gre_offload: statically build GRE offloading support

GRO/GSO layers can be enabled on a node, even if said
node is only forwarding packets.

This patch permits GSO (and upcoming GRO) support for GRE
encapsulated packets, even if the host has no GRE tunnel setup.

Signed-off-by: Eric Dumazet <[e-mail]>
Cc: H.K. Jerry Chu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/gre.h

···
 int gre_cisco_register(struct gre_cisco_protocol *proto);
 int gre_cisco_unregister(struct gre_cisco_protocol *proto);
 
-int gre_offload_init(void);
-void gre_offload_exit(void);
-
 void gre_build_header(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
 		      int hdr_len);
 

net/ipv4/Makefile

···
 	     tcp_offload.o datagram.o raw.o udp.o udplite.o \
 	     udp_offload.o arp.o icmp.o devinet.o af_inet.o igmp.o \
 	     fib_frontend.o fib_semantics.o fib_trie.o \
-	     inet_fragment.o ping.o ip_tunnel_core.o
+	     inet_fragment.o ping.o ip_tunnel_core.o gre_offload.o
 
 obj-$(CONFIG_NET_IP_TUNNEL) += ip_tunnel.o
 obj-$(CONFIG_SYSCTL) += sysctl_net_ipv4.o
···
 obj-$(CONFIG_IP_MULTIPLE_TABLES) += fib_rules.o
 obj-$(CONFIG_IP_MROUTE) += ipmr.o
 obj-$(CONFIG_NET_IPIP) += ipip.o
-gre-y := gre_demux.o gre_offload.o
+gre-y := gre_demux.o
 obj-$(CONFIG_NET_IPGRE_DEMUX) += gre.o
 obj-$(CONFIG_NET_IPGRE) += ip_gre.o
 obj-$(CONFIG_NET_IPVTI) += ip_vti.o

net/ipv4/gre_demux.c

···
 		goto err_gre;
 	}
 
-	if (gre_offload_init()) {
-		pr_err("can't add protocol offload\n");
-		goto err_gso;
-	}
-
 	return 0;
-err_gso:
-	gre_del_protocol(&ipgre_protocol, GREPROTO_CISCO);
 err_gre:
 	inet_del_protocol(&net_gre_protocol, IPPROTO_GRE);
 err:
···
 
 static void __exit gre_exit(void)
 {
-	gre_offload_exit();
-
 	gre_del_protocol(&ipgre_protocol, GREPROTO_CISCO);
 	inet_del_protocol(&net_gre_protocol, IPPROTO_GRE);
 }

net/ipv4/gre_offload.c

···
 	},
 };
 
-int __init gre_offload_init(void)
+static int __init gre_offload_init(void)
 {
 	return inet_add_offload(&gre_offload, IPPROTO_GRE);
 }
 
-void __exit gre_offload_exit(void)
+static void __exit gre_offload_exit(void)
 {
 	inet_del_offload(&gre_offload, IPPROTO_GRE);
 }
+
+module_init(gre_offload_init);
+module_exit(gre_offload_exit);