net: dsa: mt7530: fix impossible MDIO address and issue warning

The MDIO address of the MT7530 and MT7531 switch ICs can be configured
using bootstrap pins. However, there are only 4 possible options for the
switch itself: 7, 15, 23 and 31. As in MediaTek's SDK the address of the
switch is wrongly stated in the device tree as 0 (while in reality it is
31), warn the user about such broken device tree and make a good guess
what was actually intended.

This is imporant also to not break compatibility with older device trees
as with commit 868ff5f ("net: dsa: mt7530-mdio: read PHY address of
switch from device tree") the address in device tree will be taken into
account.

Fixes: b8f126a ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: Daniel Golle <[email protected]>

drivers/net/dsa/mt7530-mdio.c

@@ -193,6 +193,19 @@ mt7530_probe(struct mdio_device *mdiodev)
 			return PTR_ERR(priv->io_pwr);
 	}
 
+	/* Only MDIO bus address 7, 15, 23 and 31 are valid options */
+	if (~(mdiodev->addr & 0x7) & 0x7) {
+		/* If the address in DT must be wrong, make a good guess about
+		 * the most likely intention, and issue a warning.
+		 */
+		int correct_addr = ((((mdiodev->addr - 7) & ~0x7) % 0x20) + 7) & 0x1f;
+
+		dev_warn(&mdiodev->dev, FW_WARN
+			 "impossible switch MDIO address in device tree: %d, assuming %d\n",
+			 mdiodev->addr, correct_addr);
+		mdiodev->addr = correct_addr;
+	}
+
 	regmap_config = devm_kzalloc(&mdiodev->dev, sizeof(*regmap_config),
 				     GFP_KERNEL);
 	if (!regmap_config)