[PATCH net-next] net: dsa: offer a flowtable to the switch before the conduit

[Julius Bairaktaris <julius@bairaktaris.de>]

A netfilter flowtable bound to a DSA user port reaches hardware through
dsa_user_setup_ft_block(), which forwards the block to the conduit
netdev. That route serves a flow engine that sits on the conduit, as
mtk_eth's does. TC_SETUP_FT is the only tc setup type handled this way:
every other type falls through to ds->ops->port_setup_tc, so a switch
that owns its flow engine is never offered the flowtable at all.

Reaching back from the conduit's ndo_setup_tc through conduit->dsa_ptr
is not a substitute: dsa_tree_teardown() tears down the conduit before
the ports, so a block unbound while the tree goes down can no longer be
resolved to the port it was bound on.

Offer TC_SETUP_FT to the switch first and keep the conduit forward as
the fallback. Every driver implementing .port_setup_tc returns
-EOPNOTSUPP for a setup type it does not handle, so a conduit-side flow
engine is reached exactly as before.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Julius Bairaktaris <julius@bairaktaris.de>
---
The switch-side consumer is a DSA driver for the IPQ8074 PPE, maintained
in OpenWrt; a flowtable bound to its user ports is exercised there on
IPQ8074 hardware, against a live PPPoE uplink. The conduit path is
unchanged by inspection: every in-tree .port_setup_tc implementation
returns -EOPNOTSUPP for a setup type it does not handle, so mtk_eth
behind mt7530 keeps receiving the forward exactly as before.

 net/dsa/user.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/dsa/user.c b/net/dsa/user.c
index 041f9060c8ef..c5adb557056f 100644
--- a/net/dsa/user.c
+++ b/net/dsa/user.c
@@ -1734,11 +1734,22 @@ static int dsa_user_setup_tc(struct net_device *dev, enum tc_setup_type type,
 {
 	struct dsa_port *dp = dsa_user_to_port(dev);
 	struct dsa_switch *ds = dp->ds;
+	int err;
 
 	switch (type) {
 	case TC_SETUP_BLOCK:
 		return dsa_user_setup_tc_block(dev, type_data);
 	case TC_SETUP_FT:
+		/* A switch that owns the flow tables answers for itself; only
+		 * a conduit-side flow engine needs the block forwarded, and
+		 * that forward is gone by the time the port is torn down.
+		 */
+		if (ds->ops->port_setup_tc) {
+			err = ds->ops->port_setup_tc(ds, dp->index, type,
+						     type_data);
+			if (err != -EOPNOTSUPP)
+				return err;
+		}
 		return dsa_user_setup_ft_block(ds, dp->index, type_data);
 	default:
 		break;

base-commit: 25c1f6111034aef7fc06cfbdcf1e4f0d6e5ee74b
-- 
2.53.0