net: dsa: return per-port upstream port

The current dsa_upstream_port() helper still assumes a unique CPU port
in the whole switch fabric. This is becoming wrong, as every port in the
fabric has its dedicated CPU port, thus every port has an upstream port.

Add a port argument to the dsa_upstream_port() helper and fetch its CPU
port instead of the deprecated unique fabric CPU port. A CPU or unused
port has no dedicated CPU port, so return itself in this case.

At the same time, change the return value from u8 to unsigned int since
there is no need to limit the size here.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -1750,7 +1750,7 @@ static int mv88e6xxx_setup_upstream_port(struct mv88e6xxx_chip *chip, int port)
 	int upstream_port;
 	int err;
 
-	upstream_port = dsa_upstream_port(ds);
+	upstream_port = dsa_upstream_port(ds, port);
 	if (chip->info->ops->port_set_upstream_port) {
 		err = chip->info->ops->port_set_upstream_port(chip, port,
 							      upstream_port);

include/net/dsa.h

@@ -307,10 +307,13 @@ static inline unsigned int dsa_towards_port(struct dsa_switch *ds, int device,
 }
 
 /* Return the local port used to reach the dedicated CPU port */
-static inline u8 dsa_upstream_port(struct dsa_switch *ds)
+static inline unsigned int dsa_upstream_port(struct dsa_switch *ds, int port)
 {
-	struct dsa_switch_tree *dst = ds->dst;
-	struct dsa_port *cpu_dp = dst->cpu_dp;
+	const struct dsa_port *dp = dsa_to_port(ds, port);
+	const struct dsa_port *cpu_dp = dp->cpu_dp;
+
+	if (!cpu_dp)
+		return port;
 
 	return dsa_towards_port(ds, cpu_dp->ds->index, cpu_dp->index);
 }