net: phy: allow scanning busses with missing phys

Some MDIO busses will error out when trying to read a phy address with no
phy present at that address. In that case, probing the bus will fail
because __mdiobus_register() is scanning the bus for all possible phys
addresses.

In case MII_PHYSID1 returns -EIO or -ENODEV, consider there is no phy at
this address and set the phy ID to 0xffffffff which is then properly
handled in get_phy_device().
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy_device.c

@@ -535,8 +535,17 @@ static int get_phy_id(struct mii_bus *bus, int addr, u32 *phy_id,
 
 	/* Grab the bits from PHYIR1, and put them in the upper half */
 	phy_reg = mdiobus_read(bus, addr, MII_PHYSID1);
-	if (phy_reg < 0)
+	if (phy_reg < 0) {
+		/* if there is no device, return without an error so scanning
+		 * the bus works properly
+		 */
+		if (phy_reg == -EIO || phy_reg == -ENODEV) {
+			*phy_id = 0xffffffff;
+			return 0;
+		}
+
 		return -EIO;
+	}
 
 	*phy_id = (phy_reg & 0xffff) << 16;