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"
Signed-off-by: Johannes Zink <j.zink@pengutronix.de>
Link: https://lore.kernel.org/r/20230719-stmmac_correct_mac_delay-v2-1-3366f38ee9a6@pengutronix.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>