[PATCH net 1/2] net: dsa: mt7530: use consistent egress tag for trapped link-local frames

[Tao <gongtao0607@gmail.com>]

commit 3ac85bcfd404 ("net: dsa: mt7530: preserve VLAN tags on trapped
link-local frames") changed the egress VLAN tag attribute for trapped
link-local frames in BPC, RGAC1, and RGAC2 from MT7530_VLAN_EG_UNTAGGED
to MT7530_VLAN_EG_DISABLED, with the intention of passing frames
through without VLAN tag modification.

However, in MT7530 hardware, MT7530_VLAN_EG_DISABLED disables the
protocol-level egress tagging override, causing trapped frames to
fall back to the egress port's active VLAN table (VAWD2) rule. When a
bridge with VLAN filtering is active, the CPU port is configured with
stack mode, causing the switch to insert a VLAN tag into incoming
untagged BPDUs (01:80:C2:00:00:00) and other link-local frames before
delivering them to the CPU conduit.

Because BPDUs arrive at the CPU with a VLAN tag (EtherType 0x8100
at offset 12 instead of 802.3 length <= 1500), userspace STP daemons
(such as ustpd) and the kernel bridge drop or ignore them, breaking
STP convergence.

Use MT7530_VLAN_EG_CONSISTENT instead. This instructs the hardware to
keep egress tagging consistent with the ingress wire format: frames
received untagged on the wire remain untagged when forwarded to the
CPU, while VLAN-tagged frames preserve their original VLAN tag.

Fixes: 3ac85bcfd404 ("net: dsa: mt7530: preserve VLAN tags on trapped link-local frames")
Signed-off-by: Tao <gongtao0607@gmail.com>
---
 drivers/net/dsa/mt7530.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index 14692497e1a6..863f2079833f 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -1245,39 +1245,39 @@ static void
 mt753x_trap_frames(struct mt7530_priv *priv)
 {
 	/* Trap 802.1X PAE frames and BPDUs to the CPU port(s) and egress
-	 * them with the EG_TAG attribute set to disabled (system default)
-	 * so that any VLAN tags in the frame are not modified by the
-	 * switch egress VLAN tag processing. This preserves VLAN tags
-	 * for reception on VLAN sub-interfaces.
+	 * them with the EG_TAG attribute set to consistent so that any
+	 * VLAN tags in the frame are not modified by the switch egress
+	 * VLAN tag processing. This preserves VLAN tags for reception
+	 * on VLAN sub-interfaces.
 	 */
 	regmap_update_bits(priv->regmap, MT753X_BPC,
 			   PAE_BPDU_FR | PAE_EG_TAG_MASK | PAE_PORT_FW_MASK |
 				   BPDU_EG_TAG_MASK | BPDU_PORT_FW_MASK,
-			   PAE_BPDU_FR | PAE_EG_TAG(MT7530_VLAN_EG_DISABLED) |
+			   PAE_BPDU_FR | PAE_EG_TAG(MT7530_VLAN_EG_CONSISTENT) |
 				   PAE_PORT_FW(TO_CPU_FW_CPU_ONLY) |
-				   BPDU_EG_TAG(MT7530_VLAN_EG_DISABLED) |
+				   BPDU_EG_TAG(MT7530_VLAN_EG_CONSISTENT) |
 				   TO_CPU_FW_CPU_ONLY);
 
 	/* Trap frames with :01 and :02 MAC DAs to the CPU port(s) and
-	 * egress them with EG_TAG disabled.
+	 * egress them with EG_TAG consistent.
 	 */
 	regmap_update_bits(priv->regmap, MT753X_RGAC1,
 			   R02_BPDU_FR | R02_EG_TAG_MASK | R02_PORT_FW_MASK |
 				   R01_BPDU_FR | R01_EG_TAG_MASK | R01_PORT_FW_MASK,
-			   R02_BPDU_FR | R02_EG_TAG(MT7530_VLAN_EG_DISABLED) |
+			   R02_BPDU_FR | R02_EG_TAG(MT7530_VLAN_EG_CONSISTENT) |
 				   R02_PORT_FW(TO_CPU_FW_CPU_ONLY) | R01_BPDU_FR |
-				   R01_EG_TAG(MT7530_VLAN_EG_DISABLED) |
+				   R01_EG_TAG(MT7530_VLAN_EG_CONSISTENT) |
 				   TO_CPU_FW_CPU_ONLY);
 
 	/* Trap frames with :03 and :0E MAC DAs to the CPU port(s) and
-	 * egress them with EG_TAG disabled.
+	 * egress them with EG_TAG consistent.
 	 */
 	regmap_update_bits(priv->regmap, MT753X_RGAC2,
 			   R0E_BPDU_FR | R0E_EG_TAG_MASK | R0E_PORT_FW_MASK |
 				   R03_BPDU_FR | R03_EG_TAG_MASK | R03_PORT_FW_MASK,
-			   R0E_BPDU_FR | R0E_EG_TAG(MT7530_VLAN_EG_DISABLED) |
+			   R0E_BPDU_FR | R0E_EG_TAG(MT7530_VLAN_EG_CONSISTENT) |
 				   R0E_PORT_FW(TO_CPU_FW_CPU_ONLY) | R03_BPDU_FR |
-				   R03_EG_TAG(MT7530_VLAN_EG_DISABLED) |
+				   R03_EG_TAG(MT7530_VLAN_EG_CONSISTENT) |
 				   TO_CPU_FW_CPU_ONLY);
 }
 
-- 
2.55.0