RE: [PATCH] net/af_packet: add timestamp offloading support

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Stefan Lässer [mailto:stefan.laesser@omicronenergy.com]
> Sent: Friday, 6 September 2024 08.23
> 
> > > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > > Sent: Tuesday, 3 September 2024 18.22
> > >
> > > On Tue, 3 Sep 2024 13:43:06 +0200
> > > Stefan Laesser <stefan.laesser@omicronenergy.com> wrote:
> > >
> > > > Add the packet timestamp from TPACKET_V2 to the mbuf dynamic rx
> > > > timestamp register if offload RTE_ETH_RX_OFFLOAD_TIMESTAMP is
> > > > enabled.
> > > >
> > > > TPACKET_V2 provides the timestamp with nanosecond resolution.
> > > >
> > > > Signed-off-by: Stefan Laesser <stefan.laesser@omicronenergy.com>
> > > > ---
> > > >  .mailmap                                  |  1 +
> > > >  doc/guides/nics/af_packet.rst             |  8 ++++--
> > > >  drivers/net/af_packet/rte_eth_af_packet.c | 34
> > > > +++++++++++++++++++++-
> > > -
> > > >  3 files changed, 38 insertions(+), 5 deletions(-)
> > >
> > > Adding timestamp is good, but it would be better if the timestamp
> > > field was generic. The pcap PMD also has a timestamp, and pdump API
> > > could/should use timestamp as well.
> >
> > As far as I can see, this patch does use the existing cross-driver/generic
> > timestamp dynamic field, like the pcap driver.
> 
> Yes, I use the generic timestamp dynamic field as used in all the other PMDs I
> have looked at.
> 
> >
> > >
> > > What makes sense is for there to be a standard dynamic field for
> > > nanosecond resolution timestamp, and add a make sure that all drivers
> > > use the same base  1/1/1970 same as Linux/Unix.
> >
> > Yes, standardizing on nanosecond resolution and a common base might have
> > been a better choice than using driver-specific units for the generic
> > timestamp dynamic field.
> > If the driver can use the NIC's native clock system, the driver doesn't need
> to
> > convert to nanoseconds, which has a performance cost.
> > However, I suppose any application using timestamps needs to do this
> > conversion in the application instead, so the total performance is the same
> as
> > if the drivers did it. I.e. from a performance perspective, the drivers
> might as
> > well do the conversion, and from a usability perspective, it would be easier
> > with a standard unit and base.
> >
> > We should define a roadmap towards dynamic mbuf field timestamps using
> > fixed unit and base (instead of driver-specific) and migrate towards it.
> >
> > Perhaps start by adding an ethdev capability flag,
> > RTE_ETH_RX_OFFLOAD_TIMESTAMP_NS used together with
> > RTE_ETH_RX_OFFLOAD_TIMESTAMP to indicate that the timestamp unit and
> > base follows a common standard, i.e. nanoseconds since UNIX epoch.
> >
> > There may be other considerations, though: The NIC's clock may drift
> > compared to the CPU's clock, and compared to the clock of other NICs in the
> > same system. So the "base" and "nanoseconds" will still be using the NIC's
> > clock as reference, and it might be way out of sync with the CPU's clock.
> >
> > > Also, having
> > > standard helpers in ethdev for the conversion from TSC to NS would
> > > help.
> >
> > Helpers to convert from CPU TSC to nanoseconds have broader scope than
> > ethdev and belong in the EAL, perhaps in
> > /lib/eal/include/generic/rte_cycles.h?
> 
> Should I extend my patch to include the new RTE_ETH_RX_OFFLOAD_TIMESTAMP_NS
> capability?

That would be nice, but not a requirement. :-)

Please do it as a series of patches, maybe three:
1. This patch.
2. A patch to generally introduce TIMESTAMP_NS RX offload and capability flags.
3. A patch to implement TIMESTAMP_NS in af_packet.

The new TIMESTAMP_NS feature might trigger some discussions, and you don't want this patch caught up too much in that discussion.

> What happens if the user only enables RTE_ETH_RX_OFFLOAD_TIMESTAMP in the
> AF_PACKET PMD?
> I would suggest that in this case the timestamp will have microsecond accuracy
> and only if RTE_ETH_RX_OFFLOAD_TIMESTAMP_NS is also enabled, then the
> timestamp will have nanosecond accuracy.

There's no need for different timestamp accuracy if TIMESTAMP_NS is not enabled.
RTE_ETH_RX_OFFLOAD_TIMESTAMP means that a timestamp is present, with driver dependent clock and base.
The driver is allowed to use nanoseconds as clock and UNIX origo as base, regardless.