Re: flash bbt broken due to unitialized bitflip_threshold?

[Mike Dunn <mikedunn@newsguy.com>]

On 06/07/2012 12:36 AM, Artem Bityutskiy wrote:
> On Wed, 2012-06-06 at 19:55 +0200, Ivan Djelic wrote:
>> 1. on legacy systems with 1-bit nand and strength = 1, default bitflip_threshold is 1
>> 2. on legacy systems with 1-bit nand and strength > 1, default bitflip_threshold is 'strength'
>> 3. on new systems   with 2+ bit nand and strength > 1, default bitflip_threshold is 'strength'
> 
> Ivan, Shmulik,
> 
> I've gave this another though, and I think it is OK to leave this as it
> is now. Your replies were very helpful, thanks.


Yes, many thanks guys.  It seems I picked the wrong couple days to be away from
email.

This is not an issue on my docg4 because it does not use a flash-based BBT, but
instead scans the whole device for blocks that are marked bad in oob. EUCLEAN is
ignored in this case.  The following code is present in both scan_block_full()
and scan_block_fast():

	/* Ignore ECC errors when checking for BBM */
	if (ret && !mtd_is_bitflip_or_eccerr(ret))
		return ret;

Digging into this, it turns out this is a problem only in the case of:

  (1) nand->td != NULL (flash-based BBT present)
  (2) NAND_BBT_NO_OOB is not set

Here's the call stack for the above case, and with NAND_BBT_ABSPAGE not set
(this is true for the mxc_nand controller).  The problem occurs in
scan_read_raw_oob()...

nand_scan_bbt()
|
+-> search_read_bbts()                ignores return code
    |
    +-> search_bbt()                  always returns 1
        |
        +-> scan_read_raw()           -EUCLEAN propagated up
            |
            +-> scan_read_raw_oob()   returns without updating buf, len, offs
                |
                +-> mtd_read_oob()    -EUCLEAN returned


I addition to the patch suggested by Shmulik, I would also suggest the
following, in the interest of consistency with the bad block scanning code, and
also thoroughness:

diff --git a/drivers/mtd/nand/nand_bbt.c b/drivers/mtd/nand/nand_bbt.c
index 30d1319..ed59aa8 100644
--- a/drivers/mtd/nand/nand_bbt.c
+++ b/drivers/mtd/nand/nand_bbt.c
@@ -319,7 +319,7 @@ static int scan_read_raw_oob(struct mtd_info *mtd, uint8_t
*buf, loff_t offs,

                res = mtd_read_oob(mtd, offs, &ops);

-               if (res)
+               if (res && !mtd_is_bitflip_or_eccerr(res))
                        return res;

                buf += mtd->oobsize + mtd->writesize;

Shmulik, please let me know if yuo'd like me to submit the patch you suggested,
and I will do so promptly.  Otherwise, thanks again!

More gory details... by comparison, here's the call stack for the same case,
except NAND_BBT_NO_OOB is set.  Here, there's no problem.

nand_scan_bbt()
|
+-> search_read_bbts()                ignores return code
    |
    +-> search_bbt()                  always returns 1
        |
        +-> scan_read_raw()           -EUCLEAN propagated up
            |
            +-> scan_read_raw_data()  -EUCLEAN propagated up
                |
                +-> mtd_read_oob()    -EUCLEAN returned


Thanks, and sorry for the oversight,
Mike