Message-ID: <20140722100954.GA29623@pengutronix.de>
Date:	Tue, 22 Jul 2014 12:09:54 +0200
From:	Michael Grzeschik <mgr@...gutronix.de>
To:	Brian Norris <computersforpeace@...il.com>
Cc:	Michael Grzeschik <m.grzeschik@...gutronix.de>,
	linux-mtd@...ts.infradead.org, dwmw2@...radead.org,
	linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATH v2] mxc_nand: use our own read_page function

Hi Brian,

On Mon, Jul 14, 2014 at 12:19:47PM -0700, Brian Norris wrote:
> Hi Michael,
> 
> On Fri, Jun 27, 2014 at 12:38:44PM +0200, Michael Grzeschik wrote:
> > The current approach of the read_page function is to iterate over all
> > subpages and call the correct_data function. The correct_data function
> > currently does the same. It iterates over all subpages and checks for
> > correctable and uncorrectable data. This redundant call for each
> > subpage leads to miscalculations.
> 
> Hmm, you certainly do have some statistic bugs, but I'm not sure you're
> solving this correctly.
> 

This driver sure has.

> > This patch changes the driver to use its own read_page function in which
> > we call the correct_data function only once per page. With that we do
> > the failure and correct statistics counting inside this function.
> > 
> > Signed-off-by: Michael Grzeschik <m.grzeschik@...gutronix.de>
> > ---
> > fixed printk to pr_debug
> > 
> >  drivers/mtd/nand/mxc_nand.c | 73 ++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 69 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/mtd/nand/mxc_nand.c b/drivers/mtd/nand/mxc_nand.c
> > index a72d508..5f9e36d 100644
> > --- a/drivers/mtd/nand/mxc_nand.c
> > +++ b/drivers/mtd/nand/mxc_nand.c
> > @@ -141,6 +141,8 @@ struct mxc_nand_host;
> >  
> >  struct mxc_nand_devtype_data {
> >  	void (*preset)(struct mtd_info *);
> > +	int (*read_page)(struct mtd_info *mtd, struct nand_chip *chip,
> > +			uint8_t *buf, int oob_required, int page);
> >  	void (*send_cmd)(struct mxc_nand_host *, uint16_t, int);
> >  	void (*send_addr)(struct mxc_nand_host *, uint16_t, int);
> >  	void (*send_page)(struct mtd_info *, unsigned int);
> > @@ -649,6 +651,59 @@ static int mxc_nand_correct_data_v1(struct mtd_info *mtd, u_char *dat,
> >  	return 0;
> >  }
> >  
> > +/**
> > + * mxc_nand_read_page_hwecc_v2_v3 - [REPLACEABLE] hardware ECC based page read function
> 
> Don't include the [REPLACEABLE] language here. That's mostly just used
> for code like nand_base, where we provide some defaults / helpers that
> may or may not be intended to allow overrides. (Not really the best
> approach, IMO, but that's beside the point.) So don't mark this in your
> low-level driver.

It was a copy paste leftover. Thanks for the hint.

> 
> > + * @mtd: mtd info structure
> > + * @chip: nand chip info structure
> > + * @buf: buffer to store read data
> > + * @oob_required: caller requires OOB data read to chip->oob_poi
> > + * @page: page number to read
> > + *
> > + * Not for syndrome calculating ECC controllers which need a special oob layout.
> > + */
> > +static int
> 
> I don't think you want a line break here, to match the style of the rest
> of the driver.

I used Lindent as I didn't know how to align it correctly. I will
keep it in one line instead.

> 
> > +mxc_nand_read_page_hwecc_v2_v3(struct mtd_info *mtd,
> > +			       struct nand_chip *chip,
> > +			       uint8_t *buf, int oob_required, int page)
> > +{
> > +	int i, eccsize = chip->ecc.size;
> > +	struct nand_chip *nand_chip = mtd->priv;
> > +	struct mxc_nand_host *host = nand_chip->priv;
> > +	int eccbytes = chip->ecc.bytes;
> > +	int eccsteps = chip->ecc.steps;
> > +	uint8_t *p = buf;
> > +	uint8_t *ecc_calc = chip->buffers->ecccalc;
> > +	uint8_t *ecc_code = chip->buffers->ecccode;
> > +	uint32_t *eccpos = chip->ecc.layout->eccpos;
> > +	unsigned int max_bitflips = 0;
> > +	u32 ecc_stat, err;
> > +	int stat;
> > +
> > +	ecc_stat = host->devtype_data->get_ecc_status(host);
> > +	for (i = 0; eccsteps; eccsteps--, i += eccbytes, p += eccsize) {
> > +		err = ecc_stat & 0xf;
> 
> drivers/mtd/nand/mxc_nand.c: In function 'mxc_nand_read_page_hwecc_v2_v3':
> drivers/mtd/nand/mxc_nand.c:679:16: warning: variable 'err' set but not used [-Wunused-but-set-variable]
> 
> Is that intentional?

No, this is also an leftover. Thanks for the hint.

> 
> > +		chip->ecc.hwctl(mtd, NAND_ECC_READ);
> > +		chip->read_buf(mtd, p, eccsize);
> > +		chip->ecc.calculate(mtd, p, &ecc_calc[i]);
> > +		ecc_stat >>= 4;
> 
> You're also never using the value of ecc_stat, except to calculate other
> values which are never used.
> 
> > +
> 
> Drop the extra blank line.
> 
> > +	}
> > +	ecc_stat = host->devtype_data->get_ecc_status(host);
> 
> Result unused?
> 

Ok, this is odd. I must have messed up my patch here.

> > +	chip->read_buf(mtd, chip->oob_poi, mtd->oobsize);
> > +
> > +	for (i = 0; i < chip->ecc.total; i++)
> > +		ecc_code[i] = chip->oob_poi[eccpos[i]];
> > +
> > +	eccsteps = chip->ecc.steps;
> > +	p = buf;
> > +
> > +	stat = chip->ecc.correct(mtd, p, &ecc_code[i], &ecc_calc[i]);
> > +	max_bitflips = max_t(unsigned int, max_bitflips, stat);
> 
> This is wrong. First, you probably don't want to cast 'stat' to
> unsigned, in case it's an error. Second, absent an error, this just
> resolves to:
> 
> 	max_bitflips = stat;
> 
> So you're not actually determining the maximum per-sector bitflips,
> you're just determining the total # of bitflips.
> 
> > +
> > +	return max_bitflips;
> 
> So I think you have some leftover/unused code in this function

Indeed.

> 
> Additionally, I'm confused because your ecc.correct() function is now
> hiding some of the stat counting -- this is contrary to its usage
> elsewhere. See more comments below.
> 

You are right about the hiding functionality.

> > +}
> > +
> > +
> >  static int mxc_nand_correct_data_v2_v3(struct mtd_info *mtd, u_char *dat,
> >  				 u_char *read_ecc, u_char *calc_ecc)
> >  {
> > @@ -656,7 +711,7 @@ static int mxc_nand_correct_data_v2_v3(struct mtd_info *mtd, u_char *dat,
> >  	struct mxc_nand_host *host = nand_chip->priv;
> >  	u32 ecc_stat, err;
> >  	int no_subpages = 1;
> > -	int ret = 0;
> > +	int ret = 0, broken = 0;
> >  	u8 ecc_bit_mask, err_limit = 0x1;
> >  
> >  	ecc_bit_mask = 0xf;
> > @@ -673,15 +728,21 @@ static int mxc_nand_correct_data_v2_v3(struct mtd_info *mtd, u_char *dat,
> >  	do {
> >  		err = ecc_stat & ecc_bit_mask;
> >  		if (err > err_limit) {
> > -			printk(KERN_WARNING "UnCorrectable RS-ECC Error\n");
> > -			return -1;
> > +			broken++;
> >  		} else {
> >  			ret += err;
> >  		}
> >  		ecc_stat >>= 4;
> >  	} while (--no_subpages);
> >  
> > -	pr_debug("%d Symbol Correctable RS-ECC Error\n", ret);
> > +	mtd->ecc_stats.corrected += ret;
> > +	if (ret)
> > +		pr_debug("%d Symbol Correctable RS-ECC Error\n", ret);
> > +
> > +	mtd->ecc_stats.failed += broken;
> > +	if (broken)
> > +		printk(KERN_WARNING "%d Symbol UnCorrectable RS-ECC Error\n",
> > +			 broken);
> 
> This is wrong; either this function should not be assigned to
> ecc.correct(), or else it should not be modifying the ecc_stats. See all
> other examples of ecc.correct() callbacks. Additionally, it should be
> returning negative to indicate ECC failure, which you're not doing.
> 
> One solution: stop don't assign an chip->ecc.correct() callback any
> more, so that your correction logic is encapsulated entirely within
> chip->read_page(). You would need to make mxc_nand_read_page_hwecc_v2_v3
> call mxc_nand_correct_data_v2_v3() directly, of course. (And you could
> even try a BUG() whenever chip->ecc.correct() is called, like
> cafe_nand.c does.)
> 
> And in fact, I'd just take this one step further; kill
> mxc_nand_correct_data_v2_v3() and just merge it with
> mxc_nand_read_page_hwecc_v2_v3(). Then you can count bitflips (resolving
> the 'max_bitflips' issue I pointed out above) all in one place.
> 

I think your hints all make sense. I was poking around with the
functions we have and need to asign. Unfortunetly I ran into no clear
picture to fix it the correct way and came up with this hacky patch.


> >  
> >  	return ret;
> >  }
> > @@ -1216,6 +1277,7 @@ static const struct mxc_nand_devtype_data imx27_nand_devtype_data = {
> >  /* v21: i.MX25, i.MX35 */
> >  static const struct mxc_nand_devtype_data imx25_nand_devtype_data = {
> >  	.preset = preset_v2,
> > +	.read_page = mxc_nand_read_page_hwecc_v2_v3,
> >  	.send_cmd = send_cmd_v1_v2,
> >  	.send_addr = send_addr_v1_v2,
> >  	.send_page = send_page_v2,
> > @@ -1242,6 +1304,7 @@ static const struct mxc_nand_devtype_data imx25_nand_devtype_data = {
> >  /* v3.2a: i.MX51 */
> >  static const struct mxc_nand_devtype_data imx51_nand_devtype_data = {
> >  	.preset = preset_v3,
> > +	.read_page = mxc_nand_read_page_hwecc_v2_v3,
> >  	.send_cmd = send_cmd_v3,
> >  	.send_addr = send_addr_v3,
> >  	.send_page = send_page_v3,
> > @@ -1269,6 +1332,7 @@ static const struct mxc_nand_devtype_data imx51_nand_devtype_data = {
> >  /* v3.2b: i.MX53 */
> >  static const struct mxc_nand_devtype_data imx53_nand_devtype_data = {
> >  	.preset = preset_v3,
> > +	.read_page = mxc_nand_read_page_hwecc_v2_v3,
> >  	.send_cmd = send_cmd_v3,
> >  	.send_addr = send_addr_v3,
> >  	.send_page = send_page_v3,
> > @@ -1483,6 +1547,7 @@ static int mxcnd_probe(struct platform_device *pdev)
> >  	this->ecc.layout = host->devtype_data->ecclayout_512;
> >  
> >  	if (host->pdata.hw_ecc) {
> > +		this->ecc.read_page = host->devtype_data->read_page;
> >  		this->ecc.calculate = mxc_nand_calculate_ecc;
> >  		this->ecc.hwctl = mxc_nand_enable_hwecc;
> >  		this->ecc.correct = host->devtype_data->correct_data;
> 
> Brian
> 

Many thanks!

Michael

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