tipc: add tipc_bearer_min_mtu to calculate min mtu

As different media may requires different min mtu, and even the
same media with different net family requires different min mtu,
add tipc_bearer_min_mtu() to calculate min mtu accordingly.

This API will be used to check the new mtu when doing the link
mtu negotiation in the next patch.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/bearer.c

@@ -541,6 +541,19 @@ int tipc_bearer_mtu(struct net *net, u32 bearer_id)
 	return mtu;
 }
 
+int tipc_bearer_min_mtu(struct net *net, u32 bearer_id)
+{
+	int mtu = TIPC_MIN_BEARER_MTU;
+	struct tipc_bearer *b;
+
+	rcu_read_lock();
+	b = bearer_get(net, bearer_id);
+	if (b)
+		mtu += b->encap_hlen;
+	rcu_read_unlock();
+	return mtu;
+}
+
 /* tipc_bearer_xmit_skb - sends buffer to destination over bearer
  */
 void tipc_bearer_xmit_skb(struct net *net, u32 bearer_id,

net/tipc/bearer.h

@@ -146,6 +146,7 @@ struct tipc_media {
  * @identity: array index of this bearer within TIPC bearer array
  * @disc: ptr to link setup request
  * @net_plane: network plane ('A' through 'H') currently associated with bearer
+ * @encap_hlen: encap headers length
  * @up: bearer up flag (bit 0)
  * @refcnt: tipc_bearer reference counter
  *
@@ -170,6 +171,7 @@ struct tipc_bearer {
 	u32 identity;
 	struct tipc_discoverer *disc;
 	char net_plane;
+	u16 encap_hlen;
 	unsigned long up;
 	refcount_t refcnt;
 };
@@ -232,6 +234,7 @@ int tipc_bearer_setup(void);
 void tipc_bearer_cleanup(void);
 void tipc_bearer_stop(struct net *net);
 int tipc_bearer_mtu(struct net *net, u32 bearer_id);
+int tipc_bearer_min_mtu(struct net *net, u32 bearer_id);
 bool tipc_bearer_bcast_support(struct net *net, u32 bearer_id);
 void tipc_bearer_xmit_skb(struct net *net, u32 bearer_id,
 			  struct sk_buff *skb,

net/tipc/udp_media.c

@@ -738,8 +738,8 @@ static int tipc_udp_enable(struct net *net, struct tipc_bearer *b,
 			udp_conf.local_ip.s_addr = local.ipv4.s_addr;
 		udp_conf.use_udp_checksums = false;
 		ub->ifindex = dev->ifindex;
-		if (tipc_mtu_bad(dev, sizeof(struct iphdr) +
-				      sizeof(struct udphdr))) {
+		b->encap_hlen = sizeof(struct iphdr) + sizeof(struct udphdr);
+		if (tipc_mtu_bad(dev, b->encap_hlen)) {
 			err = -EINVAL;
 			goto err;
 		}
@@ -760,6 +760,7 @@ static int tipc_udp_enable(struct net *net, struct tipc_bearer *b,
 		else
 			udp_conf.local_ip6 = local.ipv6;
 		ub->ifindex = dev->ifindex;
+		b->encap_hlen = sizeof(struct ipv6hdr) + sizeof(struct udphdr);
 		b->mtu = 1280;
 #endif
 	} else {