Re: [PATCH] mtd: nand: omap: Fix BCH bit correction

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, Jun 07, 2017 at 08:45:08AM +0200, Sascha Hauer wrote:
> +Cc Matt Reimer <mreimer@sdgsystems.com>
> 
> On Tue, Jun 06, 2017 at 06:10:25PM +0200, Daniel Schultz wrote:
> > After commit dec7b4d2bf9 was applied our barebox only corrected the
> > first 512 Bytes of NAND pages.
> > 
> > This patch separates between Hamming and BCH when finding out the
> > eccsteps, because BCH always works with 2kB pages.
> > 
> > Before this patch:
> > 
> > barebox@Phytec phyCORE AM335x:/ nand_bitflip -r -n 5 /dev/nand0.barebox
> > nand0.barebox: Flipping bit 5 @ 1796
> > nand0.barebox: Flipping bit 6 @ 1258
> > nand0.barebox: Flipping bit 5 @ 1062
> > nand0.barebox: Flipping bit 2 @ 1399
> > nand0.barebox: Flipping bit 6 @ 1243
> > No bitflips found on block 0, offset 0x00000000
> > barebox@Phytec phyCORE AM335x:/ nand_bitflip -r -n 5 /dev/nand0.barebox
> > nand0.barebox: Flipping bit 2 @ 872
> > nand0.barebox: Flipping bit 4 @ 252
> > nand0.barebox: Flipping bit 3 @ 568
> > nand0.barebox: Flipping bit 2 @ 247
> > nand0.barebox: Flipping bit 5 @ 401
> > page at block 0, offset 0x00000000 has 3 bitflips
> > 
> > After this patch:
> > 
> > barebox@Phytec phyCORE AM335x:/ nand_bitflip -r -n 5 /dev/nand0.barebox
> > nand0.barebox: Flipping bit 2 @ 1962
> > nand0.barebox: Flipping bit 0 @ 1563
> > nand0.barebox: Flipping bit 0 @ 1808
> > nand0.barebox: Flipping bit 6 @ 1460
> > nand0.barebox: Flipping bit 7 @ 2034
> > page at block 0, offset 0x00000000 has 5 bitflips
> > barebox@Phytec phyCORE AM335x:/ nand_bitflip -r -n 5 /dev/nand0.barebox
> > nand0.barebox: Flipping bit 1 @ 1352
> > nand0.barebox: Flipping bit 7 @ 1542
> > nand0.barebox: Flipping bit 2 @ 1021
> > nand0.barebox: Flipping bit 7 @ 691
> > nand0.barebox: Flipping bit 6 @ 1196
> > page at block 0, offset 0x00000000 has 10 bitflips, needs cleanup
> > 
> > Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
> > ---
> >  drivers/mtd/nand/nand_omap_gpmc.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/mtd/nand/nand_omap_gpmc.c b/drivers/mtd/nand/nand_omap_gpmc.c
> > index 05c8486..61220da 100644
> > --- a/drivers/mtd/nand/nand_omap_gpmc.c
> > +++ b/drivers/mtd/nand/nand_omap_gpmc.c
> > @@ -302,10 +302,17 @@ static int omap_correct_bch(struct mtd_info *mtd, uint8_t *dat,
> >  	unsigned int err_loc[8];
> >  	int bitflip_count;
> >  	int bch_max_err;
> > +	int eccsteps;
> >  
> > -	int eccsteps = (nand->ecc.mode == NAND_ECC_HW) &&
> > -			(nand->ecc.size == 2048) ? 4 : 1;
> >  	int eccsize = oinfo->nand.ecc.bytes;
> > +	if (oinfo->ecc_mode == OMAP_ECC_HAMMING_CODE_HW_ROMCODE)
> 
> This is wrong. When in Hamming ECC mode you shouldn't get into this
> function. The test should always fail.
> 
> > +		if ((nand->ecc.mode == NAND_ECC_HW) &&
> > +				(nand->ecc.size == 2048))
> > +			eccsteps = 4;
> > +		else
> > +			eccsteps = 1;
> 
> The question is why ecc.size is set to the wrong value in the first
> place:
> 
> 	case OMAP_ECC_BCH8_CODE_HW:
> 		...
> 		oinfo->nand.ecc.size     = 512 * 4;
> 
> This seems to be wrong. The BCH controller works in 512 Byte chunks, so
> ecc.size should be 512. This would make the special cases in
> omap_correct_bch() unnecessary.
> 
> In dec7b4d2bf9 Matt said:
> 
> |  The fix is to pull over a bit of code from the kernel's
> |  omap_correct_data() that sets eccsteps = 4 when the page size is 2048
> |  bytes and hardware ECC is being used.
> 
> In fact, this piece is in the kernel code:
> 
> 	/* Ex NAND_ECC_HW12_2048 */
> 	if ((info->nand.ecc.mode == NAND_ECC_HW) &&
> 			(info->nand.ecc.size  == 2048))
> 		blockCnt = 4;
> 	else
> 		blockCnt = 1;
> 
> I just suspect this is never used, because ecc.size is correctly set to 512 in
> all cases. Then ecc.steps results in 4 for 2k page sizes and the framework correctly
> iterates over the ecc steps.
> 
> Please give the attached test a try. It's completely untested.

And can not work. Additionally eccsteps must be set to 1 in
omap_correct_bch(). This effectively makes the loop in this function
unnecessary which can then removed.

Sascha

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