Subject: [PATCH] MTD: at91: atmel_nand: return bit flips for the PMECC read_page()

From: [email-masked] (Mike Dunn)
Date: Mon, 26 Nov 2012 12:38:52 -0800

Hi Josh, sorry for the delay.

I guess maybe I missed this one when I did the bitflips patch?  Anyway, please
see below...

On 11/20/2012 09:14 PM, Josh Wu wrote:
> Signed-off-by: Josh Wu <[email-masked]>
> ---
>  drivers/mtd/nand/atmel_nand.c |   13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/mtd/nand/atmel_nand.c b/drivers/mtd/nand/atmel_nand.c
> index 9144557..860dd36 100644
> --- a/drivers/mtd/nand/atmel_nand.c
> +++ b/drivers/mtd/nand/atmel_nand.c
> @@ -723,6 +723,7 @@ static int pmecc_correction(struct mtd_info *mtd, u32 pmecc_stat, uint8_t *buf,
>  	struct atmel_nand_host *host = nand_chip->priv;
>  	int i, err_nbr, eccbytes;
>  	uint8_t *buf_pos;
> +	int total_err = 0;
>  
>  	eccbytes = nand_chip->ecc.bytes;
>  	for (i = 0; i < eccbytes; i++)
> @@ -750,12 +751,13 @@ normal_check:
>  				pmecc_correct_data(mtd, buf_pos, ecc, i,
>  					host->pmecc_bytes_per_sector, err_nbr);
>  				mtd->ecc_stats.corrected += err_nbr;
> +				total_err += err_nbr;
>  			}
>  		}
>  		pmecc_stat >>= 1;
>  	}
>  
> -	return 0;
> +	return total_err;
>  }
>  
>  static int atmel_nand_pmecc_read_page(struct mtd_info *mtd,
> @@ -767,6 +769,7 @@ static int atmel_nand_pmecc_read_page(struct mtd_info *mtd,
>  	uint32_t *eccpos = chip->ecc.layout->eccpos;
>  	uint32_t stat;
>  	unsigned long end_time;
> +	int bitflips = 0;
>  
>  	pmecc_writel(host->ecc, CTRL, PMECC_CTRL_RST);
>  	pmecc_writel(host->ecc, CTRL, PMECC_CTRL_DISABLE);
> @@ -789,11 +792,13 @@ static int atmel_nand_pmecc_read_page(struct mtd_info *mtd,
>  	}
>  
>  	stat = pmecc_readl_relaxed(host->ecc, ISR);
> -	if (stat != 0)
> -		if (pmecc_correction(mtd, stat, buf, &oob[eccpos[0]]) != 0)
> +	if (stat != 0) {
> +		bitflips = pmecc_correction(mtd, stat, buf, &oob[eccpos[0]]);
> +		if (bitflips < 0)
>  			return -EIO;

This is wrong.  In the case of uncorrectible bitflips, read_page() should return
0.  The nand infrastructure code is alerted to the uncorrectible bitflips case
by the ecc_stats, and returns -EBADMSG.  This is the correct return code from
the mtd layer (i.e., mtd_read()) for uncorrectible bitflips.  (See the bottom of
nand_do_read_ops() in nand_base.c.)

Returning 0 when uncorrectible bitflips occur is a bit counter-intuitive and
potentially confusing, I know.  (Fixing that would involve some rework to
nand_base.c.)  This prompted me to add this comment in nand.h:

 * @read_page:	function to read a page according to the ECC generator
 *		requirements; returns maximum number of bitflips corrected in
 *		any single ECC step, 0 if bitflips uncorrectable, -EIO hw error

I made a similiar mistake in the docg4 driver, where I returned -EBADMSG on
uncorrectible errors.  One consequence of that mistake was that mtdchar returned
0 to userspace, which user code interprets as 0 bytes read, and typically
re-issues the read.  When I ran nanddump, the read of a page containing the
uncorrectible errors was repeated over and over.  In the case of this patch as
currently implemented, -EIO will get propagated up as-is, and the entire read
operation will be aborted with an error.  -EIO should only be returned in the
case of a hardware error.

Hope this makes sense.

Mike

> +	}
>  
> -	return 0;
> +	return bitflips;
>  }
>  
>  static int atmel_nand_pmecc_write_page(struct mtd_info *mtd,