Re: [dpdk-dev] mlx5 & pdump: convert HW timestamps to nanoseconds

[PATRICK KEROULAS <patrick.keroulas@radio-canada.ca>]

> > > > mlx5 part of libibverbs includes a ts-to-ns converter which takes the
> > > > instantaneous clock info. It's unused in dpdk so far. I've tested it in the
> > > > device/port init routine and the result looks reliable. Since this approach
> > > > looks very simple, compared to the time sync mechanism, I'm trying to
> > > > integrate.
> > > >
> > > > The conversion should occur in the primary process (testpmd) I suppose.
> > > > 1) The needed clock info derives from ethernet device. Is it possible to
> > > >    access that struct from a rx callback?
> > > > 2) how to attach the nanosecond to mbuf so that `pdump` catches it?
> > > >    (workaround: copy `mbuf->udata64` in forwarded packets.)
> > > > 3) any other idea?
> > >
> > > The timestamp is carried in mbuf.
> > > Then the conversion must be done by the ethdev caller (application or
> > > any other upper layer).
> >
> > What if the converter function needs a clock_info?
> > https://github.com/linux-rdma/rdma-core/blob/7af01c79e00555207dee6132d72e7bfc1bb5485e/providers/mlx5/mlx5dv.h#L1201
> > I'm affraid this info may change by the time the converter is called
> > by upper layer.
>
> Indeed, the clock in the device is not an atomic one :)
> We need to adjust the time conversion continuously.
> I am not an expert of time synchronization, so I add more people Cc
> who could help for having a precise timestamp.

Thanks Thomas.
Not sure this is a synchronization issue. We have dedicated processes
(linuxptp) to keep both NIC and sys clocks in sync with an external clock.
It is "just" a matter of unit conversion.

If it has to be performed in dpdk-pdump, I would need some help to
retrieve mlx5_clock_info from inside a secondary process. Looking at
mlx5_read_clock(), this info is extracted from ibv_context which looks
reachable in a primary process only (segfault, if I try in pdump).