Hi ,
So, I looked in the code. yaffs_PackedTags2TagsPart is a 16-byte struct
(4 unsigned longs).
Together with the 2 byte bad block marker, this means total 18 spare
byte consumption per
2 kbyte block ?.
If I do the math, for a 15-bit BCH code (uses 15 spare bits per error
correctable)
-> For SLC flash, with 16 spare bytes/512 bytes: 46 bytes available
for ECC, max. 24 bit correction / 2kbyte sector.
-> For MLC flash, I dont find the spare area size on the web. I
think its larger, permitting stronger ECC.
Regards,
Henri Cloetens.
Charles Manning wrote:
It depends on what you're protecting
sizeof (yaffs_PackedTags2) is a full set of tags + ecc on the tags themselves.
if your ecc system is going to cover the spare area too, then youonly need to
store sizeof(yaffs_PackedTags2TagsPart).
You will also need a bad block marker. Say 2 bytes.
-- Charles
On Thursday 12 November 2009 05:28:45 Henri Cloetens wrote:
Hi,
I am looking at ways to do ECC for MLC nand flash.
I have a question in this regards on the Yaffs file system:
When used together with a NAND flash with 2kbyte sectors, how many spare
bytes are needed for the Yaffs headers ?. (#bytes / 2kbyte sector).
I need this number, so I can figure out how many bytes can be used by the
ECC. I want to design my ECC to use up all spares not needed by Yaffs.
Best Regards,
Henri Cloetens.
Charles Manning wrote:
Fixed something...
On Wednesday 11 November 2009 12:59:07 Charles Manning wrote:
Hi Ross
Thanks for that patch.
I had a look and have come up with an alternative solution to the problem:
instead of trying to unwind a half created object if the string or tnode
malloc fail it is a lot cleaner to reverse the order of allocation. ie
forst create the tnode or string, then create the object if that succeeds.
I also had a sniff around and found a few other areas where tnode creation
failures would cause issues.
The fixed stuff has been running in a stress test for almost 24 hours and
I've just checked that in.
http://www.aleph1.co.uk/cgi-bin/viewcvs.cgi/yaffs2/yaffs_guts.c?r1=1.93&r2=
1.94
Look at:
http://www.aleph1.co.uk/cgi-bin/viewcvs.cgi/yaffs2/yaffs_guts.c?r1=1.95&r2=
1.93
I also put a day or so into extending low mem testing and will expand the
tests more.
Thanks
-- Charles
On Friday 30 October 2009 06:35:32 Ross Younger wrote:
Whilst doing some stress testing on a board with limited RAM, I tripped
over an assertion failure.
In yaffs_CreateNewObject, YAFFS would sometimes successfully
AllocateEmptyObject but not be able to allocate a new tnode, so GetTnode
returned 0. It would then immediately attempt to FreeObject, but the
freshly allocated object already has a parent so it hits the YBUG trap at
line 1992 of yaffs_guts.c.
I attach a patch which fixes this assertion (and a similar case I spotted
in yaffs_MknodObject) by calling DoGenericObjectDeletion instead of
FreeObject, though it might instead be reasonable for AllocateEmptyObject
to not try to add the object to the rootDir or lostNFoundDir.
Ross
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Henri Cloetens
Freescale Semiconductor Ltd
De Waal 26
5684 PH Best
The Netherlands
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