Re: [chrony-users] hwtimestamp

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On Sun, Aug 09, 2020 at 12:51:03PM +0100, Nuno Gonçalves wrote:
> I so assume as you say that since this is not working, and the
> hardware does not support non PTP RX timestamps, it is very likely
> that it would not support this on the TX side.

Usually that's not the case, but here it seems to be like that
according to this code:

The RX filter is needed when the NIC cannot provide a timestamp
with each packet, so it needs to be told what (rarely received)
packets it should timestamp. Fortunately, it seems the trend with
modern NICs is going away from this limitation.

For TX timestamping, most NICs don't care what packet it is as the
driver can tell the NIC that it should timestamp this particular
packet when it's queued. But with some NICs that doesn't seem to be
possible, so the NIC is timestamping all PTP-event packets and the
driver needs to get check if a timestamp was requested for a
transmitted packet.

A theoretical possibility to get around this limitation would be to
wrap NTP packets in PTP packets in order to match the HW filter in the
NIC if it allows longer packets. Of course, this non-standard PTP/NTP
would need to be supported on both server and client.

Miroslav Lichvar

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