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

[Jean-Baptiste Maneyrol]

>
>________________________________________
>From: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>
>Sent: Saturday, July 18, 2026 03:14
>To: Jean-Baptiste Maneyrol via B4 Relay
>Cc: Jean-Baptiste Maneyrol; David Lechner; Nuno Sá; Andy Shevchenko; linux-iio@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>Subject: Re: [PATCH 1/2] iio: invensense: better timestamp alignment when using watermark
>
>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
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>On Fri, 17 Jul 2026 14:39:40 +0200
>Jean-Baptiste Maneyrol via B4 Relay <devnull+jean-baptiste.maneyrol.tdk.com@xxxxxxxxxx> wrote:
>
>> From: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@xxxxxxx>
>>
>> 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!
>

Hello Jonathan,

you're explanation is right. I will change the commit message and use your
rewording.

>
>>
>> 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@xxxxxxx>
>> ---
>>  .../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");
>>
>>

Thanks,
JB