net: dsa: felix: Don't restart PCS SGMII AN if not needed

Some PHYs like VSC8234 don't like it when AN restarts on their system side
and they restart line side AN too, going into an endless link up/down loop.
Don't restart PCS AN if link is up already.

Although in theory this feedback loop should be possible with the other
in-band AN modes too, for some reason it was not seen with the VSC8514
QSGMII and AQR412 USXGMII PHYs. So keep this logic only for SGMII where
the problem was found.

Fixes: bdeced75 ("net: dsa: felix: Add PCS operations for PHYLINK")
Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/ocelot/felix_vsc9959.c

@@ -645,6 +645,27 @@ static void vsc9959_pcs_init_sgmii(struct phy_device *pcs,
 				   const struct phylink_link_state *state)
 {
 	if (link_an_mode == MLO_AN_INBAND) {
+		int bmsr, bmcr;
+
+		/* Some PHYs like VSC8234 don't like it when AN restarts on
+		 * their system  side and they restart line side AN too, going
+		 * into an endless link up/down loop.  Don't restart PCS AN if
+		 * link is up already.
+		 * We do check that AN is enabled just in case this is the 1st
+		 * call, PCS detects a carrier but AN is disabled from power on
+		 * or by boot loader.
+		 */
+		bmcr = phy_read(pcs, MII_BMCR);
+		if (bmcr < 0)
+			return;
+
+		bmsr = phy_read(pcs, MII_BMSR);
+		if (bmsr < 0)
+			return;
+
+		if ((bmcr & BMCR_ANENABLE) && (bmsr & BMSR_LSTATUS))
+			return;
+
 		/* SGMII spec requires tx_config_Reg[15:0] to be exactly 0x4001
 		 * for the MAC PCS in order to acknowledge the AN.
 		 */