NAND bad blocks

[Charles Manning]

On Friday 16 April 2004 04:39, J.D. Bakker wrote:
> Hi all,
>
> There's a slight discrepancy between what mtd and mkyaffs consider a bad
> block.
>
>  From mtd/drivers/mtd/nand/nand.c:
>
>    static int nand_block_bad(struct mtd_info *mtd, unsigned long page)
>    {
>          struct nand_chip *this = mtd->priv;
>
>          this->cmdfunc (mtd, NAND_CMD_READOOB, NAND_BADBLOCK_POS, page);
>          if (this->read_byte(mtd) != 0xff)
>                  return 1;
>
>          return 0;
>    }
>
> ...which would indicate that even one bit set to zero would mark the
> block bad. However, mkyaffs.c has this code:
>
>    /* Read the OOB data to determine if the block is valid.
>     * If the block is damaged, then byte 5 of the OOB data will
>     * have at least 2 zero bits.
>     */
>
> ..snip..
>
>    if(countBits[oobbuf[5]] < 7)
>    {
> 	printf("Block at 0x08%lx is damaged and is not being
> formatted\n",addr);
>    }

The idea with the bit counting in YAFFS is to prevent a single bad bit 
causing a block to be erroneously marked bad, thereby losing your data.

In reality this situation hardly ever, if ever, arises and the two code 
sequences are pretty much equivalent.

-- Charles