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SubjectRe: [PATCH v2 5/7] iio: adc: ina2xx: Use a monotonic clock for delay calculation
On Thu, 21 Dec 2017 19:31:36 +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.
>
> Use a monotonic timestamp to track the time for the next poll iteration.
> The timestamp is advanced by the sampling interval each iteration. In case
> the conversion overrruns the register readout (i.e. fast sampling combined
> with a slow bus), one or multiple samples will be dropped.
>
> Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Applied to the togreg branch of iio.git and pushed out as testing for
the autobuilders to play with it.

Thanks,

Jonathan

>
> ---
>
> Changes in v2:
> - add a comment mentioning skipping samples on overrun
>
> drivers/iio/adc/ina2xx-adc.c | 41 +++++++++++++++++++++++++++--------------
> 1 file changed, 27 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c
> index 2621a34ee5c6..415e53b5c0a6 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 +773,28 @@ 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);
> +
> + /*
> + * Advance the timestamp for the next poll by one sampling
> + * interval, and sleep for the remainder (next - now).
> + * In case "next" has already passed, the interval is added
> + * multiple times, i.e. samples are dropped.
> + */
> + do {
> + timespec64_add_ns(&next, 1000 * sampling_us);
> + delta = timespec64_sub(next, now);
> + delay_us = timespec64_to_ns(&delta) / 1000;
> + } while (delay_us <= 0);
> +
> + usleep_range(delay_us, (delay_us * 3) >> 1);
>
> } while (!kthread_should_stop());
>

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