Re: more flexible HW ECC support for nand_base

[Charles Manning <manningc2@actrix.gen.nz>]

On Friday 30 September 2005 23:57, Vitaly Wool wrote:

> Another part of the problem is nand_read_oob()/nand_write_oob()
> functions that also make assumptions on NAND flash page layout. It's
> suggested that they go to nand_chip structure, i. e. it will become a
> specific chip driver's responsibility to provide those; and if driver
> does not provide one, the current implementation can be used. If
> nand_chip provides its own layout, it must read/write oob data according
> to the layout.
>
> These steps should provide backward compatibility and the ability for
> particular driver owners to update their drivers accordingly with as
> less pain as possible.
>
> Any comments are welcome,
>

Hi Vitaly

I'm sorry I could not access the list archive to see if there has been further 
progress on this topic, so I apologise upfront if this wrecks the flow of the 
thread.

I think this might also be just what we're looking for to fix some problems we 
see in YAFFSland.

YAFFS2 uses AUTOPLACE for read_ecc and write_ecc, with the idea that YAFFS2 
can just pass in a buffer and mtd can do all the byte placement. YAFFS2 does 
not need (and should not have) knowledge of the actual physical placement. 

That part works fine, but YAFFS2 also needs to read only the tags (in the oob) 
during mount scanning (and at a few other times like gc). We obviously don't 
want to read the entire data area and do ECC etc because that is just wasted 
work. Instead we want to just do read_oob.

At the moment we have the situation that read_oob does a raw read, so you 
don't get the same bytes as you wrote with AUTOPLACE.  Some people hope that 
the only difference [with 2k pages] is the 2 bytes of bad block marker and so 
read at an offset of 2. That works for some, but won't work for everyone.

If I understand it correctly, your proposal would use an implied AUTOPLACE for 
read_oob. In other words read_oob will provide the same oob bytes as read_ecc 
with AUTOPLACE. 

That would get you some fans in YAFFSland.

-- CHarles