Re: [PATCH] mtd: rawnand: Fix unexpected timeouts in waitrdy

From: Miquel Raynal <miquel.raynal@bootlin.com>
To: Martin DEVERA <devik@eaxlabs.cz>
Cc: Christophe Kerello <christophe.kerello@st.com>,
	Marek Vasut <marek.vasut@gmail.com>,
	Richard Weinberger <richard@nod.at>,
	linux-kernel@vger.kernel.org,
	Boris Brezillon <boris.brezillon@bootlin.com>,
	linux-mtd@lists.infradead.org, jan.pohanka@merz.cz,
	Brian Norris <computersforpeace@gmail.com>,
	David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH] mtd: rawnand: Fix unexpected timeouts in waitrdy
Date: Thu, 9 Jan 2020 18:22:42 +0100	[thread overview]
Message-ID: <20200109182242.03743cf7@xps13> (raw)
In-Reply-To: <73164aea-d889-21e4-4e7d-345ebd4e5197@eaxlabs.cz>

Hi Martin,

Martin DEVERA <devik@eaxlabs.cz> wrote on Thu, 9 Jan 2020 17:17:30
+0100:

> On 1/9/20 4:37 PM, Miquel Raynal wrote:
> > Hi Martin,
> >
> > Martin Devera <devik@eaxlabs.cz> wrote on Tue, 10 Dec 2019 16:03:18
> > +0100:
> >  
> >> The used way to compute jiffies timeout brokes when
> >> jiffie difference is 1. Simply add 1 - it has no other
> >> side effects.
> >> Fixes STM32MP1 FMC2 NAND controller which sometimes failed
> >> exactly in this way.
> >>
> >> Signed-off-by: Martin Devera <devik@eaxlabs.cz>
> >> ---
> >>   drivers/mtd/nand/raw/nand_base.c | 6 +++++-
> >>   1 file changed, 5 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/mtd/nand/raw/nand_base.c b/drivers/mtd/nand/raw/nand_base.c
> >> index d527e448ce19..beab3a775cc7 100644
> >> --- a/drivers/mtd/nand/raw/nand_base.c
> >> +++ b/drivers/mtd/nand/raw/nand_base.c
> >> @@ -721,7 +721,11 @@ int nand_soft_waitrdy(struct nand_chip *chip, unsigned long timeout_ms)
> >>   	if (ret)
> >>   		return ret;  
> >>   >> -	timeout_ms = jiffies + msecs_to_jiffies(timeout_ms);  
> >> +	/* +1 below is necessary because if we are now in the last fraction
> >> +	 * of jiffy and msecs_to_jiffies is 1 then we will wait only that
> >> +	 * small jiffy fraction - possibly leading to false timeout
> >> +	 */
> >> +	timeout_ms = jiffies + msecs_to_jiffies(timeout_ms) + 1;
> >>   	do {
> >>   		ret = nand_read_data_op(chip, &status, sizeof(status), true);
> >>   		if (ret)  
> > I don't really what you are fixing here, I suspect the root cause to be
> > a wrongly calculated timeout_ms in the calling driver.
> >
> > It is the responsibility of the caller to use this function with a
> > relevant timeout_ms parameter. Maybe Christophe can help you here?
> >  
> Hi Miquel,
> 
> assume that nand_soft_waitrdy is called with timeout_ms==1. I suppose it is
> valid case. Jiffies are 1000 for example (assume something more like 1000.99 -
> just before incrementing to 1001).
> We compute timeout_ms = 1000+msecs_to_jiffies(1) = 1001 (at least for my jiffies rate).
> nand_read_data_op is called for the first time and returns 0. During the call jiffies changes
> to 1001 thus "while loop" ends here (wrongly).
> Notice that routine was called with expected timeout 1ms but actual timeout used was something
> between 0...1ms (which I also measured by tracing & scope on the bus).
> Or is my analysis flawed somewhere ?

I agree with your analysis. Even if nand_soft_waitrdy will no longer be
used by the stm32 driver (Christophe sent a patch for that) I am fine
applying this change.

Could you add a comment to explain the '+1' and resend?

Thanks,
Miquèl

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