> -----Original Message-----
> From: Miroslav Lichvar <mlich...@redhat.com>
> Sent: Wednesday, March 16, 2022 2:32 AM
> To: Marco Davids (SIDN) <marco.dav...@sidn.nl>
> Cc: linuxptp-users@lists.sourceforge.net
> Subject: Re: [Linuxptp-users] Building a PTP grandmaster with disciplined
> oscillator
>
> On Tue, Mar 15, 2022 at 04:06:31PM +0100, Marco Davids (SIDN) via Linuxptp-
> users wrote:
> > I have a setup with an interface being a PTP client (HW timestamping, a nice
> > Meinberg GM and such) and phc2sys supplying time from it to another
> > interface's PHC in the same server.
> >
> > Is there anything to be said about the accuracy/precision on the second
> > interface's PHC with this approach?
>
> The accuracy and stability is impacted by asymmetry and jitter of the
> PCIe latency. Half of the delay printed by phc2sys gives you an upper
> bound, assuming in one direction the latency is zero and the other
> direction taking all the delay, similarly to the root delay in NTP.
>
> There is a PCIe feature supported on some HW called Precision Time
> Measurement (PTM) that uses HW timestamping of PCIe messages and
> should at least in theory significantly improve the accuracy and
> stability, but the support in the kernel doesn't seem to be finished
> yet.
>
The challenge here is that PCIe PTM also requires device support.
I know some variants of the ice hardware support this, but typically only the
ones that are built onto motherboards.
> If you need best performance, you should synchronize the PHC through
> the system clock. That is, instead of
>
> phc2sys -s eth0 -c eth1
>
> use
>
> phc2sys -s eth0 -c CLOCK_REALTIME
> phc2sys -s CLOCK_REALTIME -c eth1
>
> Ideally, the NICs should be the same model. That will cancel out
> asymmetries in the HW timestamping errors, which can be larger than
> the PCIe asymmetry.
>
This approach allows you to take advantage of PCIe PTM as well, if its
supported. The other approach does not allow any use of PCIe PTM since its
directly tied to using the platform clock.
> --
> Miroslav Lichvar
>
>
>
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