linux-kernel - Re: [PATCH v3] workaround regression in ina2xx introduced by cb47755725da("time: Prevent undefined behaviour in timespec64_to

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Date:   Thu, 30 Sep 2021 17:20:18 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Iain Hunter <drhunter95@...il.com>, lothar.felten@...il.com,
        iain@...terembedded.co.uk, Lars-Peter Clausen <lars@...afoo.de>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Gwendal Grignou <gwendal@...omium.org>,
        Matt Ranostay <matt.ranostay@...sulko.com>,
        Arnd Bergmann <arnd@...db.de>,
        Zeng Tao <prime.zeng@...ilicon.com>, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] workaround regression in ina2xx introduced by
 cb47755725da("time: Prevent undefined behaviour in timespec64_to_ns()")

On Thu, 30 Sep 2021 17:18:44 +0100
Jonathan Cameron <jic23@...nel.org> wrote:

> On Sun, 26 Sep 2021 23:18:42 +0200
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > On Sun, Sep 26 2021 at 18:16, Iain Hunter wrote:  
> > > --- a/drivers/iio/adc/ina2xx-adc.c
> > > +++ b/drivers/iio/adc/ina2xx-adc.c
> > > @@ -817,10 +817,10 @@ static int ina2xx_capture_thread(void *data)
> > >  		 */
> > >  		do {
> > >  			timespec64_add_ns(&next, 1000 * sampling_us);
> > > -			delta = timespec64_sub(next, now);
> > > -			delay_us = div_s64(timespec64_to_ns(&delta), 1000);
> > > -		} while (delay_us <= 0);
> > > +		} while (timespec64_compare(&next, &now) < 0);
> > >  
> > > +		delta = timespec64_sub(next, now);
> > > +		delay_us = div_s64(timespec64_to_ns(&delta), 1000);    
> > 
> > This whole timespec dance does not make any sense and can be completely
> > avoided by using just scalar nanoseconds. Untested patch below.
> > 
> > Thanks,
> > 
> >         tglx  
> 
> Thanks Thomas.
> 
> Iain could you test this approach?

Ah. Just seen v4, so I guess you did.

Thanks,

J
> 
> Thanks,
> 
> Jonathan
> 
> > ---
> > --- a/drivers/iio/adc/ina2xx-adc.c
> > +++ b/drivers/iio/adc/ina2xx-adc.c
> > @@ -775,7 +775,7 @@ static int ina2xx_capture_thread(void *d
> >  	struct ina2xx_chip_info *chip = iio_priv(indio_dev);
> >  	int sampling_us = SAMPLING_PERIOD(chip);
> >  	int ret;
> > -	struct timespec64 next, now, delta;
> > +	ktime_t next, now, delta;
> >  	s64 delay_us;
> >  
> >  	/*
> > @@ -785,7 +785,7 @@ static int ina2xx_capture_thread(void *d
> >  	if (!chip->allow_async_readout)
> >  		sampling_us -= 200;
> >  
> > -	ktime_get_ts64(&next);
> > +	next = ktime_get();
> >  
> >  	do {
> >  		while (!chip->allow_async_readout) {
> > @@ -798,7 +798,7 @@ static int ina2xx_capture_thread(void *d
> >  			 * reset the reference timestamp.
> >  			 */
> >  			if (ret == 0)
> > -				ktime_get_ts64(&next);
> > +				next = ktime_get();
> >  			else
> >  				break;
> >  		}
> > @@ -807,7 +807,7 @@ static int ina2xx_capture_thread(void *d
> >  		if (ret < 0)
> >  			return ret;
> >  
> > -		ktime_get_ts64(&now);
> > +		now = ktime_get();
> >  
> >  		/*
> >  		 * Advance the timestamp for the next poll by one sampling
> > @@ -816,11 +816,10 @@ static int ina2xx_capture_thread(void *d
> >  		 * multiple times, i.e. samples are dropped.
> >  		 */
> >  		do {
> > -			timespec64_add_ns(&next, 1000 * sampling_us);
> > -			delta = timespec64_sub(next, now);
> > -			delay_us = div_s64(timespec64_to_ns(&delta), 1000);
> > -		} while (delay_us <= 0);
> > +			next = ktime_add_us(next, sampling_us);
> > +		} while (next <= now);
> >  
> > +		delay_us = ktime_to_us(ktime_sub(next, now));
> >  		usleep_range(delay_us, (delay_us * 3) >> 1);
> >  
> >  	} while (!kthread_should_stop());
> > 
> >   
>