net: stmmac: Add a devlink attribute to control timestamping mode

The DWMAC1000 supports 2 timestamping configurations to configure how
frequency adjustments are made to the ptp_clock, as well as the reported
timestamp values.

There was a previous attempt at upstreaming support for configuring this
mode by Olivier Dautricourt and Julien Beraud a few years back [1]

In a nutshell, the timestamping can be either set in fine mode or in
coarse mode.

In fine mode, which is the default, we use the overflow of an accumulator to
trigger frequency adjustments, but by doing so we lose precision on the
timetamps that are produced by the timestamping unit. The main drawback
is that the sub-second increment value, used to generate timestamps, can't be
set to lower than (2 / ptp_clock_freq).

The "fine" qualification comes from the frequent frequency adjustments we are
able to do, which is perfect for a PTP follower usecase.

In Coarse mode, we don't do frequency adjustments based on an
accumulator overflow. We can therefore have very fine subsecond
increment values, allowing for better timestamping precision. However
this mode works best when the ptp clock frequency is adjusted based on
an external signal, such as a PPS input produced by a GPS clock. This
mode is therefore perfect for a Grand-master usecase.

Introduce a driver-specific devlink parameter "ts_coarse" to enable or
disable coarse mode, keeping the "fine" mode as a default.

This can then be changed with:

  devlink dev param set <dev> name ts_coarse value true cmode runtime

The associated documentation is also added.

[1] : https://lore.kernel.org/netdev/20200514102808.31163-1-olivier.dautricourt@orolia.com/

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20251024070720.71174-3-maxime.chevallier@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/devlink/index.rst

@@ -99,5 +99,6 @@ parameters, info versions, and other features it supports.
    prestera
    qed
    sfc
+   stmmac
    ti-cpsw-switch
    zl3073x