Re: Leveraging PTM

On Mon, Aug 21, 2023 at 3:28 PM Miroslav Lichvar <[email protected]> wrote:

> Do you have any recommendations for cheap switches with good PTP
> support? That seems to be the most expensive part.

In the last year, FS.com has released some cheap switches with good
PTP support. I would recommend the IES3110-8TF-R:

https://www.fs.com/products/148180.html

which goes for $159. I maintain a (short) list of cheap PTP switches
here (just updated):

https://github.com/jclark/rpi-cm4-ptp-guide/blob/main/switches.md

> I think there is a CM4 IO board with a PCIe slot which should work
> with the I210.

Yes. The standard CM4 IO board has a PCIe slot, but here are a couple
of practical difficulties:

- there are several nice cases available for the CM4 + IO board, but
none of them have room for a I210; there's an acrylic case, which
leaves the slot exposed, and then you can have a stand with a riser
off to the side
- the Raspberry Pi OS kernel doesn't include the igb driver; I could
compile it myself, but that's not going to be a nice option for the
average user; I thought of trying Fedora, but I'm not sure if its
Raspberry Pi kernel includes the igb driver

> As for the PTP-specific timestamping of the CM4 NIC, I'm interested in
> tests using the NTP-over-PTP transport with the latest chrony version
> (you might need to set hwtstimeout to 0.1 or even longer for the CM4).

That would be interesting. What kind of PTP support in a switch can
the NTP-over-PTP transport take advantage of? I'm guessing E2E
transparent.

> Yes, or even better the SOCK refclock supported in latest linuxptp.

I didn't know the latest linuxptp supported the SOCK refclock. It
looks much nicer than the SHM approach.

> Verifying accuracy is much more difficult.

I was wondering if PTM could help here. The i225-T1 can do PTM and
also output a PPS on its SDPs. So one approach to verifying the
accuracy of
would be to have a system with two NICs, one of which would be the
i225-T1. Use chrony to sync the system clock using one NIC,
and then sync the system clock to the PHC of the i225-T1 using PTM,
and compare its PPS output to the PPS from a GPS using something like
a TAPR TICC.

James

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