[Yaffs-archive] RE: yaffs: serious error in yaffs guts

[Christian Gan]

Hey guys,

I think if you want to reserve the functionality to use the eccstatus in
readChunkFromNAND then we can go to the other solution of copying (spare +
eccres1 + eccres2) in nandmtd_ReadChunkFromNand and do away with tmpSpare.
But we have to be very careful for all functions calling
nandmtd_ReadChunkFromNand to ensure that it passes a yaffs_Spare buffer that
is large enough to accomodate the extra ints.  Somebody in the future may
forget this fact since usually yaffs_Spare is defined as
YAFFS_BYTES_PER_SPARE bytes.

> -----Original Message-----
> From: Nick Bane [mailto:nick@cecomputing.co.uk]
> Sent: Thursday, February 27, 2003 4:42 PM
> To: manningc2@actrix.gen.nz; Christian Gan; yaffs list
> Subject: Re: yaffs: serious error in yaffs guts
>
>
> Charles/Christian
>
> One problem with Christian's proposal is that it fails to distinguish
> between a totally failed read and an error corrected read. If the return
> values from readChunkFromNAND can differentiate the two states that may be
> helpful in the future.
>
> Nick Bane
>
> ----- Original Message -----
> From: "Charles Manning" <manningc2@actrix.gen.nz>
> To: "Christian Gan" <cgan@iders.ca>; "Nick Bane" <nick@cecomputing.co.uk>;
> "yaffs list" <yaffs@toby-churchill.org>
> Sent: Thursday, February 27, 2003 9:01 PM
> Subject: Re: yaffs: serious error in yaffs guts
>
>
> > Hi Christian
> >
> > It is easiest for me if someone using the Linux MTD with
> NANDECC can test
> > this. Otherewise I will have to write a software emulator.
> >
> > Could you roll this together with Thomas' recent patchs for
> NANDECC, with
> > your proposed ammendments, and I can apply them as a consistent
> patch set.
> >
> > Thanx
> >
> > -- Charles
> >
> >
> > On Fri, 28 Feb 2003 06:27, Christian Gan wrote:
> > > Also,
> > >
> > > In yaffs_guts there is no real need to check eccres1 and eccres2 you
> should
> > > only need to check the return value of nandmtd_ReadChunkFromNand.  The
> two
> > > extra ints from the NAND MTD are only useful when you are performing
> > > multiple page reads when calling nand_read_ecc, since they
> will tell you
> > > which of the pages you requested has failed.  But YAFFs only a reads
> single
> > > page at a time, looking at the return value of nand_read_ecc is enough
> to
> > > know if the write was successful or if an ecc error has occurred for
> that
> > > page.  This value is checked in nandmtd_ReadChunkFromNand and
> it passes
> on
> > > a YAFFS_OK or YAFFS_FAIL (for any ecc errors) appropriately.
> > >
> > > Christian
> > >
> > > > -----Original Message-----
> > > > From: Nick Bane [mailto:nick@cecomputing.co.uk]
> > > > Sent: Thursday, February 27, 2003 10:31 AM
> > > > To: yaffs list
> > > > Subject: yaffs: serious error in yaffs guts
> > > >
> > > >
> > > > Charles
> > > >
> > > > A couple of bugs. One serious, one minor.
> > > >
> > > > in yaffs_ReadChunkFromNAND when dev->useNANDECC is true, it is
> > > > assumed that
> > > > readChunkFromNAND fills in the two ecc result integers on the end
> > > > of the oob
> > > > data.
> > > > These are then tested to see if there has been and ecc read
> > > > fail/correction
> > > > by the mtd layer.
> > > >
> > > > The problem with this is that readChunkFromNAND never
> actually passes
> the
> > > > ecc correction data back but leaves it in a local variable
> > > > tmpSpare and only
> > > > copies YAFFS_BYTES_PER_SPARE from it.
> > > >
> > > > The serious consequence of this is that all chunk reads are then
> > > > flagged as
> > > > failures and their blocks are marked as needing retiring
> once the data
> is
> > > > erased so yaffs nand becomes effectively write-once. The really
> serious
> > > > consequence of this is that one cannot now distinguish between
> > > > yaffs_retired_blocks and nand_factory_bad_blocks so one has to
> > > > speculatively
> > > > erase the entire device losing the factory erase data
> (which seems to
> be
> > > > zero on devices I have seen). Shouldn't we use a value other than 0
> for
> > > > yaffs to mark the block as bad (with more than two zero bits) so
> > > > they can be
> > > > distinguished?
> > > >
> > > > The other bug is in testing the ecc correction results in
> > > > yaffs_ReadChunkFromNAND.
> > > > if(nspare.eccres2 || nspare.eccres2)
> > > > Clearly one of them should be eccres1
> > > >
> > > > Nick
> > > >
> > > > -------------------------------------------
> > > > Nick Bane
> > > > Cambridge, UK.
> > > > +44(0)1954 719270
> > > >
> > > >
> > > >
> > > >
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