netdev - Re: [PATCH spi for-5.4 0/5] Deterministic SPI latency with NXP DSPI driver

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Date:   Thu, 22 Aug 2019 19:13:12 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Hubert Feurstein <h.feurstein@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-spi@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH spi for-5.4 0/5] Deterministic SPI latency with NXP DSPI driver

On Thu, 22 Aug 2019 at 19:05, Richard Cochran <richardcochran@...il.com> wrote:
>
> On Thu, Aug 22, 2019 at 05:58:49PM +0300, Vladimir Oltean wrote:
> > I don't think I understand the problem here.
>
> On the contrary, I do.
>

You do think that I understand the problem? But I don't!

> > You'd have something like this:
> >
> > Master (DSA master port)         Slave (switch CPU port)
> >
> >     |                            |         Tstamps known
> >     |                            |         to slave
> >     |       Local_sync_req       |
> >  t1 |------\                     |         t1
> >     |       \-----\              |
> >     |              \-----\       |
> >     |                     \----->| t2      t1, t2
> >     |                            |
> >     |     Local_sync_resp /------| t3      t1, t2, t3
> >     |              /-----/       |
> >     |       /-----/              |
> >  t4 |<-----/                     |         t1, t2, t3, t4
> >     |                            |
> >     |                            |
> >     v           time             v
>
> And who generates Local_sync_resp?
>

Local_sync_resp is the same as Local_sync_req except maybe with a
custom tag added by the switch. Irrelevant as long as the DSA master
can timestamp it.

> Also, what sort of frame is it?  PTP has no Sync request or response.
>

A frame that can be timestamped on RX and TX by the DSA switch and
master, that is not a PTP frame.

> > But you don't mean a TX timestamp at the egress of swp4 here, do you?
>
> Yes, I do.
>
> > Why would that matter?
>
> Because in order to synchronize to an external GM, you need to measure
> two things:
>
> 1. the (unchanging) delay from MAC to MAC
> 2. the (per-packet) switch residence time
>

But since when are we discussing the synchronization to an external
grandmaster? I don't see the connection.

> Thanks,
> Richard

Regards,
-Vladimir