Re: [PATCH 1/2] iio: invensense: better timestamp alignment when using watermark

[Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>]

On Fri, 17 Jul 2026 14:39:40 +0200
Jean-Baptiste Maneyrol via B4 Relay <devnull+jean-baptiste.maneyrol.tdk.com@kernel.org> wrote:

> From: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
> 
> Current interrupt timestamp alignment only change the next coming
> timestamp. For watermark where we have a batch of samples per
> interrupt, it doesn't manage to align timestamp since modification
> is limited because of jitter.
I've read this a few times and I'm not entirely following the meaning.
Please rewrite. Intent might be something close to:

"Current interrupt timestamp alignment only changes the final timestamp.
When the watermark is in use, we have a batch of samples for each interrupt.
The current code doesn't manage to align the timestamp because the
jitter is too high.

Instead modify the estimated inter interrupt period and use that
to  adjust the timestamp alignment over the batch in a linear fashion.
"

I'm not entirely sure however!


> 
> Implement a better version that instead modify the period to align
> to interrupt timestamp. Period is now computed by align timestamp
> if interrupt interval is valid, or we use the computed estimation.
> 
> Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
> ---
>  .../iio/common/inv_sensors/inv_sensors_timestamp.c | 55 ++++++++--------------
>  1 file changed, 20 insertions(+), 35 deletions(-)
> 
> diff --git a/drivers/iio/common/inv_sensors/inv_sensors_timestamp.c b/drivers/iio/common/inv_sensors/inv_sensors_timestamp.c
> index e0b10366ed2b..2c89e0edc87d 100644
> --- a/drivers/iio/common/inv_sensors/inv_sensors_timestamp.c
> +++ b/drivers/iio/common/inv_sensors/inv_sensors_timestamp.c
> @@ -10,9 +10,7 @@
>  
>  #include <linux/iio/common/inv_sensors_timestamp.h>
>  
> -/* compute jitter, min and max following jitter in per mille */
> -#define INV_SENSORS_TIMESTAMP_JITTER(_val, _jitter)		\
> -	(div_s64((_val) * (_jitter), 1000))
> +/* compute min and max following jitter in per mille */
>  #define INV_SENSORS_TIMESTAMP_MIN(_val, _jitter)		\
>  	(((_val) * (1000 - (_jitter))) / 1000)
>  #define INV_SENSORS_TIMESTAMP_MAX(_val, _jitter)		\
> @@ -102,34 +100,22 @@ static bool inv_update_chip_period(struct inv_sensors_timestamp *ts,
>  	/* update chip internal period estimation */
>  	new_chip_period = period / ts->mult;
>  	inv_update_acc(&ts->chip_period, new_chip_period);
> -	ts->period = ts->mult * ts->chip_period.val;
>  
>  	return true;
>  }
>  
> -static void inv_align_timestamp_it(struct inv_sensors_timestamp *ts)
> +static uint32_t inv_align_timestamp_it(struct inv_sensors_timestamp *ts,
> +				       unsigned int sample_nb)
>  {
> -	const int64_t period_min = (int64_t)ts->min_period * ts->mult;
> -	const int64_t period_max = (int64_t)ts->max_period * ts->mult;
> -	int64_t add_max, sub_max;
> -	int64_t delta, jitter;
> -	int64_t adjust;
> -
> -	/* delta time between last sample and last interrupt */
> -	delta = ts->it.lo - ts->timestamp;
> -
> -	/* adjust timestamp while respecting jitter */
> -	add_max = period_max - (int64_t)ts->period;
> -	sub_max = period_min - (int64_t)ts->period;
> -	jitter = INV_SENSORS_TIMESTAMP_JITTER((int64_t)ts->period, ts->chip.jitter);
> -	if (delta > jitter)
> -		adjust = add_max;
> -	else if (delta < -jitter)
> -		adjust = sub_max;
> -	else
> -		adjust = 0;
> +	const uint64_t period_min = (uint64_t)ts->min_period * ts->mult;
> +	const uint64_t period_max = (uint64_t)ts->max_period * ts->mult;
> +	uint32_t new_period;
>  
> -	ts->timestamp += adjust;
> +	/* compute new period aligning last timestamp with interrupt timestamp */
> +	new_period = div_s64(ts->it.up - ts->timestamp, sample_nb);
> +
> +	/* ensure that period never overflows the jitter */
> +	return clamp(new_period, period_min, period_max);
>  }
>  
>  void inv_sensors_timestamp_interrupt(struct inv_sensors_timestamp *ts,
> @@ -143,6 +129,13 @@ void inv_sensors_timestamp_interrupt(struct inv_sensors_timestamp *ts,
>  	if (sample_nb == 0)
>  		return;
>  
> +	/* no previous data, compute theoretical value from interrupt */
> +	if (ts->timestamp == 0) {
> +		/* elapsed time: sensor period * sensor samples number */
> +		interval = (int64_t)ts->period * (int64_t)sample_nb;
> +		ts->timestamp = timestamp - interval;
> +	}
> +
>  	/* update interrupt timestamp and compute chip and sensor periods */
>  	it = &ts->it;
>  	it->lo = it->up;
> @@ -154,17 +147,9 @@ void inv_sensors_timestamp_interrupt(struct inv_sensors_timestamp *ts,
>  		valid = inv_update_chip_period(ts, period);
>  	}
>  
> -	/* no previous data, compute theoretical value from interrupt */
> -	if (ts->timestamp == 0) {
> -		/* elapsed time: sensor period * sensor samples number */
> -		interval = (int64_t)ts->period * (int64_t)sample_nb;
> -		ts->timestamp = it->up - interval;
> -		return;
> -	}
> -
>  	/* if interrupt interval is valid, sync with interrupt timestamp */
> -	if (valid)
> -		inv_align_timestamp_it(ts);
> +	ts->period = valid ? inv_align_timestamp_it(ts, sample_nb) :
> +			     ts->mult * ts->chip_period.val;
>  }
>  EXPORT_SYMBOL_NS_GPL(inv_sensors_timestamp_interrupt, "IIO_INV_SENSORS_TIMESTAMP");
>  
>