[Yaffs] YAFFS2/MTD incompatibility isolated

[Michael Arm]

Hi all,

I've been trying to get the latest CVS version of YAFFS2 to work with a
Samsung K9F2GxxU0M chip which has 2k sector size.  I observed the problem
documented some months ago whereby files written to the flash would
disappear upon the next mount.

I traced the problem down to the fact that the nandmtd2_QueryNANDBlock()
function in yaffs_mtdif2.c is calling mtd->block_isbad, which is really
nand_block_isbad() in mtd/nand.c.  This function is checking if the first
byte of OOB data is 0xFF, and if not is returning that the block is bad.

However when data is written, the call traces through
nandmtd2_WriteChunkWithTagsToNAND() in yaffs_mtdif2.c, which in turn is
calling yaffs_PackTags2() in yaffs_packedtags2.c.  This function is using a
struct yaffs_PackedTags2 to store the OOB data for the block.  This value is
being directly written to the OOB area in the flash.  However, there is no
accomodation made for the fact that byte 0 should be 0xFF, and as a result
this is why upon mount the data disappears - YAFFS2 mistakenly thinks it is
reading bad blocks, because it didn't set byte 0 to 0xFF when it wrote the
OOB data, byte 0 is instead the LSB of the sequence number.

If we add a byte to the head of the yaffs_PacketTags2 struct, and set it to
0xFF in yaffs_PackTags2(), then everything works fine and data can be read
and written good, apparently.  However I am wondering if this is a good fix,
or if there are hidden problems that this might be cause due to the new
offset of the rest of OOB data and the possibility of a byte of OOB data
being lost at the end.

Since we are constrained to use Linux 2.4 I am currently using the version
of MTD I found on husaberg.toby-churchill.com, which was referenced
(somewhat obscurely) in earlier posts.  I'm not sure if this is the best
version, (other versions I tried showed identical symptoms) and it was in
fact cumbersome to access as there doesn't appear to be any way to fetch the
files except for one at a time through the web interface.

Can anyone comment on how good a fix shifting the yaffs_PacketTags2
structure is, or on the location of the newest MTD code which works with
YAFFS2 under Linux 2.4?

Thanks,

Michael
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