[PATCH v3] bmips: bcm6368-enetsw: Bump max MTU

[Linus Walleij]

The max frame size for this ethernet switch is 1536 bytes, excluding the
VLAN headers, which is

1536 - VLAN_ETH_HLEN - VLAN_HLEN = 1536 - 18 - 4 = 1514

not

ETH_DATA_LEN + ENETSW_TAG_SIZE = 1500 + 4 + 6 = 1510

as it is right now. The available overhead is needed when using the DSA
switch with a cascaded Marvell DSA switch, which is something that exist
in real products, in this case the Inteno XG6846.

Before this patch (on the lan1 DSA port in this case):
dsa_slave_change_mtu: master->max_mtu = 9724, dev->max_mtu = 10218, DSA overhead = 8
dsa_slave_change_mtu: master = extsw, dev = lan1
dsa_slave_change_mtu: master->max_mtu = 1510, dev->max_mtu = 9724, DSA overhead = 6
dsa_slave_change_mtu: master = eth0, dev = extsw
dsa_slave_change_mtu new_master_mtu 1514 > mtu_limit 1510
mdio_mux-0.1:00: nonfatal error -34 setting MTU to 1500 on port 0

My added debug prints before the nonfatal error: the first switch from the top
is the Marvell switch, the second in the bcm6368-enetsw with its 1510 limit.

After this patch the error is gone.

OpenWrt adds a VLAN to each port so we get VLAN tags on all frames.

So when we ping with ping -s 1472 -M do 192.168.1.220
this first turns into a 1500 byte ICMP package, when it
reaches the Marvell switch it adds VLAN_ETH_HLEN + VLAN_HLEN
1500 + 18 + 4 = 1522 and then 6 more bytes for DSA tag ending up
with 1528 bytes. Then the Broadcom switch adds another DSA tag
of 8 bytes, resulting in 1536 bytes reaching Linux, the maximum.

tcpdump (on the target eth0) gives this confirming the suspicion:

00:05:03.678271 AF Unknown (1429722180), length 1532:
        0x0000:  3d93 bcae c56b a83d 8874 02fd 0004 8100  =....k.=.t......
        0x0010:  0000 dada 0000 c000 0fff 0800 4500 05dc  ............E...
        0x0020:  0000 4000 4001 b146 c0a8 0189 c0a8 0101  .. at .@..F........
        0x0030:  0800 1fd0 0068 001f 7231 1b65 0000 0000  .....h..r1.e....
        0x0040:  f6ef 0600 0000 0000 1011 1213 1415 1617  ................
        0x0050:  1819 1a1b 1c1d 1e1f 2021 2223 2425 2627  .........!"#$%&'
        0x0060:  2829 2a2b 2c2d 2e2f 3031 3233 3435 3637  ()*+,-./01234567
        0x0070:  3839 3a3b 3c3d 3e3f 4041 4243 4445 4647  89:;<=>?@ABCDEFG

3d93 = First four bytes are the last two bytes of the destination
ethernet address I don't know why the first four are missing,
but it sure explains why the paket is 1532 bytes and not 1536.

bcae c56b a83b = source ethernet address

8874 02fd 0004 = Broadcom enetsw DSA tag

8100 0000 = VLAN 802.1Q header

dada 0000 c000 0fff 0800 = EDSA tag for the Marvell (outer) switch,
 0800 is the ethertype obviously

Next follows the contents of the ping packet as it appears if
we dump it on the DSA interface such as tcpdump -i lan1
etc, there we get the stripped out packet.

At the end 4 bytes of FCS.

Tests with larger MTUs have not been successful so this is likely the actual
hardware max MTU.

Cc: Álvaro Fernández Rojas <noltari at gmail.com>
Cc: Jonas Gorski <jonas.gorski at gmail.com>
Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
---
ChangeLog v2->v3:
- Make a more believable case for the max MTU with tcpdump
  example.
ChangeLog v1->v2:
- Do some better research after help on IRC, did some ping tests.
---
 .../files/drivers/net/ethernet/broadcom/bcm6368-enetsw.c     | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/target/linux/bmips/files/drivers/net/ethernet/broadcom/bcm6368-enetsw.c b/target/linux/bmips/files/drivers/net/ethernet/broadcom/bcm6368-enetsw.c
index 321e95dbbb3d..01e4723c176d 100644
--- a/target/linux/bmips/files/drivers/net/ethernet/broadcom/bcm6368-enetsw.c
+++ b/target/linux/bmips/files/drivers/net/ethernet/broadcom/bcm6368-enetsw.c
@@ -22,7 +22,8 @@
 #include <linux/reset.h>
 #include <linux/version.h>
 
-/* MTU */
+/* The max frame size is 1536 including VLAN headers but excluding the 6 byte DSA tag */
+#define ENETSW_MAX_MTU			(1536 - VLAN_ETH_HLEN - VLAN_HLEN)
 #define ENETSW_TAG_SIZE			(6 + VLAN_HLEN)
 #define ENETSW_MTU_OVERHEAD		(VLAN_ETH_HLEN + VLAN_HLEN + \
 					 ENETSW_TAG_SIZE)
@@ -1067,7 +1068,7 @@ static int bcm6368_enetsw_probe(struct platform_device *pdev)
 	ndev->netdev_ops = &bcm6368_enetsw_ops;
 	ndev->min_mtu = ETH_ZLEN;
 	ndev->mtu = ETH_DATA_LEN + ENETSW_TAG_SIZE;
-	ndev->max_mtu = ETH_DATA_LEN + ENETSW_TAG_SIZE;
+	ndev->max_mtu = ENETSW_MAX_MTU;
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(6,1,0)
 	netif_napi_add(ndev, &priv->napi, bcm6368_enetsw_poll);
 #else
-- 
2.34.1