dsa: mv88e6xxx: Include tagger overhead when setting MTU for DSA and CPU ports

Same members of the Marvell Ethernet switches impose MTU restrictions
on ports used for connecting to the CPU or another switch for DSA. If
the MTU is set too low, tagged frames will be discarded. Ensure the
worst case tagger overhead is included in setting the MTU for DSA and
CPU ports.

Fixes: 1baf0fac ("net: dsa: mv88e6xxx: Use chip-wide max frame size for MTU")
Reported by: 曹煜 <cao88yu@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -2944,10 +2944,10 @@ static int mv88e6xxx_get_max_mtu(struct dsa_switch *ds, int port)
 	struct mv88e6xxx_chip *chip = ds->priv;
 
 	if (chip->info->ops->port_set_jumbo_size)
-		return 10240 - VLAN_ETH_HLEN - ETH_FCS_LEN;
+		return 10240 - VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN;
 	else if (chip->info->ops->set_max_frame_size)
-		return 1632 - VLAN_ETH_HLEN - ETH_FCS_LEN;
-	return 1522 - VLAN_ETH_HLEN - ETH_FCS_LEN;
+		return 1632 - VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN;
+	return 1522 - VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN;
 }
 
 static int mv88e6xxx_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
@@ -2955,6 +2955,9 @@ static int mv88e6xxx_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
 	struct mv88e6xxx_chip *chip = ds->priv;
 	int ret = 0;
 
+	if (dsa_is_dsa_port(ds, port) || dsa_is_cpu_port(ds, port))
+		new_mtu += EDSA_HLEN;
+
 	mv88e6xxx_reg_lock(chip);
 	if (chip->info->ops->port_set_jumbo_size)
 		ret = chip->info->ops->port_set_jumbo_size(chip, port, new_mtu);

drivers/net/dsa/mv88e6xxx/chip.h

@@ -18,6 +18,7 @@
 #include <linux/timecounter.h>
 #include <net/dsa.h>
 
+#define EDSA_HLEN		8
 #define MV88E6XXX_N_FID		4096
 
 /* PVT limits for 4-bit port and 5-bit switch */