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Subject: RE: [PATCH v3 1/1] pcapng: add user-supplied timestamp support
Date: Thu, 18 Jun 2026 18:45:19 +0200
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Thread-Topic: [PATCH v3 1/1] pcapng: add user-supplied timestamp support
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References: <20260618143819.310046-1-dawid.wesierski@intel.com>
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From: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>
To: "Stephen Hemminger" <stephen@networkplumber.org>,
 "Dawid Wesierski" <dawid.wesierski@intel.com>
Cc: <dev@dpdk.org>, <thomas@monjalon.net>,
 "Marek Kasiewicz" <marek.kasiewicz@intel.com>
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> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Thursday, 18 June 2026 17.23
>=20
> On Thu, 18 Jun 2026 10:38:15 -0400
> Dawid Wesierski <dawid.wesierski@intel.com> wrote:
>=20
> > + * @param ts
> > + *   Packet timestamp in nanoseconds since the Unix epoch. If zero,
> the
> > + *   current TSC is captured and converted to epoch ns by
> > + *   rte_pcapng_write_packets() when the packet is written.
> >   *
>=20
> It might help users if a helper rte_tsc_to_epoch() was exposed.

+1

Please note that such a helper would need to compensate for rte_rdtsc() =
drift.

Simplified:

int64_t rte_tsc_to_ns(tsc)
{
  struct timespec ts =3D clock_gettime(CLOCK_REALTIME);
  int64_t now_ns =3D ts.tv_sec * NS_PER_S + ts.tv_nsec;
  int64_t now_tsc =3D rte_rdtsc();
  int64_t diff_tsc =3D tsc - tsc_now;
  return now_ns + diff_tsc * NS_PER_S / rte_get_tsc_hz();
}

A performance optimized version would take "now_ns" and "now_tsc" as =
parameters.
And with "now_ns" passed as a parameter, the function also works with =
other clocks, such as CLOCK_MONOTONIC.

Also see this discussion on the Grout mailing list:
https://inbox.dpdk.org/grout/98CBD80474FA8B44BF855DF32C47DC35F6590E@smart=
server.smartshare.dk/T/#m465a04ca2e8219612dd9c3efb4198d23d5813422