linux-kernel - Re: [PATCH net] net: stmmac: retain PTP clock time during SIOCSHWTSTAMP ioctls

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Date:   Sun, 21 Nov 2021 17:46:36 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        Yannick Vignon <yannick.vignon@....com>,
        Michael Olbrich <m.olbrich@...gutronix.de>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        Holger Assmann <h.assmann@...gutronix.de>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: Re: [PATCH net] net: stmmac: retain PTP clock time during
 SIOCSHWTSTAMP ioctls

On Fri, Nov 19, 2021 at 07:58:51PM -0800, Jakub Kicinski wrote:
> On Sat, 20 Nov 2021 01:05:42 +0200 Vladimir Oltean wrote:
> > Currently, when user space emits SIOCSHWTSTAMP ioctl calls such as
> > enabling/disabling timestamping or changing filter settings, the driver
> > reads the current CLOCK_REALTIME value and programming this into the
> > NIC's hardware clock. This might be necessary during system
> > initialization, but at runtime, when the PTP clock has already been
> > synchronized to a grandmaster, a reset of the timestamp settings might
> > result in a clock jump. Furthermore, if the clock is also controlled by
> > phc2sys in automatic mode (where the UTC offset is queried from ptp4l),
> > that UTC-to-TAI offset (currently 37 seconds in 2021) would be
> > temporarily reset to 0, and it would take a long time for phc2sys to
> > readjust so that CLOCK_REALTIME and the PHC are apart by 37 seconds
> > again.
> > 
> > To address the issue, we introduce a new function called
> > stmmac_init_tstamp_counter(), which gets called during ndo_open().
> > It contains the code snippet moved from stmmac_hwtstamp_set() that
> > manages the time synchronization. Besides, the sub second increment
> > configuration is also moved here since the related values are hardware
> > dependent and runtime invariant.
> > 
> > Furthermore, the hardware clock must be kept running even when no time
> > stamping mode is selected in order to retain the synchronized time base.
> > That way, timestamping can be enabled again at any time only with the
> > need to compensate the clock's natural drifting.
> > 
> > As a side effect, this patch fixes the issue that ptp_clock_info::enable
> > can be called before SIOCSHWTSTAMP and the driver (which looks at
> > priv->systime_flags) was not prepared to handle that ordering.
> 
> Makes build fail:
> 
> ERROR: modpost: "stmmac_init_tstamp_counter" [drivers/net/ethernet/stmicro/stmmac/stmmac-platform.ko] undefined!

You're right, I'm missing an EXPORT_SYMBOL, thanks.