/ubi.html#L_flasher_algo

If it is too long to read, in short:

1. if we write to flash, we always write to an erased eraseblock,
obviously :-)
2. erased erase block contains all 0xFFs, so skipping 0xFF NAND pages is
harmless.
3. writing 0xFF pages has side-effects - the ECC bytes in OOB will be
used
4. If we are flashing an UBIFS image, UBIFS will use the half-filled
eraseblocks, and if the "free" pages were written with 0xFFs, they'll
become unusable.

So we skip 0xFF pages at the end. Not all, only the last ones. E.g., if
your eraseblock consists of 4 pages, and you write "data, 0xFFs, data,
0xFFs", then we'll only write "data, 0xFFs, data", so that the last NAND
page can be used later.

Sorry for my poor English, and I'm writing in a hurry - have to have to
a pub to farewell several colleagues who are leaving the company :)

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)