netdev - Re: [PATCH net-next 0/9] mlxsw: Add PTP support for Spectrum-2 and newer ASICs

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Date:   Wed, 27 Jul 2022 17:44:49 +0300
From:   Ido Schimmel <idosch@...dia.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, edumazet@...gle.com, petrm@...dia.com,
        amcohen@...dia.com, danieller@...dia.com, mlxsw@...dia.com
Subject: Re: [PATCH net-next 0/9] mlxsw: Add PTP support for Spectrum-2 and
 newer ASICs

On Wed, Jul 27, 2022 at 07:16:31AM -0700, Richard Cochran wrote:
> On Wed, Jul 27, 2022 at 09:23:19AM +0300, Ido Schimmel wrote:
> 
> > Specifically, the hardware will subtract the current time stamp from the
> > correction field at the ingress port and will add the current time stamp
> > to the correction field at the egress port.
> 
> Doing this in pure HW TC mode, the time scale of the switch's clock
> does not matter at all.  It can be a free running counter.

It is done in hardware.

> 
> > For the purpose of an
> > ordinary or boundary clock (this patchset), the correction field will
> > always be adjusted between the CPU port and one of the front panel
> > ports, but never between two front panel ports.
> 
> To clarify, the only reason why you say "never between two front panel
> ports" is because the switch will configured not to forward PTP frames
> over the front panel ports, for BC mode.  For TC operation, the switch
> will apply the correction, right?

Right. The hardware can support a TC operation, but I did not find any
Linux interfaces to configure it (did I miss something?) nor got any
requirements to support it at the moment.