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

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

Hi Martin,

I forgot to mention: please don't forget to keep everyone in copy. I
re-added the mtd-list in my previous answer.

Miquel Raynal <miquel.raynal@bootlin.com> wrote on Thu, 9 Jan 2020
19:02:40 +0100:

> 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




Thanks,
Miquèl

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