netdev - Re: [PATCH] ptp: Add PTP_CLOCK_EXTTSUSR internal ptp

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Message-ID: <20210630221608.h7xdvmsc4tdgqeof@bsd-mbp.dhcp.thefacebook.com>
Date:   Wed, 30 Jun 2021 15:16:08 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     "Machnikowski, Maciej" <maciej.machnikowski@...el.com>
Cc:     Richard Cochran <richardcochran@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kernel-team@...com" <kernel-team@...com>
Subject: Re: [PATCH] ptp: Add PTP_CLOCK_EXTTSUSR internal ptp_event

On Wed, Jun 30, 2021 at 03:55:03PM +0000, Machnikowski, Maciej wrote:
> 
> 
> > -----Original Message-----
> > From: Richard Cochran <richardcochran@...il.com>
> > Sent: Wednesday, June 30, 2021 4:43 PM
> > To: Jonathan Lemon <jonathan.lemon@...il.com>
> > Cc: netdev@...r.kernel.org; kernel-team@...com
> > Subject: Re: [PATCH] ptp: Add PTP_CLOCK_EXTTSUSR internal ptp_event
> > 
> > On Tue, Jun 29, 2021 at 08:50:31PM -0700, Jonathan Lemon wrote:
> > > The PHC should be sync'd to the PPS coming from the GPS signal.
> > > However, the GPS may be in holdover, so the actual counter comes from
> > > an atomic oscillator.  As the oscillator may be ever so slightly out
> > > of sync with the GPS (or drifts with temperature), so we need to
> > > measure the phase difference between the two and steer the oscillator
> > > slightly.
> > >
> > > The phase comparision between the two signals is done in HW with a
> > > phasemeter, for precise comparisons.  The actual phase
> > > steering/adjustment is done through adjphase().
> > 
> 
> You can use different channel index in the struct ptp_clock_event to receive 
> them from more than one source. Then just calculate the difference between 
> the 1PPS from channel 0 and channel 1. Wouldn't that be sufficient?

This is what is being done right now - just in hardware for higher precision.
I asked the HW guys to check whether doing this in SW instead will provide
the same precision - I should hear back by tomorrow.
-- 
Jonathan