linux-kernel - Re: [PATCH v6 00/17] memory: omap-gpmc: mtd: nand: Support GPMC NAND on non-OMAP platforms

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Date:	Mon, 18 Apr 2016 16:10:01 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Roger Quadros <rogerq@...com>
Cc:	Tony Lindgren <tony@...mide.com>, <javier@...hile0.org>,
	<fcooper@...com>, <nsekhar@...com>,
	<linux-mtd@...ts.infradead.org>, <linux-omap@...r.kernel.org>,
	<computersforpeace@...il.com>, <dwmw2@...radead.org>,
	<ezequiel@...guardiasur.com.ar>, <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 00/17] memory: omap-gpmc: mtd: nand: Support GPMC
 NAND on non-OMAP platforms

On Mon, 18 Apr 2016 16:48:26 +0300
Roger Quadros <rogerq@...com> wrote:

> Boris,
> 
> On 18/04/16 16:13, Boris Brezillon wrote:
> > Hi Roger,
> > 
> > On Mon, 18 Apr 2016 15:52:58 +0300
> > Roger Quadros <rogerq@...com> wrote:
> > 
> >> On 18/04/16 15:31, Roger Quadros wrote:
> >>> On 16/04/16 11:57, Boris Brezillon wrote:
> >>>> On Fri, 15 Apr 2016 09:19:51 -0700
> >>>> Tony Lindgren <tony@...mide.com> wrote:
> >>>>
> >>>>>
> >>>>>> Or should I just pull this immutable branch in my current nand/next and
> >>>>>> let you pull the same immutable branch in omap-soc. I mean, would this
> >>>>>> prevent conflicts when our branches are merged into linux-next, no
> >>>>>> matter the order.
> >>>>>
> >>>>> Ideally just one or more branches with just minimal changes in
> >>>>> them against -rc1. But you may have other dependencies in
> >>>>> your NAND tree so that may no longer be doable :) Usually if
> >>>>> I merge something that may need to get merged into other
> >>>>> branches, I just apply them into a separate branch against -rc1
> >>>>> to start with, then merge that branch in.
> >>>>
> >>>> Okay, in this case, that's pretty much what I did from the beginning,
> >>>> except the immutable branch was provided by Roger (based on 4.6-rc1).
> >>>> Thanks for this detailed explanation, I'll try to remember that when
> >>>> I'll need to provide an immutable branch for another subsystem.
> >>>>
> >>>> Roger, my request remains, could you check/test my conflict resolution
> >>>> (branch nand/next-with-gpmc-rework)?
> >>>
> >>> I couldn't test that branch yet as nand/next is broken on omap platforms
> >>> (at least on dra7-evm).
> >>>
> >>> The commit where it breaks is:
> >>> a662ef4 mtd: nand: omap2: use mtd_ooblayout_xxx() helpers where appropriate
> >>>
> >>> I'm trying to figure out what went wrong there. Failure log below.
> >>
> >> OK. I was able to fix it when at commit a662ef4 with the below patch.
> > 
> > Thanks for debugging that.
> > 
> >>
> >> Looks like we need to read exactly the ECC bytes through the ECC engine and not
> >> the entire OOB region.
> > 
> > Hm, it looks like there's a bug somewhere else, because I don't see any
> > reason why the controller wouldn't be able to read the full OOB region.
> 
> The controller can read the full OOB region but we only want it to read just
> the ECC bytes because that is the way the ELM ECC engine works.

Ok, I think I got it: the ECC correction is pipelined with data read,
and the controller expect to have ECC bytes right after the in-band
data, is that correct?

> 
> >>
> >> diff --git a/drivers/mtd/nand/omap2.c b/drivers/mtd/nand/omap2.c
> >> index e622a1b..46b61d2 100644
> >> --- a/drivers/mtd/nand/omap2.c
> >> +++ b/drivers/mtd/nand/omap2.c
> >> @@ -1547,8 +1547,8 @@ static int omap_read_page_bch(struct mtd_info *mtd, struct nand_chip *chip,
> >>  	chip->read_buf(mtd, buf, mtd->writesize);
> >>  
> >>  	/* Read oob bytes */
> >> -	chip->cmdfunc(mtd, NAND_CMD_RNDOUT, mtd->writesize, -1);
> >> -	chip->read_buf(mtd, chip->oob_poi, mtd->oobsize);
> >> +	chip->cmdfunc(mtd, NAND_CMD_RNDOUT, mtd->writesize + chip->ecc.layout->eccpos[0], -1);
> > 
> > The whole point of this series is to get rid of chip->ecc.layout, so
> > we'd rather use the mtd_ooblayout_find_eccregion() instead of
> > chip->ecc.layout->eccpos[0].
> 
> We just need the position of the first ECC byte offset.
> Is that the most optimal way to get it?

For the BCH case, it seems that ECC bytes always start at offset
BADBLOCK_MARKER_LENGTH, so you can just pass
mtd->writesize + BADBLOCK_MARKER_LENGTH.

Let me know if this works, and I'll squash those changes into the
faulty commit (I know this implies a rebase + push -f, but IMO that's
better than breaking bisectability).


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com