[PATCH net] net: stmmac: retain PTP clock time during SIOCSHWTSTAMP ioctls

[Jakub Kicinski]

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!