net: dsa: Fix load order between DSA drivers and taggers

The DSA core, DSA taggers and DSA drivers all make use of
module_init(). Hence they get initialised at device_initcall() time.
The ordering is non-deterministic. It can be a DSA driver is bound to
a device before the needed tag driver has been initialised, resulting
in the message:

No tagger for this switch

Rather than have this be fatal, return -EPROBE_DEFER so that it is
tried again later once all the needed drivers have been loaded.

Fixes: d3b8c049 ("dsa: Add boilerplate helper to register DSA tag driver modules")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -623,6 +623,8 @@ static int dsa_port_parse_cpu(struct dsa_port *dp, struct net_device *master)
 	tag_protocol = ds->ops->get_tag_protocol(ds, dp->index);
 	tag_ops = dsa_tag_driver_get(tag_protocol);
 	if (IS_ERR(tag_ops)) {
+		if (PTR_ERR(tag_ops) == -ENOPROTOOPT)
+			return -EPROBE_DEFER;
 		dev_warn(ds->dev, "No tagger for this switch\n");
 		return PTR_ERR(tag_ops);
 	}