[PATCH net v2] net: dsa: mt7530: fix impossible MDIO address and issue warning

[Daniel Golle]

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 868ff5f4944a ("net: dsa: mt7530-mdio: read PHY address of
switch from device tree") the address in device tree will be taken into
account.

Fixes: b8f126a8d543 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: Daniel Golle <daniel at makrotopia.org>
---
Changes since v1:
 - use FW_WARN as suggested.
 - fix build on net tree which doesn't have 'mdiodev' as member of the
   priv struct. Imho including this patch as fix makes sense to warn
   users about broken firmware, even if the change introducing the
   actual breakage is only present in net-next for now.

 drivers/net/dsa/mt7530-mdio.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/net/dsa/mt7530-mdio.c b/drivers/net/dsa/mt7530-mdio.c
index fa3ee85a99c1..3c21c8d074c7 100644
--- a/drivers/net/dsa/mt7530-mdio.c
+++ b/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)
-- 
2.44.0