net: Have netpoll bring-up DSA management interface

DSA network devices rely on having their DSA management interface up and
running otherwise their ndo_open() will return -ENETDOWN. Without doing
this it would not be possible to use DSA devices as netconsole when
configured on the command line. These devices also do not utilize the
upper/lower linking so the check about the netpoll device having upper
is not going to be a problem.

The solution adopted here is identical to the one done for
net/ipv4/ipconfig.c with 728c020 ("net: ipv4: handle DSA enabled
master network devices"), with the network namespace scope being
restricted to that of the process configuring netpoll.

Fixes: 04ff53f ("net: dsa: Add netconsole support")
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20201117035236.22658-1-f.fainelli@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/netpoll.c

@@ -29,6 +29,7 @@
 #include <linux/slab.h>
 #include <linux/export.h>
 #include <linux/if_vlan.h>
+#include <net/dsa.h>
 #include <net/tcp.h>
 #include <net/udp.h>
 #include <net/addrconf.h>
@@ -657,22 +658,35 @@ EXPORT_SYMBOL_GPL(__netpoll_setup);
 
 int netpoll_setup(struct netpoll *np)
 {
-	struct net_device *ndev = NULL;
+	struct net_device *ndev = NULL, *dev = NULL;
+	struct net *net = current->nsproxy->net_ns;
 	struct in_device *in_dev;
 	int err;
 
 	rtnl_lock();
-	if (np->dev_name[0]) {
-		struct net *net = current->nsproxy->net_ns;
+	if (np->dev_name[0])
 		ndev = __dev_get_by_name(net, np->dev_name);
-	}
+
 	if (!ndev) {
 		np_err(np, "%s doesn't exist, aborting\n", np->dev_name);
 		err = -ENODEV;
 		goto unlock;
 	}
 	dev_hold(ndev);
 
+	/* bring up DSA management network devices up first */
+	for_each_netdev(net, dev) {
+		if (!netdev_uses_dsa(dev))
+			continue;
+
+		err = dev_change_flags(dev, dev->flags | IFF_UP, NULL);
+		if (err < 0) {
+			np_err(np, "%s failed to open %s\n",
+			       np->dev_name, dev->name);
+			goto put;
+		}
+	}
+
 	if (netdev_master_upper_dev_get(ndev)) {
 		np_err(np, "%s is a slave device, aborting\n", np->dev_name);
 		err = -EBUSY;