bridge: make user modified path cost sticky

Keep a STP port path cost value if it was set by a user.
Don't replace it with the link-speed based path cost
whenever the link goes down and comes back up.
Reported-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NStephen Hemminger <stephen@networkplumber.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_if.c

@@ -67,7 +67,8 @@ void br_port_carrier_check(struct net_bridge_port *p)
 	struct net_device *dev = p->dev;
 	struct net_bridge *br = p->br;
 
-	if (netif_running(dev) && netif_oper_up(dev))
+	if (!(p->flags & BR_ADMIN_COST) &&
+	    netif_running(dev) && netif_oper_up(dev))
 		p->path_cost = port_cost(dev);
 
 	if (!netif_running(br->dev))

net/bridge/br_private.h

@@ -156,6 +156,7 @@ struct net_bridge_port
 #define BR_BPDU_GUARD           0x00000002
 #define BR_ROOT_BLOCK		0x00000004
 #define BR_MULTICAST_FAST_LEAVE	0x00000008
+#define BR_ADMIN_COST		0x00000010
 
 #ifdef CONFIG_BRIDGE_IGMP_SNOOPING
 	u32				multicast_startup_queries_sent;

net/bridge/br_stp_if.c

@@ -288,6 +288,7 @@ int br_stp_set_path_cost(struct net_bridge_port *p, unsigned long path_cost)
 	    path_cost > BR_MAX_PATH_COST)
 		return -ERANGE;
 
+	p->flags |= BR_ADMIN_COST;
 	p->path_cost = path_cost;
 	br_configuration_update(p->br);
 	br_port_state_selection(p->br);