net: dsa: tag_brcm: legacy: fix daisy-chained switches

When BCM63xx internal switches are connected to switches with a 4-byte
Broadcom tag, it does not identify the packet as VLAN tagged, so it adds one
based on its PVID (which is likely 0).
Right now, the packet is received by the BCM63xx internal switch and the 6-byte
tag is properly processed. The next step would to decode the corresponding
4-byte tag. However, the internal switch adds an invalid VLAN tag after the
6-byte tag and the 4-byte tag handling fails.
In order to fix this we need to remove the invalid VLAN tag after the 6-byte
tag before passing it to the 4-byte tag decoding.

Fixes: 964dbf18 ("net: dsa: tag_brcm: add support for legacy tags")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230319095540.239064-1-noltari@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/tag_brcm.c

@@ -7,6 +7,7 @@
 
 #include <linux/dsa/brcm.h>
 #include <linux/etherdevice.h>
+#include <linux/if_vlan.h>
 #include <linux/list.h>
 #include <linux/slab.h>
 
@@ -252,6 +253,7 @@ static struct sk_buff *brcm_leg_tag_xmit(struct sk_buff *skb,
 static struct sk_buff *brcm_leg_tag_rcv(struct sk_buff *skb,
 					struct net_device *dev)
 {
+	int len = BRCM_LEG_TAG_LEN;
 	int source_port;
 	u8 *brcm_tag;
 
@@ -266,12 +268,16 @@ static struct sk_buff *brcm_leg_tag_rcv(struct sk_buff *skb,
 	if (!skb->dev)
 		return NULL;
 
+	/* VLAN tag is added by BCM63xx internal switch */
+	if (netdev_uses_dsa(skb->dev))
+		len += VLAN_HLEN;
+
 	/* Remove Broadcom tag and update checksum */
-	skb_pull_rcsum(skb, BRCM_LEG_TAG_LEN);
+	skb_pull_rcsum(skb, len);
 
 	dsa_default_offload_fwd_mark(skb);
 
-	dsa_strip_etype_header(skb, BRCM_LEG_TAG_LEN);
+	dsa_strip_etype_header(skb, len);
 
 	return skb;
 }