Currently when a switchdev port joins a bridge, we replay all FDB
entries pointing towards that port or towards the bridge.

However, this is insufficient in certain situations:

(a) DSA, through its assisted_learning_on_cpu_port logic, snoops
    dynamically learned FDB entries on foreign interfaces.
    These are FDB entries that are pointing neither towards the newly
    joined switchdev port, nor towards the bridge. So these addresses
    would be missed when joining a bridge where a foreign interface has
    already learned some addresses, and they would also linger on if the
    DSA port leaves the bridge before the foreign interface forgets them.
    None of this happens if we replay the entire FDB when the port joins.

(b) There is a desire to treat local FDB entries on a port (i.e. the
    port's termination MAC address) identically to FDB entries pointing
    towards the bridge itself. More details on the reason behind this in
    the next patch. The point is that this cannot be done given the
    current structure of br_fdb_replay() in this situation:
      ip link set swp0 master br0  # br0 inherits its MAC address from swp0
      ip link set swp1 master br0
    What is desirable is that when swp1 joins the bridge, br_fdb_replay()
    also notifies swp1 of br0's MAC address, but this won't in fact
    happen because the MAC address of br0 does not have fdb->dst == NULL
    (it doesn't point towards the bridge), but it has fdb->dst == swp0.
    So our current logic makes it impossible for that address to be
    replayed. But if we dump the entire FDB instead of just the entries
    with fdb->dst == swp1 and fdb->dst == NULL, then the inherited MAC
    address of br0 will be replayed too, which is what we need.

A natural question arises: say there is an FDB entry to be replayed,
like a MAC address dynamically learned on a foreign interface that
belongs to a bridge where no switchdev port has joined yet. If 10
switchdev ports belonging to the same driver join this bridge, one by
one, won't every port get notified 10 times of the foreign FDB entry,
amounting to a total of 100 notifications for this FDB entry in the
switchdev driver?

Well, yes, but this is where the "void *ctx" argument for br_fdb_replay
is useful: every port of the switchdev driver is notified whenever any
other port requests an FDB replay, but because the replay was initiated
by a different port, its context is different from the initiating port's
context, so it ignores those replays.

So the foreign FDB entry will be installed only 10 times, once per port.
This is done so that the following 4 code paths are always well balanced:
(a) addition of foreign FDB entry is replayed when port joins bridge
(b) deletion of foreign FDB entry is replayed when port leaves bridge
(c) addition of foreign FDB entry is notified to all ports currently in bridge
(c) deletion of foreign FDB entry is notified to all ports currently in bridge
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>