net: mtk: eth-mux: fix rcu_lock in mux

rcu lock caused by phy not ready yet when phylink_start is called for it
switch to sfp works too but switch back causes deadlock (no rcu-stall)

Author: frank-w

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -4919,6 +4919,37 @@ static const struct net_device_ops mtk_netdev_ops = {
 	.ndo_select_queue	= mtk_select_queue,
 };
 
+static struct phylink *mtk_mux_create_phylink(struct mtk_mux *mux, unsigned int channel)
+{
+	struct mtk_mux_data *data = mux->data[channel];
+	struct mtk_mac *mac = mux->mac;
+	struct mtk_eth *eth = mac->hw;
+	phy_interface_t phy_mode;
+	struct phylink *pl;
+	int err;
+
+	if (!data || !data->of_node)
+		return ERR_PTR(-EINVAL);
+
+	err = of_get_phy_mode(data->of_node, &phy_mode);
+	if (err) {
+		dev_err(eth->dev, "invalid phy-mode for mux channel %u\n", channel);
+		return ERR_PTR(err);
+	}
+
+	pl = phylink_create(&mac->phylink_config,
+			    of_fwnode_handle(data->of_node),
+			    phy_mode, &mtk_phylink_ops);
+	if (IS_ERR(pl)) {
+		dev_err(eth->dev, "failed to create phylink for channel %u\n", channel);
+		return pl;
+	}
+
+	dev_info(eth->dev, "Created phylink for channel %u\n", channel);
+	data->phylink = pl;
+	return pl;
+}
+
 static void mux_poll(struct work_struct *work)
 {
 	struct mtk_mux *mux = container_of(work, struct mtk_mux, poll.work);
@@ -4939,25 +4970,50 @@ static void mux_poll(struct work_struct *work)
 		goto reschedule;
 
 	dev_info(eth->dev, "ethernet mux: line:%d new channel:%d,sfp:%d\n",__LINE__, new_channel,sfp_present);
-	rtnl_lock();
 
+	rtnl_lock();
 	mtk_stop(dev);
 
-	mac->of_node = mux->data[new_channel]->of_node;
-	mac->phylink = mux->data[new_channel]->phylink;
+	/* Destroy old phylink if it exists */
+	if (mux->data[mux->channel] && mux->data[mux->channel]->phylink) {
+		dev_info(eth->dev, "Destroying phylink for channel %u\n", mux->channel);
+		phylink_destroy(mux->data[mux->channel]->phylink);
+		phylink_stop(mux->data[mux->channel]->phylink);
+		phylink_disconnect_phy(mux->data[mux->channel]->phylink);
+		phylink_destroy(mux->data[mux->channel]->phylink);
+		mux->data[mux->channel]->phylink = NULL;
+	}
+	rtnl_unlock();
 
 	dev_info(eth->dev, "ethernet mux: switch to channel%d\n", new_channel);
 
 	gpiod_set_value_cansleep(mux->chan_sel_gpio, new_channel);
 
 	usleep_range(100000,200000);
 
+	/* Create new phylink if not yet present */
+	if (!mux->data[new_channel]->phylink) {
+		mux->data[new_channel]->phylink = mtk_mux_create_phylink(mux, new_channel);
+		if (IS_ERR(mux->data[new_channel]->phylink)) {
+			dev_err(eth->dev, "Failed to create new phylink\n");
+			mux->data[new_channel]->phylink=NULL;
+			goto out_unlock;
+		}
+	}
+
+	mac->of_node = mux->data[new_channel]->of_node;
+	mac->phylink = mux->data[new_channel]->phylink;
+
+	rtnl_lock();
 	mtk_open(dev);
 
 	rtnl_unlock();
 
 	mux->channel = new_channel;
+	goto reschedule;
 
+out_unlock:
+	rtnl_unlock();
 reschedule:
 	mod_delayed_work(system_wq, &mux->poll, msecs_to_jiffies(100));
 }
@@ -4995,18 +5051,18 @@ static int mtk_add_mux_channel(struct mtk_mux *mux, struct device_node *np)
 		goto err_free_data;
 	}
 
-	phylink = phylink_create(&mux->mac->phylink_config,
+	/*phylink = phylink_create(&mux->mac->phylink_config,
 				 of_fwnode_handle(np),
 				 phy_mode, &mtk_phylink_ops);
 	if (IS_ERR(phylink)) {
 		dev_err(eth->dev, "failed to create phylink structure\n");
 		err = PTR_ERR(phylink);
 		goto err_free_data;
-	}
+	}*/
 
 	dev_info(eth->dev, "ethernet mux: line:%d added new channel:%d\n",__LINE__,id);
 	data->of_node = np;
-	data->phylink = phylink;
+	data->phylink = NULL;//phylink;
 	mux->data[id] = data;
 
 	return 0;