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>

diff --git a/drivers/net/ethernet/sfc/ptp.c b/drivers/net/ethernet/sfc/ptp.c
index af15a73..59b4f16 100644 (file)
--- a/drivers/net/ethernet/sfc/ptp.c
+++ b/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);