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sfc: Rewrite adjustment of PPS event in a clearer way

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There is substantial latency in generation and handling of PPS events
from the NIC, which we have to correct for before passing a host
timestamp to the PPS subsystem.  We compare clocks with the MC,
giving us two offsets to subtract from the timestamp generated by
pps_get_ts():

(a) Time from the last good sync (where we got host and NIC timestamps
    for nearly the same instant) to the time we called pps_get_ts()
(b) Time from NIC top of second to the last good sync

We currently calculate (a) + (b) in a quite confusing way.
Instead, calculate (a) completely, then add (b) to it.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Ben Hutchings 2014-02-12 18:58:46 +00:00 committed by David S. Miller
parent ce320f44d6
commit 92d8f766ec
1 changed files with 25 additions and 26 deletions
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51
drivers/net/ethernet/sfc/ptp.c
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@ -766,37 +766,36 @@ efx_ptp_process_times(struct efx_nic *efx, MCDI_DECLARE_STRUCT_PTR(synch_buf),
return -EAGAIN;
}
/* Convert the NIC time into kernel time. No correction is required-
* this time is the output of a firmware process.
/* Calculate delay from last good sync (host time) to last_time.
* It is possible that the seconds rolled over between taking
* the start reading and the last value written by the host. The
* timescales are such that a gap of more than one second is never
* expected. delta is *not* normalised.
*/
start_sec = ptp->timeset[last_good].host_start >> MC_NANOSECOND_BITS;
last_sec = last_time->ts_real.tv_sec & MC_SECOND_MASK;
if (start_sec != last_sec &&
((start_sec + 1) & MC_SECOND_MASK) != last_sec) {
netif_warn(efx, hw, efx->net_dev,
"PTP bad synchronisation seconds\n");
return -EAGAIN;
}
delta.tv_sec = (last_sec - start_sec) & 1;
delta.tv_nsec =
last_time->ts_real.tv_nsec -
(ptp->timeset[last_good].host_start & MC_NANOSECOND_MASK);
/* Convert the NIC time at last good sync into kernel time.
* No correction is required - this time is the output of a
* firmware process.
*/
mc_time = ptp->nic_to_kernel_time(ptp->timeset[last_good].major,
ptp->timeset[last_good].minor, 0);
/* Calculate delay from actual PPS to last_time */
delta = ktime_to_timespec(mc_time);
delta.tv_nsec +=
last_time->ts_real.tv_nsec -
(ptp->timeset[last_good].host_start & MC_NANOSECOND_MASK);
/* It is possible that the seconds rolled over between taking
* the start reading and the last value written by the host. The
* timescales are such that a gap of more than one second is never
* expected.
*/
start_sec = ptp->timeset[last_good].host_start >> MC_NANOSECOND_BITS;
last_sec = last_time->ts_real.tv_sec & MC_SECOND_MASK;
if (start_sec != last_sec) {
if (((start_sec + 1) & MC_SECOND_MASK) != last_sec) {
netif_warn(efx, hw, efx->net_dev,
"PTP bad synchronisation seconds\n");
return -EAGAIN;
} else {
delta.tv_sec = 1;
}
} else {
delta.tv_sec = 0;
}
/* Calculate delay from NIC top of second to last_time */
delta.tv_nsec += ktime_to_timespec(mc_time).tv_nsec;
/* Set PPS timestamp to match NIC top of second */
ptp->host_time_pps = *last_time;
pps_sub_ts(&ptp->host_time_pps, delta);