net: bridge: add helper to retrieve the current ageing time

The SWITCHDEV_ATTR_ID_BRIDGE_AGEING_TIME attribute is only emitted from:

sysfs/ioctl/netlink
-> br_set_ageing_time
   -> __set_ageing_time

therefore not at bridge port creation time, so:
(a) switchdev drivers have to hardcode the initial value for the address
    ageing time, because they didn't get any notification
(b) that hardcoded value can be out of sync, if the user changes the
    ageing time before enslaving the port to the bridge

We need a helper in the bridge, such that switchdev drivers can query
the current value of the bridge ageing time when they start offloading
it.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_bridge.h

@@ -137,6 +137,7 @@ struct net_device *br_fdb_find_port(const struct net_device *br_dev,
 void br_fdb_clear_offload(const struct net_device *dev, u16 vid);
 bool br_port_flag_is_set(const struct net_device *dev, unsigned long flag);
 u8 br_port_get_stp_state(const struct net_device *dev);
+clock_t br_get_ageing_time(struct net_device *br_dev);
 #else
 static inline struct net_device *
 br_fdb_find_port(const struct net_device *br_dev,
@@ -160,6 +161,11 @@ static inline u8 br_port_get_stp_state(const struct net_device *dev)
 {
 	return BR_STATE_DISABLED;
 }
+
+static inline clock_t br_get_ageing_time(struct net_device *br_dev)
+{
+	return 0;
+}
 #endif
 
 #endif

net/bridge/br_stp.c

@@ -639,6 +639,19 @@ int br_set_ageing_time(struct net_bridge *br, clock_t ageing_time)
 	return 0;
 }
 
+clock_t br_get_ageing_time(struct net_device *br_dev)
+{
+	struct net_bridge *br;
+
+	if (!netif_is_bridge_master(br_dev))
+		return 0;
+
+	br = netdev_priv(br_dev);
+
+	return jiffies_to_clock_t(br->ageing_time);
+}
+EXPORT_SYMBOL_GPL(br_get_ageing_time);
+
 /* called under bridge lock */
 void __br_set_topology_change(struct net_bridge *br, unsigned char val)
 {