net: phylink: Force retrigger in case of latched link-fail indicator

On mv88e6xxx 1G/2.5G PCS, the SerDes register 4.2001.2 has the following
description:
  This register bit indicates when link was lost since the last
  read. For the current link status, read this register
  back-to-back.

Thus to get current link state, we need to read the register twice.

But doing that in the link change interrupt handler would lead to
potentially ignoring link down events, which we really want to avoid.

Thus this needs to be solved in phylink's resolve, by retriggering
another resolve in the event when PCS reports link down and previous
link was up, and by re-reading PCS state if the previous link was down.

The wrong value is read when phylink requests change from sgmii to
2500base-x mode, and link won't come up. This fixes the bug.

Fixes: 9525ae83 ("phylink: add phylink infrastructure")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/phylink.c

@@ -741,6 +741,19 @@ static void phylink_resolve(struct work_struct *w)
 		case MLO_AN_INBAND:
 			phylink_mac_pcs_get_state(pl, &link_state);
 
+			/* The PCS may have a latching link-fail indicator.
+			 * If the link was up, bring the link down and
+			 * re-trigger the resolve. Otherwise, re-read the
+			 * PCS state to get the current status of the link.
+			 */
+			if (!link_state.link) {
+				if (cur_link_state)
+					retrigger = true;
+				else
+					phylink_mac_pcs_get_state(pl,
+								  &link_state);
+			}
+
 			/* If we have a phy, the "up" state is the union of
 			 * both the PHY and the MAC
 			 */