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authorAlex Maftei (amaftei) <amaftei@solarflare.com>2020-02-26 17:33:19 +0000
committerDavid S. Miller <davem@davemloft.net>2020-02-27 12:04:49 -0800
commit23797b98909f34b75fd130369bde86f760db69d0 (patch)
tree8ab3f20d8390108d66db979b51aa460bc14aa2b2
parent3f74957fcbeab703297ed0f135430414ed7e0dd0 (diff)
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>
-rw-r--r--drivers/net/ethernet/sfc/ptp.c38
1 files changed, 35 insertions, 3 deletions
diff --git a/drivers/net/ethernet/sfc/ptp.c b/drivers/net/ethernet/sfc/ptp.c
index af15a737c675..59b4f16896a8 100644
--- 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);

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