[U-Boot] handling of bad blocks in nand

[Grant Edwards]

On 2010-08-16, Scott Wood <scottwood at freescale.com> wrote:
> On Fri, Aug 13, 2010 at 09:22:21PM +0000, Grant Edwards wrote:

> At one point in the legacy NAND code, a distinction was made when
> reading between completely skipping bad blocks, and filling the
> buffer with zeroes in place of the bad blocks.  It looks like
> ".jffs2" or ".e" would get you zeroes, and ".jffs2s" or ".i" would
> get you block skipping.
>
> The zero-fill mode is no longer supported.  In 1.3.4, assuming you're
> not using the legacy NAND code, you need to specify one of ".jffs2",
> ".e", or ".i" to get block skipping.  It doesn't matter which one.
> In current u-boot these suffixes are accepted and ignored as legacy
> (bad blocks are always skipped), except for "jffs2s" which apparently
> never made an appearance outside the legacy code.

OK, that clears up quite a bit of my confusion.

> Yes, it's a bit of a mess.

It does appear to be improving. :)

>>  3) In the 1.3.4 source code, there are lots of instances where there
>>     are boolean flags with names like "jffs2".  AFAICT those flags
>>     have nothing to do with the JFFS2 filesystem, but simply control
>>     whether or not bad flash blocks are skipped during read/write
>>     operations.  Am I reading the code correctly?
>
> The user interface .jffs2 suffix was to indicate bad block skipping,
> but opts->jffs2 (which is still around) refers to writing JFFS2
> cleanmarkers after erasing a block.

Thanks, that helps.

>>  4) If in a custom command, I want to read/write a large block of
>>     data to/from nand flash while skipping bad blocks what functions
>>     do I call?  Right now I'm doing something like this:
>> 
>>       nand_info[0].read(nand_info+0, offset, sizeof(buffer), &len, buffer)
>>     
>>       nand_info[0].write(nand_info+0, offset, bytecount, &len, buffer)
>>     
>>     but, I have a feeling that's not right (though it seems to work,
>>     I suspect it's not going to skip bad blocks).
>
> You suspect correctly.
>
> In current code there is nand_write_skip_bad() and
> nand_read_skip_bad() in drivers/mtd/nand/nand_util.c.

Yup, I found those, and the read version is exactly what I need.

I've still got to figure out how to write.

The problem is that the data to be written is streaming. I don't know
how much there is, and I don't want to allocate arbitrarily large
buffers.  So I want to write it one erase-block at a time as it
arrives.  AFAICT nand_write_skip_bad() can't be used in that
situation. Since it provides no indication of how many blocks were
skipped, I have no way of knowing where the next block should be
written.

I guess that means I need to use the same lower-level API that is
being used by nand_write_skip_bad().

> In 1.3.4 use nand_read_opts()/nand_write_opts().  Or convince Atmel
> to support something more recent.  :-)

It turns out that Atmel's patches have been merged into the main
sources and 2010.6 works fine for the board in question.  I don't know
why the official Atmel web-page still instructs you to use
1.3.4+patches.

-- 
Grant Edwards               grant.b.edwards        Yow! I'm encased in the
                                  at               lining of a pure pork
                              gmail.com            sausage!!