net: phy: add GBit master / slave error detection

Certain PHY's have issues when operating in GBit slave mode and can
be forced to master mode. Examples are RTL8211C, also the Micrel PHY
driver has a DT setting to force master mode.
If two such chips are link partners the autonegotiation will fail.
Standard defines a self-clearing on read, latched-high bit to
indicate this error. Check this bit to inform the user.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy_device.c

@@ -1555,6 +1555,14 @@ int genphy_read_status(struct phy_device *phydev)
 			if (adv < 0)
 				return adv;
 
+			if (lpagb & LPA_1000MSFAIL) {
+				if (adv & CTL1000_ENABLE_MASTER)
+					phydev_err(phydev, "Master/Slave resolution failed, maybe conflicting manual settings?\n");
+				else
+					phydev_err(phydev, "Master/Slave resolution failed\n");
+				return -ENOLINK;
+			}
+
 			phydev->lp_advertising =
 				mii_stat1000_to_ethtool_lpa_t(lpagb);
 			common_adv_gb = lpagb & adv << 2;

include/uapi/linux/mii.h

@@ -136,6 +136,7 @@
 #define CTL1000_ENABLE_MASTER	0x1000
 
 /* 1000BASE-T Status register */
+#define LPA_1000MSFAIL		0x8000	/* Master/Slave resolution failure */
 #define LPA_1000LOCALRXOK	0x2000	/* Link partner local receiver status */
 #define LPA_1000REMRXOK		0x1000	/* Link partner remote receiver status */
 #define LPA_1000FULL		0x0800	/* Link partner 1000BASE-T full duplex */