netdev - [PATCH net-next v2 3/3] net: dsa: mv88e6xxx: default to multicast and unicast flooding

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Message-Id: <E1gvPMu-0000Ax-Ba@rmk-PC.armlinux.org.uk>
Date:   Sun, 17 Feb 2019 16:32:40 +0000
From:   Russell King <rmk+kernel@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: [PATCH net-next v2 3/3] net: dsa: mv88e6xxx: default to multicast and
 unicast flooding

Switches work by learning the MAC address for each attached station by
monitoring traffic from each station.  When a station sends a packet,
the switch records which port the MAC address is connected to.

With IPv4 networking, before communication commences with a neighbour,
an ARP packet is broadcasted to all stations asking for the MAC address
corresponding with the IPv4.  The desired station responds with an ARP
reply, and the ARP reply causes the switch to learn which port the
station is connected to.

With IPv6 networking, the situation is rather different.  Rather than
broadcasting ARP packets, a "neighbour solicitation" is multicasted
rather than broadcasted.  This multicast needs to reach the intended
station in order for the neighbour to be discovered.

Once a neighbour has been discovered, and entered into the sending
stations neighbour cache, communication can restart at a point later
without sending a new neighbour solicitation, even if the entry in
the neighbour cache is marked as stale.  This can be after the MAC
address has expired from the forwarding cache of the DSA switch -
when that occurs, there is a long pause in communication.

Our DSA implementation for mv88e6xxx switches has defaulted to having
multicast and unicast flooding disabled.  As per the above description,
this is fine for IPv4 networking, since the broadcasted ARP queries
will be sent to and received by all stations on the same network.
However, this breaks IPv6 very badly - blocking neighbour solicitations
and later causing connections to stall.

The defaults that the Linux bridge code expect from bridges are that
unknown unicast frames and unknown multicast frames are flooded to
all stations, which is at odds to the defaults adopted by our DSA
implementation for mv88e6xxx switches.

This commit enables by default flooding of both unknown unicast and
unknown multicast frames.  This means that mv88e6xxx DSA switches now
behave as per the bridge(8) man page, and IPv6 works flawlessly through
such a switch.

Signed-off-by: Russell King <rmk+kernel@...linux.org.uk>
---
 drivers/net/dsa/mv88e6xxx/chip.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
index 72db6e74be48..c1bcd13af13f 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.c
+++ b/drivers/net/dsa/mv88e6xxx/chip.c
@@ -2143,14 +2143,13 @@ static int mv88e6xxx_setup_message_port(struct mv88e6xxx_chip *chip, int port)

 static int mv88e6xxx_setup_egress_floods(struct mv88e6xxx_chip *chip, int port)
 {
-	struct dsa_switch *ds = chip->ds;
-	bool flood;
-
-	/* Upstream ports flood frames with unknown unicast or multicast DA */
-	flood = dsa_is_cpu_port(ds, port) || dsa_is_dsa_port(ds, port);
+	/* Linux bridges are expected to flood unknown multicast and
+	 * unicast frames to all ports - as per the defaults specified
+	 * in the iproute2 bridge(8) man page. Not doing this causes
+	 * stalls and failures with IPv6 over Marvell bridges. */
 	if (chip->info->ops->port_set_egress_floods)
 		return chip->info->ops->port_set_egress_floods(chip, port,
-							       flood, flood);
+							       true, true);

 	return 0;
 }
-- 
2.7.4