iio: imu: inv_icm42600: avoid frequent timestamp jitter

We are currently synchronizing every time the data timestamp with
the IT timestamp, leading to system jitter jamming timestamps.
To fix that and keep it simple, let's just synchronize when the
delta is bigger than the acceptable jitter, and keep
synchronization at the jitter value.

The result is much stable timestamps reflecting better the real
physical value. Example @50Hz delta timestamp,
* before: 20.123ms, 19.721ms, 20.023ms, 20.353ms, 19.821ms, ...
* after: 20.173ms, 20.173ms, 20.173ms, 20.40ms, 20.173ms, ...

Refactorize code and delete the unnecessary handling of multiple
FIFO data.
Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Link: https://lore.kernel.org/r/20230522093210.817212-1-inv.git-commit@tdk.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/inv_icm42600/inv_icm42600_timestamp.c

@@ -93,8 +93,8 @@ static bool inv_validate_period(uint32_t period, uint32_t mult)
 		return false;
 }
 
-static bool inv_compute_chip_period(struct inv_icm42600_timestamp *ts,
-				    uint32_t mult, uint32_t period)
+static bool inv_update_chip_period(struct inv_icm42600_timestamp *ts,
+				   uint32_t mult, uint32_t period)
 {
 	uint32_t new_chip_period;
 
@@ -104,10 +104,31 @@ static bool inv_compute_chip_period(struct inv_icm42600_timestamp *ts,
 	/* update chip internal period estimation */
 	new_chip_period = period / 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_icm42600_timestamp *ts)
+{
+	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 */
+	jitter = div_s64((int64_t)ts->period * INV_ICM42600_TIMESTAMP_JITTER, 100);
+	if (delta > jitter)
+		adjust = jitter;
+	else if (delta < -jitter)
+		adjust = -jitter;
+	else
+		adjust = 0;
+
+	ts->timestamp += adjust;
+}
+
 void inv_icm42600_timestamp_interrupt(struct inv_icm42600_timestamp *ts,
 				      uint32_t fifo_period, size_t fifo_nb,
 				      size_t sensor_nb, int64_t timestamp)
@@ -116,7 +137,6 @@ void inv_icm42600_timestamp_interrupt(struct inv_icm42600_timestamp *ts,
 	int64_t delta, interval;
 	const uint32_t fifo_mult = fifo_period / INV_ICM42600_TIMESTAMP_PERIOD;
 	uint32_t period = ts->period;
-	int32_t m;
 	bool valid = false;
 
 	if (fifo_nb == 0)
@@ -130,10 +150,7 @@ void inv_icm42600_timestamp_interrupt(struct inv_icm42600_timestamp *ts,
 	if (it->lo != 0) {
 		/* compute period: delta time divided by number of samples */
 		period = div_s64(delta, fifo_nb);
-		valid = inv_compute_chip_period(ts, fifo_mult, period);
-		/* update sensor period if chip internal period is updated */
-		if (valid)
-			ts->period = ts->mult * ts->chip_period.val;
+		valid = inv_update_chip_period(ts, fifo_mult, period);
 	}
 
 	/* no previous data, compute theoritical value from interrupt */
@@ -145,22 +162,8 @@ void inv_icm42600_timestamp_interrupt(struct inv_icm42600_timestamp *ts,
 	}
 
 	/* if interrupt interval is valid, sync with interrupt timestamp */
-	if (valid) {
-		/* compute measured fifo_period */
-		fifo_period = fifo_mult * ts->chip_period.val;
-		/* delta time between last sample and last interrupt */
-		delta = it->lo - ts->timestamp;
-		/* if there are multiple samples, go back to first one */
-		while (delta >= (fifo_period * 3 / 2))
-			delta -= fifo_period;
-		/* compute maximal adjustment value */
-		m = INV_ICM42600_TIMESTAMP_MAX_PERIOD(ts->period) - ts->period;
-		if (delta > m)
-			delta = m;
-		else if (delta < -m)
-			delta = -m;
-		ts->timestamp += delta;
-	}
+	if (valid)
+		inv_align_timestamp_it(ts);
 }
 
 void inv_icm42600_timestamp_apply_odr(struct inv_icm42600_timestamp *ts,