Re: [PATCH 2/3] mtd: nand: implement 'max_bad_blocks' mtd function

[Richard Weinberger <richard@nod.at>]

Am 11.05.2015 um 17:19 schrieb Ben Shelton:
> From: Jeff Westfahl <jeff.westfahl@ni.com>
> 
> Implement the new mtd function 'max_bad_blocks'. Use the "bad blocks
> maximum per LUN" field in the ONFI parameter page to find the maximum
> number of bad blocks to reserve for an MTD, taking into account the
> number of LUNs in the NAND device and how many LUNs the MTD spans.
> 
> From the ONFI 1.0 spec: "This field contains the maximum number of
> blocks that may be defective at manufacture and over the life of the
> device per LUN. The maximum rating assumes that the host is following
> the block endurance requirements and the ECC requirements reported in
> the parameter page."
> 
> Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
> ---
>  drivers/mtd/nand/nand_base.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/drivers/mtd/nand/nand_base.c b/drivers/mtd/nand/nand_base.c
> index c2e1232..a97d08e 100644
> --- a/drivers/mtd/nand/nand_base.c
> +++ b/drivers/mtd/nand/nand_base.c
> @@ -2884,6 +2884,40 @@ static int nand_block_markbad(struct mtd_info *mtd, loff_t ofs)
>  }
>  
>  /**
> + * nand_max_bad_blocks - [MTD Interface] Max number of bad blocks for an mtd
> + * @mtd: MTD device structure
> + * @ofs: offset relative to mtd start
> + * @len: length of mtd
> + */
> +static int nand_max_bad_blocks(struct mtd_info *mtd, loff_t ofs, size_t len)
> +{
> +	struct nand_chip *chip = mtd->priv;
> +	uint32_t part_start_block;
> +	uint32_t part_end_block;
> +	uint32_t part_start_lun;
> +	uint32_t part_end_lun;
> +
> +	/* ONFI is used to determine the maximum bad block count. */
> +	if (!chip->onfi_version)
> +		return -ENOTSUPP;
> +
> +	/* Get the start and end of the partition in erase blocks. */
> +	part_start_block = mtd_div_by_eb(ofs, mtd);
> +	part_end_block = mtd_div_by_eb(len, mtd) + part_start_block - 1;
> +
> +	/* Get the start and end LUNs of the partition. */
> +	part_start_lun = part_start_block / chip->onfi_params.blocks_per_lun;
> +	part_end_lun = part_end_block / chip->onfi_params.blocks_per_lun;
> +
> +	/*
> +	 * Look up the bad blocks per unit and multiply by the number of units
> +	 * that the partition spans.
> +	 */
> +	return chip->onfi_params.bb_per_lun *
> +		(part_end_lun - part_start_lun + 1);

Hmm, doesn't that calculation assume that all bad blocks are uniformly distributed across
the whole chip?
Not sure if is a good idea to focus on the best case. Currently UBI assumes the worst case.

Thanks,
//richard