net: dsa: tag_8021q: Fix dsa_8021q_restore_pvid for an absent pvid

This sequence of operations:
ip link set dev br0 type bridge vlan_filtering 1
bridge vlan del dev swp2 vid 1
ip link set dev br0 type bridge vlan_filtering 1
ip link set dev br0 type bridge vlan_filtering 0

apparently fails with the message:

[   31.305716] sja1105 spi0.1: Reset switch and programmed static config. Reason: VLAN filtering
[   31.322161] sja1105 spi0.1: Couldn't determine PVID attributes (pvid 0)
[   31.328939] sja1105 spi0.1: Failed to setup VLAN tagging for port 1: -2
[   31.335599] ------------[ cut here ]------------
[   31.340215] WARNING: CPU: 1 PID: 194 at net/switchdev/switchdev.c:157 switchdev_port_attr_set_now+0x9c/0xa4
[   31.349981] br0: Commit of attribute (id=6) failed.
[   31.354890] Modules linked in:
[   31.357942] CPU: 1 PID: 194 Comm: ip Not tainted 5.4.0-rc6-01792-gf4f632e07665-dirty #2062
[   31.366167] Hardware name: Freescale LS1021A
[   31.370437] [<c03144dc>] (unwind_backtrace) from [<c030e184>] (show_stack+0x10/0x14)
[   31.378153] [<c030e184>] (show_stack) from [<c11d1c1c>] (dump_stack+0xe0/0x10c)
[   31.385437] [<c11d1c1c>] (dump_stack) from [<c034c730>] (__warn+0xf4/0x10c)
[   31.392373] [<c034c730>] (__warn) from [<c034c7bc>] (warn_slowpath_fmt+0x74/0xb8)
[   31.399827] [<c034c7bc>] (warn_slowpath_fmt) from [<c11ca204>] (switchdev_port_attr_set_now+0x9c/0xa4)
[   31.409097] [<c11ca204>] (switchdev_port_attr_set_now) from [<c117036c>] (__br_vlan_filter_toggle+0x6c/0x118)
[   31.418971] [<c117036c>] (__br_vlan_filter_toggle) from [<c115d010>] (br_changelink+0xf8/0x518)
[   31.427637] [<c115d010>] (br_changelink) from [<c0f8e9ec>] (__rtnl_newlink+0x3f4/0x76c)
[   31.435613] [<c0f8e9ec>] (__rtnl_newlink) from [<c0f8eda8>] (rtnl_newlink+0x44/0x60)
[   31.443329] [<c0f8eda8>] (rtnl_newlink) from [<c0f89f20>] (rtnetlink_rcv_msg+0x2cc/0x51c)
[   31.451477] [<c0f89f20>] (rtnetlink_rcv_msg) from [<c1008df8>] (netlink_rcv_skb+0xb8/0x110)
[   31.459796] [<c1008df8>] (netlink_rcv_skb) from [<c1008648>] (netlink_unicast+0x17c/0x1f8)
[   31.468026] [<c1008648>] (netlink_unicast) from [<c1008980>] (netlink_sendmsg+0x2bc/0x3b4)
[   31.476261] [<c1008980>] (netlink_sendmsg) from [<c0f43858>] (___sys_sendmsg+0x230/0x250)
[   31.484408] [<c0f43858>] (___sys_sendmsg) from [<c0f44c84>] (__sys_sendmsg+0x50/0x8c)
[   31.492209] [<c0f44c84>] (__sys_sendmsg) from [<c0301000>] (ret_fast_syscall+0x0/0x28)
[   31.500090] Exception stack(0xedf47fa8 to 0xedf47ff0)
[   31.505122] 7fa0:                   00000002 b6f2e060 00000003 beabd6a4 00000000 00000000
[   31.513265] 7fc0: 00000002 b6f2e060 5d6e3213 00000128 00000000 00000001 00000006 000619c4
[   31.521405] 7fe0: 00086078 beabd658 0005edbc b6e7ce68

The reason is the implementation of br_get_pvid:

static inline u16 br_get_pvid(const struct net_bridge_vlan_group *vg)
{
	if (!vg)
		return 0;

	smp_rmb();
	return vg->pvid;
}

Since VID 0 is an invalid pvid from the bridge's point of view, let's
add this check in dsa_8021q_restore_pvid to avoid restoring a pvid that
doesn't really exist.

Fixes: 5f33183b ("net: dsa: tag_8021q: Restore bridge VLANs when enabling vlan_filtering")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_8021q.c

@@ -106,7 +106,7 @@ static int dsa_8021q_restore_pvid(struct dsa_switch *ds, int port)
 	slave = ds->ports[port].slave;
 
 	err = br_vlan_get_pvid(slave, &pvid);
-	if (err < 0)
+	if (!pvid || err < 0)
 		/* There is no pvid on the bridge for this port, which is
 		 * perfectly valid. Nothing to restore, bye-bye!
 		 */