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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>
This commit is contained in:
Alex Maftei (amaftei) 2020-02-26 17:33:19 +00:00 committed by David S. Miller
parent 3f74957fcb
commit 23797b9890
1 changed files with 35 additions and 3 deletions
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38
drivers/net/ethernet/sfc/ptp.c
View File

@ -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);