sfc: fix timestamp reconstruction at 16-bit rollover points

We can't just use the top bits of the last sync event as they could be
off-by-one every 65,536 seconds, giving an error in reconstruction of
65,536 seconds.

This patch uses the difference in the bottom 16 bits (mod 2^16) to
calculate an offset that needs to be applied to the last sync event to
get to the current time.
Signed-off-by: Alexandru-Mihai Maftei <amaftei@solarflare.com>
Acked-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/ptp.c

@@ -560,13 +560,45 @@ efx_ptp_mac_nic_to_ktime_correction(struct efx_nic *efx,
 				    u32 nic_major, u32 nic_minor,
 				    s32 correction)
 {
+	u32 sync_timestamp;
 	ktime_t kt = { 0 };
+	s16 delta;
 
 	if (!(nic_major & 0x80000000)) {
 		WARN_ON_ONCE(nic_major >> 16);
-		/* Use the top bits from the latest sync event. */
-		nic_major &= 0xffff;
-		nic_major |= (last_sync_timestamp_major(efx) & 0xffff0000);
+
+		/* Medford provides 48 bits of timestamp, so we must get the top
+		 * 16 bits from the timesync event state.
+		 *
+		 * We only have the lower 16 bits of the time now, but we do
+		 * have a full resolution timestamp at some point in past. As
+		 * long as the difference between the (real) now and the sync
+		 * is less than 2^15, then we can reconstruct the difference
+		 * between those two numbers using only the lower 16 bits of
+		 * each.
+		 *
+		 * Put another way
+		 *
+		 * a - b = ((a mod k) - b) mod k
+		 *
+		 * when -k/2 < (a-b) < k/2. In our case k is 2^16. We know
+		 * (a mod k) and b, so can calculate the delta, a - b.
+		 *
+		 */
+		sync_timestamp = last_sync_timestamp_major(efx);
+
+		/* Because delta is s16 this does an implicit mask down to
+		 * 16 bits which is what we need, assuming
+		 * MEDFORD_TX_SECS_EVENT_BITS is 16. delta is signed so that
+		 * we can deal with the (unlikely) case of sync timestamps
+		 * arriving from the future.
+		 */
+		delta = nic_major - sync_timestamp;
+
+		/* Recover the fully specified time now, by applying the offset
+		 * to the (fully specified) sync time.
+		 */
+		nic_major = sync_timestamp + delta;
 
 		kt = ptp->nic_to_kernel_time(nic_major, nic_minor,
 					     correction);