When a network interface is brought down, the associated PHY is stopped.
However, several link-specific parameters within the phy_device struct
are not cleared. This leads to userspace tools like ethtool reporting
stale information from the last active connection, which is misleading
as the link is no longer active.

For example, after running `ip link set dev lan2 down`, ethtool might
show the following outdated information, indicating a 1000Mb/s Full
duplex link with a link partner, even though the link is detected as down:

  # ethtool lan2
  Settings for lan2:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        ...
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
                                             1000baseT/Full
        ...
        Speed: 1000Mb/s
        Duplex: Full
        Auto-negotiation: on
        master-slave cfg: preferred master
        master-slave status: slave
        ...
        Link detected: no

This patch fixes the issue by clearing all outdated link parameters within
the `phy_link_down()` function. This seems to be the correct place to reset
this data, as it is called whenever the link transitions to a down state.

The following parameters are now reset:
- Speed and duplex are set to UNKNOWN.
- Link partner advertising information is zeroed out.
- Master/slave state is set to UNKNOWN.
- MDI-X status is set to INVALID.
- EEE active status is set to false.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
changes v2:
- rebase and resned against net-next

 drivers/net/phy/phy.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/net/phy/phy.c b/drivers/net/phy/phy.c
index 13df28445f02..6bdd49d93740 100644
--- a/drivers/net/phy/phy.c
+++ b/drivers/net/phy/phy.c
@@ -83,6 +83,16 @@ static void phy_link_down(struct phy_device *phydev)
 {
 	phydev->phy_link_change(phydev, false);
 	phy_led_trigger_change_speed(phydev);
+
+	/* Clear the outdated link parameters */
+	phydev->speed = SPEED_UNKNOWN;
+	phydev->duplex = DUPLEX_UNKNOWN;
+	if (phydev->master_slave_state != MASTER_SLAVE_STATE_UNSUPPORTED)
+		phydev->master_slave_state = MASTER_SLAVE_STATE_UNKNOWN;
+	phydev->mdix = ETH_TP_MDI_INVALID;
+	linkmode_zero(phydev->lp_advertising);
+	phydev->eee_active = false;
+
 	WRITE_ONCE(phydev->link_down_events, phydev->link_down_events + 1);
 }

--
2.39.5