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SubjectRe: [PATCH v1 5/7] iio: adc: ina2xx: Use a monotonic clock for delay calculation
On Tue, 12 Dec 2017 20:21:14 +0000
Jonathan Cameron <jic23@kernel.org> wrote:

> On Sun, 10 Dec 2017 21:47:37 +0100
> Stefan Brüns <stefan.bruens@rwth-aachen.de> wrote:
>
> > 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@rwth-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@rwth-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.
>
> So this is deliberately skipping a sample if this happens? It was this
> element that I wasn't understanding previously.
> Add a comment in the code to explain this and I'm happy. It's horrible,
> but not much we can do if things are simply going too fast.
I still want to see a comment in the code making it clear what is
happening in that loop. So for now I'm going to stop here in applying
this series.

Jonathan

>
> Thanks for the info.
>
> Jonathan
> >
> > 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
> >
>
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