net: mdio_bus: make mdio_bus_type const

Since commit d492cc25 ("driver core: device.h: make struct
bus_type a const *"), the driver core can properly handle constant
struct bus_type, move the mdio_bus_type variable to be a constant
structure as well, placing it into read-only memory which can not be
modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240213-bus_cleanup-mdio-v1-1-f9e799da7fda@marliere.netSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/phy/mdio_bus.c

@@ -1426,7 +1426,7 @@ static const struct attribute_group *mdio_bus_dev_groups[] = {
 	NULL,
 };
 
-struct bus_type mdio_bus_type = {
+const struct bus_type mdio_bus_type = {
 	.name		= "mdio_bus",
 	.dev_groups	= mdio_bus_dev_groups,
 	.match		= mdio_bus_match,

drivers/net/phy/phy_device.c

@@ -1604,7 +1604,6 @@ EXPORT_SYMBOL(phy_attach_direct);
 struct phy_device *phy_attach(struct net_device *dev, const char *bus_id,
 			      phy_interface_t interface)
 {
-	struct bus_type *bus = &mdio_bus_type;
 	struct phy_device *phydev;
 	struct device *d;
 	int rc;
@@ -1615,7 +1614,7 @@ struct phy_device *phy_attach(struct net_device *dev, const char *bus_id,
 	/* Search the list of PHY devices on the mdio bus for the
 	 * PHY with the requested name
 	 */
-	d = bus_find_device_by_name(bus, NULL, bus_id);
+	d = bus_find_device_by_name(&mdio_bus_type, NULL, bus_id);
 	if (!d) {
 		pr_err("PHY %s not found\n", bus_id);
 		return ERR_PTR(-ENODEV);

include/linux/phy.h

@@ -2129,7 +2129,7 @@ static inline bool phy_package_probe_once(struct phy_device *phydev)
 	return __phy_package_set_once(phydev, PHY_SHARED_F_PROBE_DONE);
 }
 
-extern struct bus_type mdio_bus_type;
+extern const struct bus_type mdio_bus_type;
 
 struct mdio_board_info {
 	const char	*bus_id;