net: phy: mdio-bcm-unimac: handle broken turn-around for specific PHYs

Some Ethernet PHYs/switches such as Broadcom's BCM53125 have a hardware bug
which makes them not release the MDIO line during turn-around time.  This gets
flagged by the UniMAC MDIO controller as a read failure, and we fail the read
transaction.

Check the MDIO bus phy_ignore_ta_mask bitmask for the PHY we are reading
from and if it is listed in this bitmask, ignore the read failure and
proceed with returning the data we read out of the controller.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mdio-bcm-unimac.c

@@ -81,7 +81,13 @@ static int unimac_mdio_read(struct mii_bus *bus, int phy_id, int reg)
 		return -ETIMEDOUT;
 
 	cmd = __raw_readl(priv->base + MDIO_CMD);
-	if (cmd & MDIO_READ_FAIL)
+
+	/* Some broken devices are known not to release the line during
+	 * turn-around, e.g: Broadcom BCM53125 external switches, so check for
+	 * that condition here and ignore the MDIO controller read failure
+	 * indication.
+	 */
+	if (!(bus->phy_ignore_ta_mask & 1 << phy_id) && (cmd & MDIO_READ_FAIL))
 		return -EIO;
 
 	return cmd & 0xffff;