Re: [PATCH v3 4/4] mtd: rawnand: micron: support 8/512 on-die ECC

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Wed, 20 Jun 2018 17:05:44 +1200
Chris Packham <chris.packham@alliedtelesis.co.nz> wrote:

> Micron MT29F1G08ABAFAWP-ITE:F supports an on-die ECC with 8 bits
> per 512 bytes. Add support for this combination.
> 
> Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
> ---
> Changes in v2:
> - New
> Changes in v3:
> - Handle reporting of corrected errors that don't require a rewrite, expand
>   comment for the ECC status bits.
> 
>  drivers/mtd/nand/raw/nand_micron.c | 34 ++++++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/mtd/nand/raw/nand_micron.c b/drivers/mtd/nand/raw/nand_micron.c
> index 5cec79372181..0c2bde4411d7 100644
> --- a/drivers/mtd/nand/raw/nand_micron.c
> +++ b/drivers/mtd/nand/raw/nand_micron.c
> @@ -18,10 +18,24 @@
>  #include <linux/mtd/rawnand.h>
>  
>  /*
> - * Special Micron status bit that indicates when the block has been
> - * corrected by on-die ECC and should be rewritten
> + * Special Micron status bit 3 indicates that the block has been
> + * corrected by on-die ECC and should be rewritten.
> + *
> + * On chips with 8-bit ECC and additional bit can be used to distinguish
> + * cases where a errors were corrected without needing a rewrite
> + *
> + * Bit 4 Bit 3 Bit 0 Description
> + * ----- ----- ----- -----------
> + * 0     0     0     No Errors
> + * 0     0     1     Multiple uncorrected errors
> + * 0     1     0     4 - 6 errors corrected, recommend rewrite
> + * 0     0     1     Reserved
> + * 1     0     0     1 - 3 errors corrected
> + * 1     0     1     Reserved
> + * 1     1     0     7 - 8 errors corrected, recommend rewrite
>   */
>  #define NAND_STATUS_WRITE_RECOMMENDED	BIT(3)
> +#define NAND_STATUS_ERRORS_CORRECTED	BIT(4)
>  
>  struct nand_onfi_vendor_micron {
>  	u8 two_plane_read;
> @@ -141,7 +155,7 @@ micron_nand_read_page_on_die_ecc(struct mtd_info *mtd, struct nand_chip *chip,
>  		mtd->ecc_stats.failed++;
>  
>  	/*
> -	 * The internal ECC doesn't tell us the number of bitflips
> +	 * The internal 4-bit ECC doesn't tell us the number of bitflips
>  	 * that have been corrected, but tells us if it recommends to
>  	 * rewrite the block. If it's the case, then we pretend we had
>  	 * a number of bitflips equal to the ECC strength, which will
> @@ -149,6 +163,12 @@ micron_nand_read_page_on_die_ecc(struct mtd_info *mtd, struct nand_chip *chip,
>  	 */
>  	else if (status & NAND_STATUS_WRITE_RECOMMENDED)
>  		max_bitflips = chip->ecc.strength;
> +	/*
> +	 * Chips with 8-bit internal ECC do tell us if errors 1 to 3 bit
> +	 * errors have been corrected without recommending a rewrite.
> +	 */
> +	else if (status & NAND_STATUS_ERRORS_CORRECTED)
> +		max_bitflips = 3;

Why not masking bit 3, 4 and 0 and having a switch-case block?

Also, you should update ecc_stats.corrected (see the patch I just sent
[1]).

>  
>  	ret = nand_read_data_op(chip, buf, mtd->writesize, false);
>  	if (!ret && oob_required)
> @@ -240,9 +260,9 @@ static int micron_supports_on_die_ecc(struct nand_chip *chip)
>  
>  	/*
>  	 * Some Micron NANDs have an on-die ECC of 4/512, some other
> -	 * 8/512. We only support the former.
> +	 * 8/512.
>  	 */
> -	if (chip->ecc_strength_ds != 4)
> +	if (chip->ecc_strength_ds != 4 && chip->ecc_strength_ds != 8)
>  		return MICRON_ON_DIE_UNSUPPORTED;

Given that our on-die-support detection procedure is not reliable, I'd
recommend changing the way we do it and instead base this detection
logic on the model name (in the ONFI param page) or the READ_ID bytes.

>  
>  	return MICRON_ON_DIE_SUPPORTED;
> @@ -274,9 +294,9 @@ static int micron_nand_init(struct nand_chip *chip)
>  			return -EINVAL;
>  		}
>  
> -		chip->ecc.bytes = 8;
> +		chip->ecc.bytes = chip->ecc_strength_ds * 2;
>  		chip->ecc.size = 512;
> -		chip->ecc.strength = 4;
> +		chip->ecc.strength = chip->ecc_strength_ds;
>  		chip->ecc.algo = NAND_ECC_BCH;
>  		chip->ecc.read_page = micron_nand_read_page_on_die_ecc;
>  		chip->ecc.write_page = micron_nand_write_page_on_die_ecc;

[1]http://patchwork.ozlabs.org/patch/932006/