net: stmmac: the XPCS obscures a potential "PHY not found" error

stmmac_mdio_register() has logic to search for PHYs on the MDIO bus and
assign them IRQ lines, as well as to set priv->plat->phy_addr.

If no PHY is found, the "found" variable remains set to 0 and the
function errors out.

After the introduction of commit f213bbe ("net: stmmac: Integrate
it with DesignWare XPCS"), the "found" variable was immediately reused
for searching for a PCS on the same MDIO bus.

This can result in 2 types of potential problems (none of them seems to
be seen on the only Intel system that sets has_xpcs = true, otherwise it
would have been reported):

1. If a PCS is found but a PHY is not, then the code happily exits with
   no error. One might say "yes, but this is not possible, because
   of_mdiobus_register will probe a PHY for all MDIO addresses,
   including for the XPCS, so if an XPCS exists, then a PHY certainly
   exists too". Well, that is not true, see intel_mgbe_common_data():

	/* Ensure mdio bus scan skips intel serdes and pcs-xpcs */
	plat->mdio_bus_data->phy_mask = 1 << INTEL_MGBE_ADHOC_ADDR;
	plat->mdio_bus_data->phy_mask |= 1 << INTEL_MGBE_XPCS_ADDR;

2. A PHY is found but an MDIO device with the XPCS PHY ID isn't, and in
   that case, the error message will be "No PHY found". Confusing.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20210527155959.3270478-1-olteanv@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c

@@ -503,6 +503,12 @@ int stmmac_mdio_register(struct net_device *ndev)
 		found = 1;
 	}
 
+	if (!found && !mdio_node) {
+		dev_warn(dev, "No PHY found\n");
+		err = -ENODEV;
+		goto no_phy_found;
+	}
+
 	/* Try to probe the XPCS by scanning all addresses. */
 	if (priv->hw->xpcs) {
 		struct mdio_xpcs_args *xpcs = &priv->hw->xpcs_args;
@@ -511,6 +517,7 @@ int stmmac_mdio_register(struct net_device *ndev)
 
 		xpcs->bus = new_bus;
 
+		found = 0;
 		for (addr = 0; addr < max_addr; addr++) {
 			xpcs->addr = addr;
 
@@ -520,20 +527,22 @@ int stmmac_mdio_register(struct net_device *ndev)
 				break;
 			}
 		}
-	}
 
-	if (!found && !mdio_node) {
-		dev_warn(dev, "No PHY found\n");
-		mdiobus_unregister(new_bus);
-		mdiobus_free(new_bus);
-		return -ENODEV;
+		if (!found && !mdio_node) {
+			dev_warn(dev, "No XPCS found\n");
+			err = -ENODEV;
+			goto no_xpcs_found;
+		}
 	}
 
 bus_register_done:
 	priv->mii = new_bus;
 
 	return 0;
 
+no_xpcs_found:
+no_phy_found:
+	mdiobus_unregister(new_bus);
 bus_register_fail:
 	mdiobus_free(new_bus);
 	return err;