[PATCH net-next 2/3] net: dsa: Allow default tag protocol to be overridden from DT

[Tobias Waldekranz <tobias@waldekranz.com>]

Some combinations of tag protocols and Ethernet controllers are
incompatible, and it is hard for the driver to keep track of these.

Therefore, allow the device tree author (typically the board vendor)
to inform the driver of this fact by selecting an alternate protocol
that is known to work.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
---

This deviates from the advise given by Andrew in that we store the
switches' default protocol on the tree rather than the override
value. I chose this because you want to make sure that the tagger
(default or overwritten) is available at probe time, and since that
means we actually load it, it made more sense to keep it loaded.

Then we just check during setup if we are running a different tagger
than what the driver would expect.

Somewhat related: since we know the default now, users could easily
revert to it via sysfs: `echo >/sys/class/eth0/dsa/tagging`. Not sure
if that would be useful.

 include/net/dsa.h |  5 +++
 net/dsa/dsa2.c    | 95 +++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 84 insertions(+), 16 deletions(-)

diff --git a/include/net/dsa.h b/include/net/dsa.h
index 57b2c49f72f4..2385ba317888 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -149,6 +149,11 @@ struct dsa_switch_tree {
 	/* Tagging protocol operations */
 	const struct dsa_device_ops *tag_ops;
 
+	/* Default tagging protocol preferred by the switches in this
+	 * tree.
+	 */
+	enum dsa_tag_protocol default_proto;
+
 	/*
 	 * Configuration data for the platform device that owns
 	 * this dsa switch tree instance.
diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
index 4d4956ed303b..52afe70f75af 100644
--- a/net/dsa/dsa2.c
+++ b/net/dsa/dsa2.c
@@ -668,6 +668,35 @@ static const struct devlink_ops dsa_devlink_ops = {
 	.sb_occ_tc_port_bind_get	= dsa_devlink_sb_occ_tc_port_bind_get,
 };
 
+static int dsa_switch_setup_tag_protocol(struct dsa_switch *ds)
+{
+	const struct dsa_device_ops *tag_ops = ds->dst->tag_ops;
+	struct dsa_switch_tree *dst = ds->dst;
+	int port, err;
+
+	if (tag_ops->proto == dst->default_proto)
+		return 0;
+
+	if (!ds->ops->change_tag_protocol) {
+		dev_err(ds->dev, "Tag protocol cannot be modified\n");
+		return -EINVAL;
+	}
+
+	for (port = 0; port < ds->num_ports; port++) {
+		if (!(dsa_is_dsa_port(ds, port) || dsa_is_cpu_port(ds, port)))
+			continue;
+
+		err = ds->ops->change_tag_protocol(ds, port, tag_ops->proto);
+		if (err) {
+			dev_err(ds->dev, "Tag protocol \"%s\" is not supported\n",
+				tag_ops->name);
+			return err;
+		}
+	}
+
+	return 0;
+}
+
 static int dsa_switch_setup(struct dsa_switch *ds)
 {
 	struct dsa_devlink_priv *dl_priv;
@@ -718,6 +747,10 @@ static int dsa_switch_setup(struct dsa_switch *ds)
 	if (err < 0)
 		goto unregister_notifier;
 
+	err = dsa_switch_setup_tag_protocol(ds);
+	if (err)
+		goto teardown;
+
 	devlink_params_publish(ds->devlink);
 
 	if (!ds->slave_mii_bus && ds->ops->phy_read) {
@@ -1062,32 +1095,60 @@ static enum dsa_tag_protocol dsa_get_tag_protocol(struct dsa_port *dp,
 	return ds->ops->get_tag_protocol(ds, dp->index, tag_protocol);
 }
 
-static int dsa_port_parse_cpu(struct dsa_port *dp, struct net_device *master)
+static int dsa_port_parse_cpu(struct dsa_port *dp, struct net_device *master,
+			      const char *user_protocol)
 {
 	struct dsa_switch *ds = dp->ds;
 	struct dsa_switch_tree *dst = ds->dst;
-	enum dsa_tag_protocol tag_protocol;
+	const struct dsa_device_ops *tag_ops;
+	enum dsa_tag_protocol default_proto;
+
+	/* Find out which protocol the switch would prefer. */
+	default_proto = dsa_get_tag_protocol(dp, master);
+	if (dst->default_proto) {
+		if (dst->default_proto != default_proto) {
+			dev_err(ds->dev,
+				"A DSA switch tree can have only one tagging protovol\n");
+			return -EINVAL;
+		}
+	} else {
+		dst->default_proto = default_proto;
+	}
+
+	/* See if the user wants to override that preference. */
+	if (user_protocol && ds->ops->change_tag_protocol) {
+		tag_ops = dsa_find_tagger_by_name(user_protocol);
+	} else {
+		if (user_protocol)
+			dev_warn(ds->dev,
+				 "Tag protocol cannot be modified, using default\n");
+
+		tag_ops = dsa_tag_driver_get(default_proto);
+	}
+
+	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);
+	}
 
-	tag_protocol = dsa_get_tag_protocol(dp, master);
 	if (dst->tag_ops) {
-		if (dst->tag_ops->proto != tag_protocol) {
+		if (dst->tag_ops != tag_ops) {
 			dev_err(ds->dev,
 				"A DSA switch tree can have only one tagging protocol\n");
+
+			dsa_tag_driver_put(tag_ops);
 			return -EINVAL;
 		}
+
 		/* In the case of multiple CPU ports per switch, the tagging
-		 * protocol is still reference-counted only per switch tree, so
-		 * nothing to do here.
+		 * protocol is still reference-counted only per switch tree.
 		 */
+		dsa_tag_driver_put(tag_ops);
 	} else {
-		dst->tag_ops = dsa_tag_driver_get(tag_protocol);
-		if (IS_ERR(dst->tag_ops)) {
-			if (PTR_ERR(dst->tag_ops) == -ENOPROTOOPT)
-				return -EPROBE_DEFER;
-			dev_warn(ds->dev, "No tagger for this switch\n");
-			dp->master = NULL;
-			return PTR_ERR(dst->tag_ops);
-		}
+		dst->tag_ops = tag_ops;
 	}
 
 	dp->master = master;
@@ -1108,12 +1169,14 @@ static int dsa_port_parse_of(struct dsa_port *dp, struct device_node *dn)
 
 	if (ethernet) {
 		struct net_device *master;
+		const char *user_protocol;
 
 		master = of_find_net_device_by_node(ethernet);
 		if (!master)
 			return -EPROBE_DEFER;
 
-		return dsa_port_parse_cpu(dp, master);
+		user_protocol = of_get_property(dn, "dsa,tag-protocol", NULL);
+		return dsa_port_parse_cpu(dp, master, user_protocol);
 	}
 
 	if (link)
@@ -1225,7 +1288,7 @@ static int dsa_port_parse(struct dsa_port *dp, const char *name,
 
 		dev_put(master);
 
-		return dsa_port_parse_cpu(dp, master);
+		return dsa_port_parse_cpu(dp, master, NULL);
 	}
 
 	if (!strcmp(name, "dsa"))
-- 
2.25.1