platform/chrome: cros_ec_sensorhub: Simplify legacy timestamp spreading

On some machines (nami), interrupt latency cause samples to appear
to be from the future and are pegged to the current time.
We would see samples with this pattern:

[t, t + ~5ms, t + ~10ms, t + ~10ms + 100us, t + ~10ms + 200us],
                             (current now)      (current now)
(t is the last timestamp time)

Last 2 samples would be barely spread, causing applications to
complain.

We now spread the entire sequence. This is not great: in the example
the sensor was supposed to send samples every 5ms, it now appears to
send one every 2.5ms, but it is slightly closer to reality:

sampling time in the example above
At sensor level

1             2             3             4            5
+-----5ms-----+-----5ms-----+-----5ms-----+----5ms-----+---> t

Before, at host level
                       1             2             3 4 5
--interrupt delay------+-----5ms-----+-----5ms-----+-+-+---> t

Afer, at host level
                       1       2       3       4       5
--interrupt delay------+-2.5ms-+-2.5ms-+-2.5ms-+-2.5ms-+---> t
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

drivers/platform/chrome/cros_ec_sensorhub_ring.c

@@ -673,29 +673,22 @@ cros_ec_sensor_ring_spread_add(struct cros_ec_sensorhub *sensorhub,
  * cros_ec_sensor_ring_spread_add_legacy: Calculate proper timestamps then
  * add to ringbuffer (legacy).
  *
- * Note: This assumes we're running old firmware, where every sample's timestamp
- * is after the sample. Run if tight_timestamps == false.
- *
- * If there is a sample with a proper timestamp
+ * Note: This assumes we're running old firmware, where timestamp
+ * is inserted after its sample(s)e. There can be several samples between
+ * timestamps, so several samples can have the same timestamp.
  *
  *                        timestamp | count
  *                        -----------------
- * older_unprocess_out --> TS1      | 1
- *                         TS1      | 2
- *                out -->  TS1      | 3
- *           next_out -->  TS2      |
- *
- * We spread time for the samples [older_unprocess_out .. out]
- * between TS1 and TS2: [TS1+1/4, TS1+2/4, TS1+3/4, TS2].
+ *          1st sample --> TS1      | 1
+ *                         TS2      | 2
+ *                         TS2      | 3
+ *                         TS3      | 4
+ *           last_out -->
  *
- * If we reach the end of the samples, we compare with the
- * current timestamp:
  *
- * older_unprocess_out --> TS1      | 1
- *                         TS1      | 2
- *                 out --> TS1      | 3
+ * We spread time for the samples using perod p = (current - TS1)/4.
+ * between TS1 and TS2: [TS1+p/4, TS1+2p/4, TS1+3p/4, current_timestamp].
  *
- * We know have [TS1+1/3, TS1+2/3, current timestamp]
  */
 static void
 cros_ec_sensor_ring_spread_add_legacy(struct cros_ec_sensorhub *sensorhub,
@@ -708,58 +701,37 @@ cros_ec_sensor_ring_spread_add_legacy(struct cros_ec_sensorhub *sensorhub,
 	int i;
 
 	for_each_set_bit(i, &sensor_mask, sensorhub->sensor_num) {
-		s64 older_timestamp;
 		s64 timestamp;
-		struct cros_ec_sensors_ring_sample *older_unprocess_out =
-			sensorhub->ring;
-		struct cros_ec_sensors_ring_sample *next_out;
-		int count = 1;
-
-		for (out = sensorhub->ring; out < last_out; out = next_out) {
+		int count = 0;
 		s64 time_period;
 
-			next_out = out + 1;
+		for (out = sensorhub->ring; out < last_out; out++) {
 			if (out->sensor_id != i)
 				continue;
 
 			/* Timestamp to start with */
-			older_timestamp = out->timestamp;
-
-			/* Find next sample. */
-			while (next_out < last_out && next_out->sensor_id != i)
-				next_out++;
-
-			if (next_out >= last_out) {
-				timestamp = current_timestamp;
-			} else {
-				timestamp = next_out->timestamp;
-				if (timestamp == older_timestamp) {
-					count++;
-					continue;
+			timestamp = out->timestamp;
+			out++;
+			count = 1;
+			break;
 		}
+		for (; out < last_out; out++) {
+			/* Find last sample. */
+			if (out->sensor_id != i)
+				continue;
+			count++;
 		}
+		if (count == 0)
+			continue;
 
-			/*
-			 * The next sample has a new timestamp, spread the
-			 * unprocessed samples.
-			 */
-			if (next_out < last_out)
-				count++;
-			time_period = div_s64(timestamp - older_timestamp,
-					      count);
+		/* Spread uniformly between the first and last samples. */
+		time_period = div_s64(current_timestamp - timestamp, count);
 
-			for (; older_unprocess_out <= out;
-					older_unprocess_out++) {
-				if (older_unprocess_out->sensor_id != i)
+		for (out = sensorhub->ring; out < last_out; out++) {
+			if (out->sensor_id != i)
 				continue;
-				older_timestamp += time_period;
-				older_unprocess_out->timestamp =
-					older_timestamp;
-			}
-			count = 1;
-			/* The next_out sample has a valid timestamp, skip. */
-			next_out++;
-			older_unprocess_out = next_out;
+			timestamp += time_period;
+			out->timestamp = timestamp;
 		}
 	}