phy state machine: failsafe leave invalid RUNNING state

While in RUNNING state, phy_state_machine() checks for link changes by
comparing phydev->link before and after calling phy_read_status().
This works as long as it is guaranteed that phydev->link is never
changed outside the phy_state_machine().

If in some setups this happens, it causes the state machine to miss
a link loss and remain RUNNING despite phydev->link being 0.

This has been observed running a dsa setup with a process continuously
polling the link states over ethtool each second (SNMPD RFC-1213
agent). Disconnecting the link on a phy followed by a ETHTOOL_GSET
causes dsa_slave_get_settings() / dsa_slave_get_link_ksettings() to
call phy_read_status() and with that modify the link status - and
with that bricking the phy state machine.

This patch adds a fail-safe check while in RUNNING, which causes to
move to CHANGELINK when the link is gone and we are still RUNNING.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy.c

@@ -1065,6 +1065,15 @@ void phy_state_machine(struct work_struct *work)
 			if (old_link != phydev->link)
 				phydev->state = PHY_CHANGELINK;
 		}
+		/*
+		 * Failsafe: check that nobody set phydev->link=0 between two
+		 * poll cycles, otherwise we won't leave RUNNING state as long
+		 * as link remains down.
+		 */
+		if (!phydev->link && phydev->state == PHY_RUNNING) {
+			phydev->state = PHY_CHANGELINK;
+			phydev_err(phydev, "no link in PHY_RUNNING\n");
+		}
 		break;
 	case PHY_CHANGELINK:
 		err = phy_read_status(phydev);