[PATCH 2/2] net: mdio: reset PHY before attempting to access registers in fwnode_mdiobus_register_phy

[Buday Csaba <buday.csaba@prolan.hu>]

When the ID of an ethernet PHY is not provided by the 'compatible'
string in the device tree, its actual ID is read via the MDIO bus.
For some PHYs this could be unsafe, since a hard reset may be
necessary to safely access the MDIO registers.
This patch makes it possible to hard-reset an ethernet PHY before
attempting to read the ID, via a new device tree property, called:
`reset-phy-before-probe`.

There were previous attempts to implement such functionality, I
tried to collect a few of these (see links).

Link: https://lore.kernel.org/lkml/1499346330-12166-2-git-send-email-richard.leitner@skidata.com/
Link: https://lore.kernel.org/all/20230405-net-next-topic-net-phy-reset-v1-0-7e5329f08002@pengutronix.de/
Link: https://lore.kernel.org/netdev/20250709133222.48802-4-buday.csaba@prolan.hu/
Signed-off-by: Buday Csaba <buday.csaba@prolan.hu>
---
 drivers/net/mdio/fwnode_mdio.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/drivers/net/mdio/fwnode_mdio.c b/drivers/net/mdio/fwnode_mdio.c
index ba7091518..6f405df98 100644
--- a/drivers/net/mdio/fwnode_mdio.c
+++ b/drivers/net/mdio/fwnode_mdio.c
@@ -114,6 +114,31 @@ int fwnode_mdiobus_phy_device_register(struct mii_bus *mdio,
 }
 EXPORT_SYMBOL(fwnode_mdiobus_phy_device_register);
 
+static int fwnode_reset_phy_before_probe(struct mii_bus *bus, u32 addr,
+					 struct fwnode_handle *phy_node)
+{
+	struct mdio_device *tmpdev;
+	int err;
+
+	tmpdev = mdio_device_create(bus, addr);
+
+	fwnode_handle_get(phy_node);
+	device_set_node(&tmpdev->dev, phy_node);
+	err = mdio_device_register_reset(tmpdev);
+	if (err)
+		return err;
+
+	mdio_device_reset(tmpdev, 1);
+	mdio_device_reset(tmpdev, 0);
+
+	mdio_device_unregister_reset(tmpdev);
+
+	mdio_device_free(tmpdev);
+	fwnode_handle_put(phy_node);
+
+	return 0;
+}
+
 int fwnode_mdiobus_register_phy(struct mii_bus *bus,
 				struct fwnode_handle *child, u32 addr)
 {
@@ -129,8 +154,11 @@ int fwnode_mdiobus_register_phy(struct mii_bus *bus,
 		return PTR_ERR(mii_ts);
 
 	is_c45 = fwnode_device_is_compatible(child, "ethernet-phy-ieee802.3-c45");
-	if (is_c45 || fwnode_get_phy_id(child, &phy_id))
+	if (is_c45 || fwnode_get_phy_id(child, &phy_id)) {
+		if (fwnode_property_present(child, "reset-phy-before-probe"))
+			fwnode_reset_phy_before_probe(bus, addr, child);
 		phy = get_phy_device(bus, addr, is_c45);
+	}
 	else
 		phy = phy_device_create(bus, addr, phy_id, 0, NULL);
 	if (IS_ERR(phy)) {
-- 
2.39.5