phy: Don't increment MDIO bus refcount unless it's a different owner

Commit 3e3aaf64 ("phy: fix mdiobus module safety") fixed the way we
dealt with MDIO bus module reference count, but sort of introduced a
regression in that, if an Ethernet driver registers its own MDIO bus
driver, as is common, we will end up with the Ethernet driver's
module->refnct set to 1, thus preventing this driver from any removal.

Fix this by comparing the network device's device driver owner against
the MDIO bus driver owner, and only if they are different, increment the
MDIO bus module refcount.

Fixes: 3e3aaf64 ("phy: fix mdiobus module safety")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy_device.c

@@ -857,11 +857,17 @@ EXPORT_SYMBOL(phy_attached_print);
 int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
 		      u32 flags, phy_interface_t interface)
 {
+	struct module *ndev_owner = dev->dev.parent->driver->owner;
 	struct mii_bus *bus = phydev->mdio.bus;
 	struct device *d = &phydev->mdio.dev;
 	int err;
 
-	if (!try_module_get(bus->owner)) {
+	/* For Ethernet device drivers that register their own MDIO bus, we
+	 * will have bus->owner match ndev_mod, so we do not want to increment
+	 * our own module->refcnt here, otherwise we would not be able to
+	 * unload later on.
+	 */
+	if (ndev_owner != bus->owner && !try_module_get(bus->owner)) {
 		dev_err(&dev->dev, "failed to get the bus module\n");
 		return -EIO;
 	}
@@ -921,6 +927,7 @@ int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
 
 error:
 	put_device(d);
+	if (ndev_owner != bus->owner)
 		module_put(bus->owner);
 	return err;
 }
@@ -971,6 +978,8 @@ EXPORT_SYMBOL(phy_attach);
  */
 void phy_detach(struct phy_device *phydev)
 {
+	struct net_device *dev = phydev->attached_dev;
+	struct module *ndev_owner = dev->dev.parent->driver->owner;
 	struct mii_bus *bus;
 	int i;
 
@@ -998,6 +1007,7 @@ void phy_detach(struct phy_device *phydev)
 	bus = phydev->mdio.bus;
 
 	put_device(&phydev->mdio.dev);
+	if (ndev_owner != bus->owner)
 		module_put(bus->owner);
 }
 EXPORT_SYMBOL(phy_detach);