[net-rfc 12/16] can: peak_usb: fix timestamp wrapping

[Marc Kleine-Budde <mkl@pengutronix.de>]

From: Stephane Grosjean <s.grosjean@peak-system.com>

Fabian Inostroza <fabianinostrozap@gmail.com> has discovered a potential
problem in the hardware timestamp reporting from the PCAN-USB USB CAN
interface (only), related to the fact that a timestamp of an event may
precede the timestamp used for synchronization when both records are part
of the same USB packet. However, this case was used to detect the wrapping
of the time counter.

This patch details and fixes the two identified cases where this problem
can occur.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Link: https://lore.kernel.org/r/20201014085631.15128-1-s.grosjean@peak-system.com
Fixes: bb4785551f64 ("can: usb: PEAK-System Technik USB adapters driver core")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
---
 drivers/net/can/usb/peak_usb/pcan_usb_core.c | 51 ++++++++++++++++++--
 1 file changed, 46 insertions(+), 5 deletions(-)

diff --git a/drivers/net/can/usb/peak_usb/pcan_usb_core.c b/drivers/net/can/usb/peak_usb/pcan_usb_core.c
index d91df34e7fa8..c2764799f9ef 100644
--- a/drivers/net/can/usb/peak_usb/pcan_usb_core.c
+++ b/drivers/net/can/usb/peak_usb/pcan_usb_core.c
@@ -130,14 +130,55 @@ void peak_usb_get_ts_time(struct peak_time_ref *time_ref, u32 ts, ktime_t *time)
 	/* protect from getting time before setting now */
 	if (ktime_to_ns(time_ref->tv_host)) {
 		u64 delta_us;
+		s64 delta_ts = 0;
+
+		/* General case: dev_ts_1 < dev_ts_2 < ts, with:
+		 *
+		 * - dev_ts_1 = previous sync timestamp
+		 * - dev_ts_2 = last sync timestamp
+		 * - ts = event timestamp
+		 * - ts_period = known sync period (theoretical)
+		 *             ~ dev_ts2 - dev_ts1
+		 * *but*:
+		 *
+		 * - time counters wrap (see adapter->ts_used_bits)
+		 * - sometimes, dev_ts_1 < ts < dev_ts2
+		 *
+		 * "normal" case (sync time counters increase):
+		 * must take into account case when ts wraps (tsw)
+		 *
+		 *      < ts_period > <          >
+		 *     |             |            |
+		 *  ---+--------+----+-------0-+--+-->
+		 *     ts_dev_1 |    ts_dev_2  |
+		 *              ts             tsw
+		 */
+		if (time_ref->ts_dev_1 < time_ref->ts_dev_2) {
+			/* case when event time (tsw) wraps */
+			if (ts < time_ref->ts_dev_1)
+				delta_ts = 1 << time_ref->adapter->ts_used_bits;
+
+		/* Otherwise, sync time counter (ts_dev_2) has wrapped:
+		 * handle case when event time (tsn) hasn't.
+		 *
+		 *      < ts_period > <          >
+		 *     |             |            |
+		 *  ---+--------+--0-+---------+--+-->
+		 *     ts_dev_1 |    ts_dev_2  |
+		 *              tsn            ts
+		 */
+		} else if (time_ref->ts_dev_1 < ts) {
+			delta_ts = -(1 << time_ref->adapter->ts_used_bits);
+		}
 
-		delta_us = ts - time_ref->ts_dev_2;
-		if (ts < time_ref->ts_dev_2)
-			delta_us &= (1 << time_ref->adapter->ts_used_bits) - 1;
+		/* add delay between last sync and event timestamps */
+		delta_ts += (signed int)(ts - time_ref->ts_dev_2);
 
-		delta_us += time_ref->ts_total;
+		/* add time from beginning to last sync */
+		delta_ts += time_ref->ts_total;
 
-		delta_us *= time_ref->adapter->us_per_ts_scale;
+		/* convert ticks number into microseconds */
+		delta_us = delta_ts * time_ref->adapter->us_per_ts_scale;
 		delta_us >>= time_ref->adapter->us_per_ts_shift;
 
 		*time = ktime_add_us(time_ref->tv_host_0, delta_us);
-- 
2.28.0