[RFC net-next 1/2] net: dsa: tag_mtk: skip address learning on transmit to standalone ports

[DENG Qingfang]

Consider the following bridge configuration, where bond0 is not
offloaded:

         +-- br0 --+
        / /   |     \
       / /    |      \
      /  |    |     bond0
     /   |    |     /   \
   swp0 swp1 swp2 swp3 swp4
     .        .       .
     .        .       .
     A        B       C

Address learning is enabled on offloaded ports (swp0~2) and the CPU
port, so when client A sends a packet to C, the following will happen:

1. The switch learns that client A can be reached at swp0.
2. The switch probably already knows that client C can be reached at the
   CPU port, so it forwards the packet to the CPU.
3. The bridge core knows client C can be reached at bond0, so it
   forwards the packet back to the switch.
4. The switch learns that client A can be reached at the CPU port.
5. The switch forwards the packet to either swp3 or swp4, according to
   the packet's tag.

That makes client A's MAC address flap between swp0 and the CPU port. If
client B sends a packet to A, it is possible that the packet is
forwarded to the CPU. With offload_fwd_mark = 1, the bridge core won't
forward it back to the switch, resulting in packet loss.

To avoid that, skip address learning on the CPU port when the destination
port is standalone, which can be done by setting the SA_DIS bit of the
MTK tag, if bridge_dev of the destination port is not set.

Signed-off-by: DENG Qingfang <dqfext at gmail.com>
---
 net/dsa/tag_mtk.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/dsa/tag_mtk.c b/net/dsa/tag_mtk.c
index cc3ba864ad5b..8c361812e21b 100644
--- a/net/dsa/tag_mtk.c
+++ b/net/dsa/tag_mtk.c
@@ -15,8 +15,7 @@
 #define MTK_HDR_XMIT_TAGGED_TPID_8100	1
 #define MTK_HDR_XMIT_TAGGED_TPID_88A8	2
 #define MTK_HDR_RECV_SOURCE_PORT_MASK	GENMASK(2, 0)
-#define MTK_HDR_XMIT_DP_BIT_MASK	GENMASK(5, 0)
-#define MTK_HDR_XMIT_SA_DIS		BIT(6)
+#define MTK_HDR_XMIT_SA_DIS_SHIFT	6
 
 static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
 				    struct net_device *dev)
@@ -50,7 +49,8 @@ static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
 	 * whether that's a combined special tag with 802.1Q header.
 	 */
 	mtk_tag[0] = xmit_tpid;
-	mtk_tag[1] = (1 << dp->index) & MTK_HDR_XMIT_DP_BIT_MASK;
+	mtk_tag[1] = BIT(dp->index) |
+		     (!dp->bridge_dev << MTK_HDR_XMIT_SA_DIS_SHIFT);
 
 	/* Tag control information is kept for 802.1Q */
 	if (xmit_tpid == MTK_HDR_XMIT_UNTAGGED) {
-- 
2.25.1