net: phylink: introduce helpers for replaying link callbacks

net: phylink: introduce helpers for replaying link callbacks

Some drivers of MAC + tightly integrated PCS (example: SJA1105 + XPCS
covered by same reset domain) need to perform resets at runtime.

The reset is triggered by the MAC driver, and it needs to restore its
and the PCS' registers, all invisible to phylink.

However, there is a desire to simplify the API through which the MAC and
the PCS interact, so this becomes challenging.

Phylink holds all the necessary state to help with this operation, and
can offer two helpers which walk the MAC and PCS drivers again through
the callbacks required during a destructive reset operation. The
procedure is as follows:

Before reset, MAC driver calls phylink_replay_link_begin():
- Triggers phylink mac_link_down() and pcs_link_down() methods

After reset, MAC driver calls phylink_replay_link_end():
- Triggers phylink mac_config() -> pcs_config() -> mac_link_up() ->
  pcs_link_up() methods.

MAC and PCS registers are restored with no other custom driver code.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20260119121954.1624535-3-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>