net: dsa: tag_sja1105: fix source port decoding in vlan_filtering=0 bridge mode

There was a regression introduced by the blamed commit, where pinging to
a VLAN-unaware bridge would fail with the repeated message "Couldn't
decode source port" coming from the tagging protocol driver.

When receiving packets with a bridge_vid as determined by
dsa_tag_8021q_bridge_join(), dsa_8021q_rcv() will decode:
- source_port = 0 (which isn't really valid, more like "don't know")
- switch_id = 0 (which isn't really valid, more like "don't know")
- vbid = value in range 1-7

Since the blamed patch has reversed the order of the checks, we are now
going to believe that source_port != -1 and switch_id != -1, so they're
valid, but they aren't.

The minimal solution to the problem is to only populate source_port and
switch_id with what dsa_8021q_rcv() came up with, if the vbid is zero,
i.e. the source port information is trustworthy.

Fixes: c1ae02d ("net: dsa: tag_sja1105: always prefer source port information from INCL_SRCPT")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: vladimiroltean

net/dsa/tag_sja1105.c

@@ -573,11 +573,14 @@ static struct sk_buff *sja1105_rcv(struct sk_buff *skb,
 		 * if available. This allows us to not overwrite a valid source
 		 * port and switch ID with zeroes when receiving link-local
 		 * frames from a VLAN-unaware bridged port (non-zero vbid) or a
-		 * VLAN-aware bridged port (non-zero vid).
+		 * VLAN-aware bridged port (non-zero vid). Furthermore, the
+		 * tag_8021q source port information is only of trust when the
+		 * vbid is 0 (precise port). Otherwise, tmp_source_port and
+		 * tmp_switch_id will be zeroes.
 		 */
-		if (source_port == -1)
+		if (vbid == 0 && source_port == -1)
 			source_port = tmp_source_port;
-		if (switch_id == -1)
+		if (vbid == 0 && switch_id == -1)
 			switch_id = tmp_switch_id;
 	} else if (source_port == -1 && switch_id == -1) {
 		/* Packets with no source information have no chance of