netdev - Re: [RFC PATCH net-next 0/6] ptp: Support hardware clocks with additional free running time

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Date:   Mon, 7 Mar 2022 17:01:31 +0100
From:   Gerhard Engleder <gerhard@...leder-embedded.com>
To:     Michael Walle <michael@...le.cc>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, mlichvar@...hat.com,
        netdev <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        yangbo.lu@....com
Subject: Re: [RFC PATCH net-next 0/6] ptp: Support hardware clocks with
 additional free running time

> From what I understand, you have a second PHC which is just a second
> PHC you cannot control; i.e. it is equivalent to a PHC in free running
> mode. This PHC will take the timestamps for the PTP frames. You can
> create multiple vclocks and you can use ptp4l to synchronize these.
>
> The first (controlable) PHC is used to do the Qbv scheduling, thus
> needs a synchronized time.
>
> How do you synchronize the vclock with this PHC? And how precise
> is it? I know that some cards can do cross timestamping in hardware
> to aid the synchronization (but I think that is not supported right
> now in linux).

There is no need to synchronize the first (controlable) PHC with the vclock.
A ptp4l instance is running and synchronizing the time for Qbv/TAPRIO.
vclocks are used for other time domains, which do not affect Qbv/TAPRIO.

> > You are adding eight bytes per frame for what is arguably an extreme
> > niche case.
>
> I don't think it is a niche case, btw. I was always wondering why
> NXP introduced the vclock thingy. And apparently it is for
> 802.1AS-rev, but one use case for that is 802.1Qbv and there you'll
> need a (synchronized) hardware clock to control the gates. So while
> we can have multiple time domain support with the vclock, we cannot
> use Qbv with them. That was something I have always wondered about.

I agree that most people using Linux have no interest in TSN. For the few
people who are interested in TSN, I assume using two time domains in
combination with Qbv/TAPRIO is a common goal. Is there anyone else who
wants to use two time domains in combination with Qbv/TAPRIO?

Gerhard