linux-kernel - Re: [PATCH v2] net: stmmac: correct MAC propagation delay

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Message-ID: <20230726-dreamboat-cornhusk-1bd71d19d0d4-mkl@pengutronix.de>
Date:   Wed, 26 Jul 2023 08:04:37 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Johannes Zink <j.zink@...gutronix.de>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Russell King <linux@...linux.org.uk>,
        kernel test robot <lkp@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-arm-kernel@...ts.infradead.org, patchwork-jzi@...gutronix.de
Subject: Re: [PATCH v2] net: stmmac: correct MAC propagation delay

On 25.07.2023 20:22:53, Richard Cochran wrote:
> On Tue, Jul 25, 2023 at 08:06:06PM -0700, Jakub Kicinski wrote:
> 
> > any opinion on this one?
> 
> Yeah, I saw it, but I can't get excited about drivers trying to
> correct delays.  I don't think this can be done automatically in a
> reliable way,

At least the datasheet of the IP core tells to read the MAC delay from
the IP core (1), add the PHY delay (2) and the clock domain crossing
delay (3) and write it to the time stamp correction register.

(1) added in this patch
(2) future work
(3) already in the driver,
    though corrected manually when reading the timestamp

At least in our measurements the peer delay is better with this patch
(measured with ptp4linux) and the end-to-end delay (comparison of 2 PPS
signals on a scope) is also better.

> and so I expect that the few end users who are really
> getting into the microseconds and nanoseconds will calibrate their
> systems end to end, maybe even patching out this driver nonsense in
> their kernels.

What issues make you think this change/approach is counterproductive?

> Having said that, I won't stand in the way of such driver stuff.
> After all, who cares about a few microseconds time error one way or
> the other?

There are several companies that use or plan to use PTP in their
products and are striving to achieve sub-microsecond synchronization.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
Vertretung Nürnberg              | Phone: +49-5121-206917-129 |
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