linux-kernel - Re: [PATCH v1 5/7] iio: adc: ina2xx: Use a monotonic clock for delay calculation

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Date:   Sun, 10 Dec 2017 21:47:37 +0100
From:   Stefan Brüns <stefan.bruens@...h-aachen.de>
To:     Jonathan Cameron <jic23@...nel.org>
CC:     <linux-iio@...r.kernel.org>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Maciej Purski <m.purski@...sung.com>,
        <linux-kernel@...r.kernel.org>, "Andrew F . Davis" <afd@...com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Hartmut Knaack <knaack.h@....de>,
        Javier Martinez Canillas <javier@....samsung.com>
Subject: Re: [PATCH v1 5/7] iio: adc: ina2xx: Use a monotonic clock for delay calculation

On Sunday, December 10, 2017 6:31:57 PM CET Jonathan Cameron wrote:
> On Fri, 8 Dec 2017 18:41:50 +0100
> 
> Stefan Brüns <stefan.bruens@...h-aachen.de> wrote:
> > The iio timestamp clock is user selectable and may be non-monotonic. Also,
> > only part of the acquisition time is measured, thus the delay was longer
> > than intended.
> > 
> > Signed-off-by: Stefan Brüns <stefan.bruens@...h-aachen.de>
> > ---
> > 
> >  drivers/iio/adc/ina2xx-adc.c | 35 +++++++++++++++++++++--------------
> >  1 file changed, 21 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c
> > index 2621a34ee5c6..65bd9e69faf2 100644
> > --- a/drivers/iio/adc/ina2xx-adc.c
> > +++ b/drivers/iio/adc/ina2xx-adc.c
> > @@ -703,10 +703,10 @@ static int ina2xx_work_buffer(struct iio_dev
> > *indio_dev)> 
> >  	/* data buffer needs space for channel data and timestap */
> >  	unsigned short data[4 + sizeof(s64)/sizeof(short)];
> >  	int bit, ret, i = 0;
> > 
> > -	s64 time_a, time_b;
> > +	s64 time;
> > 
> >  	unsigned int alert;
> > 
> > -	time_a = iio_get_time_ns(indio_dev);
> > +	time = iio_get_time_ns(indio_dev);
> > 
> >  	/*
> >  	
> >  	 * Because the timer thread and the chip conversion clock
> > 
> > @@ -752,11 +752,9 @@ static int ina2xx_work_buffer(struct iio_dev
> > *indio_dev)> 
> >  		data[i++] = val;
> >  	
> >  	}
> > 
> > -	time_b = iio_get_time_ns(indio_dev);
> > +	iio_push_to_buffers_with_timestamp(indio_dev, data, time);
> > 
> > -	iio_push_to_buffers_with_timestamp(indio_dev, data, time_a);
> > -
> > -	return (unsigned long)(time_b - time_a) / 1000;
> > +	return 0;
> > 
> >  };
> >  
> >  static int ina2xx_capture_thread(void *data)
> > 
> > @@ -764,7 +762,9 @@ static int ina2xx_capture_thread(void *data)
> > 
> >  	struct iio_dev *indio_dev = data;
> >  	struct ina2xx_chip_info *chip = iio_priv(indio_dev);
> >  	int sampling_us = SAMPLING_PERIOD(chip);
> > 
> > -	int buffer_us, delay_us;
> > +	int ret;
> > +	struct timespec64 next, now, delta;
> > +	s64 delay_us;
> > 
> >  	/*
> >  	
> >  	 * Poll a bit faster than the chip internal Fs, in case
> > 
> > @@ -773,15 +773,22 @@ static int ina2xx_capture_thread(void *data)
> > 
> >  	if (!chip->allow_async_readout)
> >  	
> >  		sampling_us -= 200;
> > 
> > +	ktime_get_ts64(&next);
> > +
> > 
> >  	do {
> > 
> > -		buffer_us = ina2xx_work_buffer(indio_dev);
> > -		if (buffer_us < 0)
> > -			return buffer_us;
> > +		ret = ina2xx_work_buffer(indio_dev);
> > +		if (ret < 0)
> > +			return ret;
> > 
> > -		if (sampling_us > buffer_us) {
> > -			delay_us = sampling_us - buffer_us;
> > -			usleep_range(delay_us, (delay_us * 3) >> 1);
> > -		}
> > +		ktime_get_ts64(&now);
> > +
> > +		do {
> > +			timespec64_add_ns(&next, 1000 * sampling_us);
> > +			delta = timespec64_sub(next, now);
> > +			delay_us = timespec64_to_ns(&delta) / 1000;
> > +		} while (delay_us <= 0);
> 
> Umm. I'm lost, what is the purpose of the above dance?
> A comment perhaps.

next is the timestamp for the next read to happen, now is the current time. 
Obviously we have to sleep for the remainder.

Each sampling interval the "next" timestamp is pushed back by sampling_us. 
Normally this happens exactly once per read, i.e. we schedule the reads to 
happen exactly each sampling interval.

The sampling inteval is *only* added multiple times if it is faster than the 
bus can deliver the data (at 100 kBits/s, each register read takes about 400 
us, so sampling faster than every ~1 ms is not possible.

The old code measured the time spent for reading the registers and slept for 
the remainder of the interval. This way the sampling drifts, as there is some 
time not accounted for - usleep_range, function call overhead, kthread 
interrupted.

Using a timestamp avoids the drift. It also allows simple readjustment of the 
"next" sampling time when polling the status register.

Kind regards,

Stefan

-- 
Stefan Brüns  /  Bergstraße 21  /  52062 Aachen
home: +49 241 53809034     mobile: +49 151 50412019
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