[RFC net] net: dsa: Add missing reference counting

[Florian Fainelli <f.fainelli@gmail.com>]

If we are probed through platform_data we would be intentionally
dropping the reference count on master after dev_to_net_device()
incremented it. If we are probed through Device Tree,
of_find_net_device() does not do a dev_hold() at all.

Ensure that the DSA master device is properly reference counted by
holding it as soon as the CPU port is successfully initialized and later
released during dsa_switch_release_ports(). dsa_get_tag_protocol() does
a short de-reference, so we hold and release the master at that time,
too.

Fixes: 83c0afaec7b7 ("net: dsa: Add new binding implementation")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 net/dsa/dsa2.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
index d90665b465b8..875231252ada 100644
--- a/net/dsa/dsa2.c
+++ b/net/dsa/dsa2.c
@@ -626,6 +626,7 @@ static enum dsa_tag_protocol dsa_get_tag_protocol(struct dsa_port *dp,
 	 * happens the switch driver may want to know if its tagging protocol
 	 * is going to work in such a configuration.
 	 */
+	dev_hold(master);
 	if (dsa_slave_dev_check(master)) {
 		mdp = dsa_slave_to_port(master);
 		mds = mdp->ds;
@@ -633,6 +634,7 @@ static enum dsa_tag_protocol dsa_get_tag_protocol(struct dsa_port *dp,
 		tag_protocol = mds->ops->get_tag_protocol(mds, mdp_upstream,
 							  DSA_TAG_PROTO_NONE);
 	}
+	dev_put(master);
 
 	/* If the master device is not itself a DSA slave in a disjoint DSA
 	 * tree, then return immediately.
@@ -657,6 +659,7 @@ static int dsa_port_parse_cpu(struct dsa_port *dp, struct net_device *master)
 		return PTR_ERR(tag_ops);
 	}
 
+	dev_hold(master);
 	dp->master = master;
 	dp->type = DSA_PORT_TYPE_CPU;
 	dp->filter = tag_ops->filter;
@@ -858,6 +861,8 @@ static void dsa_switch_release_ports(struct dsa_switch *ds)
 		if (dp->ds != ds)
 			continue;
 		list_del(&dp->list);
+		if (dsa_port_is_cpu(dp))
+			dev_put(dp->master);
 		kfree(dp);
 	}
 }
-- 
2.20.1