net: dsa: Add lockdep class to tx queues to avoid lockdep splat

DSA stacks an Ethernet device on top of an Ethernet device. This can
cause false positive lockdep splats for the transmit queue:
Acked-by: NFlorian Fainelli <f.fainelli@gmail.com>

=============================================
[ INFO: possible recursive locking detected ]
4.0.0-rc7-01838-g70621a215fc7 #386 Not tainted
---------------------------------------------
kworker/0:0/4 is trying to acquire lock:
 (_xmit_ETHER#2){+.-...}, at: [<c040e95c>] sch_direct_xmit+0xa8/0x1fc

but task is already holding lock:
 (_xmit_ETHER#2){+.-...}, at: [<c03f4208>] __dev_queue_xmit+0x4d4/0x56c

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(_xmit_ETHER#2);
  lock(_xmit_ETHER#2);

To avoid this, walk the tq queues of the dsa slaves and set a lockdep
class.
Signed-off-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -810,12 +810,19 @@ static int dsa_slave_phy_setup(struct dsa_slave_priv *p,
 	return 0;
 }
 
+static struct lock_class_key dsa_slave_netdev_xmit_lock_key;
+static void dsa_slave_set_lockdep_class_one(struct net_device *dev,
+					    struct netdev_queue *txq,
+					    void *_unused)
+{
+	lockdep_set_class(&txq->_xmit_lock,
+			  &dsa_slave_netdev_xmit_lock_key);
+}
+
 int dsa_slave_suspend(struct net_device *slave_dev)
 {
 	struct dsa_slave_priv *p = netdev_priv(slave_dev);
 
-	netif_device_detach(slave_dev);
-
 	if (p->phy) {
 		phy_stop(p->phy);
 		p->old_pause = -1;
@@ -861,6 +868,9 @@ int dsa_slave_create(struct dsa_switch *ds, struct device *parent,
 	slave_dev->netdev_ops = &dsa_slave_netdev_ops;
 	slave_dev->swdev_ops = &dsa_slave_swdev_ops;
 
+	netdev_for_each_tx_queue(slave_dev, dsa_slave_set_lockdep_class_one,
+				 NULL);
+
 	SET_NETDEV_DEV(slave_dev, parent);
 	slave_dev->dev.of_node = ds->pd->port_dn[port];
 	slave_dev->vlan_features = master->vlan_features;