[PATCH] mtd: nand: default bitflip-reporting threshold to 75% of correction strength

[Huang Shijie]

On Mon, Jan 12, 2015 at 12:51:29PM -0800, Brian Norris wrote:
> The MTD API reports -EUCLEAN only if the maximum number of bitflips
> found in any ECC block exceeds a certain threshold. This is done to
> avoid excessive -EUCLEAN reports to MTD users, which may induce
> additional scrubbing of data, even when the ECC algorithm in use is
> perfectly capable of handling the bitflips.
> 
> This threshold can be controlled by user-space (via sysfs), to allow
> users to determine what they are willing to tolerate in their
> application. But it still helps to have sane defaults.
> 
> In recent discussion [1], it was pointed out that our default threshold
> is equal to the correction strength. That means that we won't actually
> report any -EUCLEAN (i.e., "bitflips were corrected") errors until there
> are almost too many to handle. It was determined that 3/4 of the
> correction strength is probably a better default.
> 
> [1] http://lists.infradead.org/pipermail/linux-mtd/2015-January/057259.html
> 
> Signed-off-by: Brian Norris <computersforpeace at gmail.com>
> ---
>  drivers/mtd/nand/nand_base.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/nand/nand_base.c b/drivers/mtd/nand/nand_base.c
> index 816b5c1fd416..3f24b587304f 100644
> --- a/drivers/mtd/nand/nand_base.c
> +++ b/drivers/mtd/nand/nand_base.c
> @@ -4171,7 +4171,7 @@ int nand_scan_tail(struct mtd_info *mtd)
>  	 * properly set.
>  	 */
>  	if (!mtd->bitflip_threshold)
> -		mtd->bitflip_threshold = mtd->ecc_strength;
> +		mtd->bitflip_threshold = DIV_ROUND_UP(mtd->ecc_strength * 3, 4);
After this patch, we have to change the bitflip_threshold to
ecc_strength manually when we do the mtd_biterrors.ko test.
Anyway, I think this patch makes sense.

>  
>  	/* Check, if we should skip the bad block table scan */
>  	if (chip->options & NAND_SKIP_BBTSCAN)
> -- 
Acked-by: Huang Shijie <shijie.huang at intel.com>