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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.

Fixes: 92ba6888

 ("stmmac: add the support for PTP hw clock driver")
Reported-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Holger Assmann <h.assmann@pengutronix.de>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>