net: dsa: tag_sja1105: absorb logic for not overwriting precise info into dsa_8021q_rcv()

In both sja1105_rcv() and sja1110_rcv(), we may have precise source port
information coming from parallel hardware mechanisms, in addition to the
tag_8021q header.

Only sja1105_rcv() has extra logic to not overwrite that precise info
with what's present in the VLAN tag. This is because sja1110_rcv() gets
by, by having a reversed set of checks when assigning skb->dev. When the
source port is imprecise (vbid >=1), source_port and switch_id will be
set to zeroes by dsa_8021q_rcv(), which might be problematic. But by
checking for vbid >= 1 first, sja1110_rcv() fends that off.

We would like to make more code common between sja1105_rcv() and
sja1110_rcv(), and for that, we need to make sure that sja1110_rcv()
also goes through the precise source port preservation logic.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20240713211620.1125910-4-paweldembicki@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

net/dsa/tag_8021q.c

@@ -497,9 +497,21 @@ struct net_device *dsa_tag_8021q_find_port_by_vbid(struct net_device *conduit,
 }
 EXPORT_SYMBOL_GPL(dsa_tag_8021q_find_port_by_vbid);
 
+/**
+ * dsa_8021q_rcv - Decode source information from tag_8021q header
+ * @skb: RX socket buffer
+ * @source_port: pointer to storage for precise source port information.
+ *	If this is known already from outside tag_8021q, the pre-initialized
+ *	value is preserved. If not known, pass -1.
+ * @switch_id: similar to source_port.
+ * @vbid: pointer to storage for imprecise bridge ID. Must be pre-initialized
+ *	with -1. If a positive value is returned, the source_port and switch_id
+ *	are invalid.
+ */
 void dsa_8021q_rcv(struct sk_buff *skb, int *source_port, int *switch_id,
 		   int *vbid)
 {
+	int tmp_source_port, tmp_switch_id, tmp_vbid;
 	u16 vid, tci;
 
 	if (skb_vlan_tag_present(skb)) {
@@ -513,11 +525,25 @@ void dsa_8021q_rcv(struct sk_buff *skb, int *source_port, int *switch_id,
 
 	vid = tci & VLAN_VID_MASK;
 
-	*source_port = dsa_8021q_rx_source_port(vid);
-	*switch_id = dsa_8021q_rx_switch_id(vid);
+	tmp_source_port = dsa_8021q_rx_source_port(vid);
+	tmp_switch_id = dsa_8021q_rx_switch_id(vid);
+	tmp_vbid = dsa_tag_8021q_rx_vbid(vid);
+
+	/* Precise source port information is unknown when receiving from a
+	 * VLAN-unaware bridging domain, and tmp_source_port and tmp_switch_id
+	 * are zeroes in this case.
+	 *
+	 * Preserve the source information from hardware-specific mechanisms,
+	 * if available. This allows us to not overwrite a valid source port
+	 * and switch ID with less precise values.
+	 */
+	if (tmp_vbid == 0 && *source_port == -1)
+		*source_port = tmp_source_port;
+	if (tmp_vbid == 0 && *switch_id == -1)
+		*switch_id = tmp_switch_id;
 
 	if (vbid)
-		*vbid = dsa_tag_8021q_rx_vbid(vid);
+		*vbid = tmp_vbid;
 
 	skb->priority = (tci & VLAN_PRIO_MASK) >> VLAN_PRIO_SHIFT;
 }

net/dsa/tag_sja1105.c

@@ -524,30 +524,13 @@ static struct sk_buff *sja1105_rcv(struct sk_buff *skb,
 	/* Normal data plane traffic and link-local frames are tagged with
 	 * a tag_8021q VLAN which we have to strip
 	 */
-	if (sja1105_skb_has_tag_8021q(skb)) {
-		int tmp_source_port = -1, tmp_switch_id = -1;
-
-		sja1105_vlan_rcv(skb, &tmp_source_port, &tmp_switch_id, &vbid,
-				 &vid);
-		/* Preserve the source information from the INCL_SRCPT option,
-		 * 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). 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 (vbid == 0 && source_port == -1)
-			source_port = tmp_source_port;
-		if (vbid == 0 && switch_id == -1)
-			switch_id = tmp_switch_id;
-	} else if (source_port == -1 && switch_id == -1) {
+	if (sja1105_skb_has_tag_8021q(skb))
+		sja1105_vlan_rcv(skb, &source_port, &switch_id, &vbid, &vid);
+	else if (source_port == -1 && switch_id == -1)
 		/* Packets with no source information have no chance of
 		 * getting accepted, drop them straight away.
 		 */
 		return NULL;
-	}
 
 	if (source_port != -1 && switch_id != -1)
 		skb->dev = dsa_conduit_find_user(netdev, switch_id, source_port);