net: mdio: Fix lockdep falls positive splat

MDIO devices can be stacked upon each other. The current code supports
two levels, which until recently has been enough for a DSA mdio bus on
top of another bus. Now we have hardware which has an MDIO mux in the
middle.

Define an MDIO MUTEX class with three levels.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mdio-mux.c

@@ -45,13 +45,7 @@ static int mdio_mux_read(struct mii_bus *bus, int phy_id, int regnum)
 	struct mdio_mux_parent_bus *pb = cb->parent;
 	int r;
 
-	/* In theory multiple mdio_mux could be stacked, thus creating
-	 * more than a single level of nesting.  But in practice,
-	 * SINGLE_DEPTH_NESTING will cover the vast majority of use
-	 * cases.  We use it, instead of trying to handle the general
-	 * case.
-	 */
-	mutex_lock_nested(&pb->mii_bus->mdio_lock, SINGLE_DEPTH_NESTING);
+	mutex_lock_nested(&pb->mii_bus->mdio_lock, MDIO_MUTEX_MUX);
 	r = pb->switch_fn(pb->current_child, cb->bus_number, pb->switch_data);
 	if (r)
 		goto out;
@@ -76,7 +70,7 @@ static int mdio_mux_write(struct mii_bus *bus, int phy_id,
 
 	int r;
 
-	mutex_lock_nested(&pb->mii_bus->mdio_lock, SINGLE_DEPTH_NESTING);
+	mutex_lock_nested(&pb->mii_bus->mdio_lock, MDIO_MUTEX_MUX);
 	r = pb->switch_fn(pb->current_child, cb->bus_number, pb->switch_data);
 	if (r)
 		goto out;

drivers/net/phy/mdio_bus.c

@@ -457,7 +457,7 @@ int mdiobus_read_nested(struct mii_bus *bus, int addr, u32 regnum)
 
 	BUG_ON(in_interrupt());
 
-	mutex_lock_nested(&bus->mdio_lock, SINGLE_DEPTH_NESTING);
+	mutex_lock_nested(&bus->mdio_lock, MDIO_MUTEX_NESTED);
 	retval = bus->read(bus, addr, regnum);
 	mutex_unlock(&bus->mdio_lock);
 
@@ -509,7 +509,7 @@ int mdiobus_write_nested(struct mii_bus *bus, int addr, u32 regnum, u16 val)
 
 	BUG_ON(in_interrupt());
 
-	mutex_lock_nested(&bus->mdio_lock, SINGLE_DEPTH_NESTING);
+	mutex_lock_nested(&bus->mdio_lock, MDIO_MUTEX_NESTED);
 	err = bus->write(bus, addr, regnum, val);
 	mutex_unlock(&bus->mdio_lock);
 

include/linux/mdio.h

@@ -13,6 +13,17 @@
 
 struct mii_bus;
 
+/* Multiple levels of nesting are possible. However typically this is
+ * limited to nested DSA like layer, a MUX layer, and the normal
+ * user. Instead of trying to handle the general case, just define
+ * these cases.
+ */
+enum mdio_mutex_lock_class {
+	MDIO_MUTEX_NORMAL,
+	MDIO_MUTEX_MUX,
+	MDIO_MUTEX_NESTED,
+};
+
 struct mdio_device {
 	struct device dev;