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.

quoted

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