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

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Message-ID: <20230726-subsector-unguided-8f1fc1edb037-mkl@pengutronix.de>
Date:   Wed, 26 Jul 2023 07:50:11 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Richard Cochran <richardcochran@...il.com>,
        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 20:06:06, Jakub Kicinski wrote:
> On Mon, 24 Jul 2023 12:01:31 +0200 Johannes Zink wrote:
> > The IEEE1588 Standard specifies that the timestamps of Packets must be
> > captured when the PTP message timestamp point (leading edge of first
> > octet after the start of frame delimiter) crosses the boundary between
> > the node and the network. As the MAC latches the timestamp at an
> > internal point, the captured timestamp must be corrected for the
> > additional path latency, as described in the publicly available
> > datasheet [1].
> > 
> > This patch only corrects for the MAC-Internal delay, which can be read
> > out from the MAC_Ingress_Timestamp_Latency register, since the Phy
> > framework currently does not support querying the Phy ingress and egress
> > latency. The Closs Domain Crossing Circuits errors as indicated in [1]
> > are already being accounted in the stmmac_get_tx_hwtstamp() function and
> > are not corrected here.
> > 
> > As the Latency varies for different link speeds and MII
> > modes of operation, the correction value needs to be updated on each
> > link state change.
> > 
> > As the delay also causes a phase shift in the timestamp counter compared
> > to the rest of the network, this correction will also reduce phase error
> > when generating PPS outputs from the timestamp counter.
> > 
> > [1] i.MX8MP Reference Manual, rev.1 Section 11.7.2.5.3 "Timestamp
> > correction"
> 
> Hi Richard,
> 
> any opinion on this one?
> 
> The subject read to me like it's about *MII clocking delays, I figured
> you may have missed it, too.

The patch description clarifies what is being corrected, namely the
"MAC-internal delay, which can be read out from the
MAC_Ingress_Timestamp_Latency register".

The next step would be to correct PHY latency, but there is no support
for querying PHY latency yet.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
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