[PATCH v5 6/6] platform/chrome: cros_ec_sensorhub: Drop sensor samples with past timestamps

[Satyam Gupta <sgsatyam@google.com>]

From: Scott Collyer <scollyer@google.com>

When a sensor event timestamp is strictly earlier than the newest
timestamp previously seen for that sensor, drop the Out Of Order (OOO)
sample instead of clamping its timestamp to last_ts. Clamping past
samples to last_ts previously caused downstream batch spreading logic
(cros_ec_sensor_ring_spread_add()) to treat stale events as a batch of
identical timestamps.

Rename cros_ec_sensor_ring_check_for_past_timestamp() to
cros_ec_sensor_ring_is_past_timestamp() returning true when the sample
timestamp is in the past.

Signed-off-by: Scott Collyer <scollyer@google.com>
Signed-off-by: Satyam Gupta <sgsatyam@google.com>
---
 .../platform/chrome/cros_ec_sensorhub_ring.c  | 36 +++++++++++--------
 1 file changed, 22 insertions(+), 14 deletions(-)

diff --git a/drivers/platform/chrome/cros_ec_sensorhub_ring.c b/drivers/platform/chrome/cros_ec_sensorhub_ring.c
index 7740bb42b1df..a5215955470f 100644
--- a/drivers/platform/chrome/cros_ec_sensorhub_ring.c
+++ b/drivers/platform/chrome/cros_ec_sensorhub_ring.c
@@ -427,23 +427,29 @@ cros_ec_sensor_ring_fix_overflow(s64 *ts,
 	state->last = *ts;
 }
 
-static void
-cros_ec_sensor_ring_check_for_past_timestamp(struct cros_ec_sensorhub
-					     *sensorhub,
-					     struct cros_ec_sensors_ring_sample
-					     *sample)
+static bool
+cros_ec_sensor_ring_is_past_timestamp(struct cros_ec_sensorhub *sensorhub,
+				      struct cros_ec_sensors_ring_sample *sample)
 {
 	const u8 sensor_id = sample->sensor_id;
 
-	/* If this event is earlier than one we saw before... */
+	/*
+	 * If this event is strictly earlier than one we saw before, it is truly
+	 * out-of-order and must be dropped to preserve monotonicity.
+	 *
+	 * We intentionally allow duplicate timestamps (==) to pass through here
+	 * because the EC/ISH may report a batch of samples with the same raw
+	 * timestamp. These duplicates are required by the downstream spreading
+	 * logic (cros_ec_sensor_ring_spread_add) to interpolate unique,
+	 * monotonic timestamps for each sample in the batch.
+	 */
 	if (sensorhub->batch_state[sensor_id].newest_sensor_event >
 	    sample->timestamp)
-		/* mark it for spreading. */
-		sample->timestamp =
-			sensorhub->batch_state[sensor_id].last_ts;
-	else
-		sensorhub->batch_state[sensor_id].newest_sensor_event =
-			sample->timestamp;
+		return true;
+
+	sensorhub->batch_state[sensor_id].newest_sensor_event =
+		sample->timestamp;
+	return false;
 }
 
 /**
@@ -576,8 +582,10 @@ cros_ec_sensor_ring_process_event(struct cros_ec_sensorhub *sensorhub,
 	for (axis = 0; axis < 3; axis++)
 		out->vector[axis] = in->data[axis];
 
-	if (sensorhub->tight_timestamps)
-		cros_ec_sensor_ring_check_for_past_timestamp(sensorhub, out);
+	if (sensorhub->tight_timestamps) {
+		if (cros_ec_sensor_ring_is_past_timestamp(sensorhub, out))
+			return false;
+	}
 	return true;
 }
 
-- 
2.56.0.385.gd3acb90ef8-goog