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

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Martin,

Martin DEVERA <devik@eaxlabs.cz> wrote on Thu, 9 Jan 2020 18:43:46
+0100:

> On 1/9/20 6:22 PM, Miquel Raynal wrote:
> > 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?
> >  
> Can you give me some guidance please ? Where should I add more comment to
> the git commit or to the C code ? I tried to comment both commit and code, probably
> you find the comments not clear enough ?

Sorry for not explaining: Could you add the example to the commit
message? The comment is fine, besides the fact that it should start
like this:

	/*
	 * Bla bla bla

Also please change the commit title, maybe

	mtd: rawnand: Ensure nand_soft_waitrdy wait period is enough

Thanks,
Miquèl

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