Monitor the following events and notify the driver when:

- A DSA port joins/leaves a LAG.
- A LAG, made up of DSA ports, joins/leaves a bridge.
- A DSA port in a LAG is enabled/disabled (enabled meaning
  "distributing" in 802.3ad LACP terms).

Each LAG interface to which a DSA port is attached is represented by a
`struct dsa_lag` which is globally reachable from the switch tree and
from each associated port.

When a LAG joins a bridge, the DSA subsystem will treat that as each
individual port joining the bridge. The driver may look at the port's
LAG pointer to see if it is associated with any LAG, if that is
required. This is analogue to how switchdev events are replicated out
to all lower devices when reaching e.g. a LAG.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
---
 include/net/dsa.h  |  97 +++++++++++++++++++++++++++++++
 net/dsa/dsa2.c     |  51 ++++++++++++++++
 net/dsa/dsa_priv.h |  31 ++++++++++
 net/dsa/port.c     | 141 +++++++++++++++++++++++++++++++++++++++++++++
 net/dsa/slave.c    |  83 ++++++++++++++++++++++++--
 net/dsa/switch.c   |  49 ++++++++++++++++
 6 files changed, 446 insertions(+), 6 deletions(-)

+static inline struct dsa_lag *dsa_lag_by_id(struct dsa_switch_tree *dst, int id)
+{
+	if (!test_bit(id, dst->lags.busy))
+		return NULL;
+
+	return &dst->lags.pool[id];
+}
+
+static inline struct net_device *dsa_lag_dev_by_id(struct dsa_switch_tree *dst,
+						   int id)
+{
+	struct dsa_lag *lag = dsa_lag_by_id(dst, id);
+
+	return lag ? rcu_dereference(lag->dev) : NULL;
+}
+
+static inline struct dsa_lag *dsa_lag_by_dev(struct dsa_switch_tree *dst,
+					     struct net_device *dev)
+{
+	struct dsa_lag *lag;
+	int id;
+
+	dsa_lag_foreach(id, dst) {
+		lag = dsa_lag_by_id(dst, id);
+
+		if (rtnl_dereference(lag->dev) == dev)
+			return lag;
+	}
+
+	return NULL;
+}
 

diff --git a/net/dsa/port.c b/net/dsa/port.c
index 73569c9af3cc..c2332ee5f5c7 100644
--- a/net/dsa/port.c
+++ b/net/dsa/port.c

@@ -193,6 +193,147 @@ void dsa_port_bridge_leave(struct dsa_port *dp, struct net_device *br)
 	dsa_port_set_state_now(dp, BR_STATE_FORWARDING);
 }
 
+static struct dsa_lag *dsa_lag_get(struct dsa_switch_tree *dst,
+				   struct net_device *dev)
+{
+	struct dsa_lag *lag;
+	int id;
+
+	lag = dsa_lag_by_dev(dst, dev);
+	if (lag) {
+		kref_get(&lag->refcount);
+		return lag;
+	}
+
+	id = find_first_zero_bit(dst->lags.busy, dst->lags.num);
+	if (id >= dst->lags.num) {
+		WARN(1, "No LAGs available");
+		return NULL;
+	}
+
+	set_bit(id, dst->lags.busy);
+
+	lag = &dst->lags.pool[id];
+	kref_init(&lag->refcount);
+	lag->id = id;
+	INIT_LIST_HEAD(&lag->ports);
+	INIT_LIST_HEAD(&lag->tx_ports);
+
+	rcu_assign_pointer(lag->dev, dev);
+	return lag;
+}
+
+static void dsa_lag_release(struct kref *refcount)
+{
+	struct dsa_lag *lag = container_of(refcount, struct dsa_lag, refcount);
+
+	rcu_assign_pointer(lag->dev, NULL);
+	synchronize_rcu();
+	memset(lag, 0, sizeof(*lag));
+}