linux-kernel - Re: [PATCH v2] axs_nand - add driver for NAND controller used on Synopsys AXS dev boards

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Message-ID: <1397227889.3284.17.camel@abrodkin-8560l.internal.synopsys.com>
Date:	Fri, 11 Apr 2014 14:51:29 +0000
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
	"Francois.Bedard@...opsys.com" <Francois.Bedard@...opsys.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"computersforpeace@...il.com" <computersforpeace@...il.com>
Subject: Re: [PATCH v2] axs_nand - add driver for NAND controller used on
 Synopsys AXS dev boards

Hi Ezequiel,

On Mon, 2014-04-07 at 10:13 +0400, Alexey Brodkin wrote:
> Hi Ezequiel,
> 
> On Fri, 2014-04-04 at 11:09 -0300, Ezequiel Garcia wrote:
> > On Apr 04, Alexey Brodkin wrote:
> > > Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> > > 
> > 
> > Maybe it would be nice adding some driver description here, so the commit
> > log actually says something useful about the commit.
> 
> Well, I thought about it but frankly I'm not sure if there's anything
> else to add to commit title "driver for NAND controller used on Synopsys
> AXS dev boards".
> 
> Do you think more info may make sense?
> 
> > > +/**
> > > + * axs_flag_wait_and_reset - Waits until requested flag in INT_STATUS register
> > > + *              is set by HW and resets it by writing "1" in INT_CLR_STATUS.
> > > + * @host:	Pointer to private data structure.
> > > + * @flag:	Bit/flag offset in INT_STATUS register
> > > + */
> > > +static void axs_flag_wait_and_reset(struct axs_nand_host *host, int flag)
> > > +{
> > > +	unsigned int i;
> > > +
> > > +	for (i = 0; i < AXS_FLAG_WAIT_DELAY * 100; i++) {
> > > +		unsigned int status = reg_get(host, INT_STATUS);
> > > +
> > > +		if (status & (1 << flag)) {
> > > +			reg_set(host, INT_CLR_STATUS, 1 << flag);
> > > +			return;
> > > +		}
> > > +
> > > +		udelay(10);
> > > +	}
> > > +
> > > +	/*
> > > +	 * Since we cannot report this problem any further than
> > > +	 * axs_nand_{write|read}_buf() letting user know there's a problem.
> > > +	 */
> > > +	dev_err(host->dev, "Waited too long (%d s.) for flag/bit %d\n",
> > > +		AXS_FLAG_WAIT_DELAY, flag);
> > > +}
> > 
> > Hm... I'm not sure the above is really true.
> > 
> > The NAND core uses the replaceable chip->waitfunc callback to check the
> > status of issued commands. See for instance:
> > 
> > static int nand_write_oob_std(struct mtd_info *mtd, struct nand_chip *chip,
> >                               int page)
> > {
> >         int status = 0;
> >         const uint8_t *buf = chip->oob_poi;
> >         int length = mtd->oobsize;
> > 
> >         chip->cmdfunc(mtd, NAND_CMD_SEQIN, mtd->writesize, page);
> >         chip->write_buf(mtd, buf, length);
> >         /* Send command to program the OOB data */
> >         chip->cmdfunc(mtd, NAND_CMD_PAGEPROG, -1, -1);
> > 
> >         status = chip->waitfunc(mtd, chip);
> > 
> >         return status & NAND_STATUS_FAIL ? -EIO : 0;
> > }
> > 
> > On the other side, if you are clearing the flags in axs_flag_wait_and_reset()
> > it might be a bit hard for you to get this right.
> > 
> > IOW, I'm not saying you *must* do this, but instead suggesting that you take
> > a look at waitfunc() and see if it helps report a proper error in the
> > read/write path.
> 
> As I may understand from generic implementation of "waitfunc" in
> "nand_base.c" it checks status of NAND chip itself - which IMHO makes
> sense. And this is done via NAND_CMD_STATUS command sent to chip through
> NAND controller.
> 
> In AXS NAND driver you may see 2 types of wait checks:
> 1. NAND_ISR_CMDDONE - this flag means that NAND controller has just
> executed provided command (i.e. set proper signal on control lines of
> NAND chip etc), but it doesn't mean NAND chip itself has completed
> command execution. That's why "waitfunc" is not direct replacement here.
> 
> 2. NAND_ISR_{RX|TX}DMACOMPLETE - this flag indicates completion of DMA
> transfer to/from NAND chip to RAM. "waitfunc" won't work here as well.
> 
> I hope this explanation makes sense.

Please treat this message as a gentle reminder.
I'm wondering if my explanation in the previous email makes any sense or
I still need to fix stuff.

Regards,
Alexey
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