Add version number to API between netback and accel plugin

diff -r 330deb0c6935 drivers/xen/netback/accel.c
--- a/drivers/xen/netback/accel.c	Wed Oct 03 13:30:20 2007 +0100
+++ b/drivers/xen/netback/accel.c	Wed Oct 03 13:32:27 2007 +0100
@@ -103,18 +103,35 @@ static int netback_accelerator_tell_back
  * Entry point for an netback accelerator plugin module.  Called to
  * advertise its presence, and connect to any suitable backends.
  */
-void netback_connect_accelerator(int id, const char *eth_name, 
-				 struct netback_accel_hooks *hooks)
-{
-	struct netback_accelerator *new_accelerator = 
+int netback_connect_accelerator(unsigned version, int id, const char *eth_name, 
+				struct netback_accel_hooks *hooks)
+{
+	struct netback_accelerator *new_accelerator;
+	unsigned eth_name_len, flags;
+
+	if (version != NETBACK_ACCEL_VERSION) {
+		if (version > NETBACK_ACCEL_VERSION) {
+			/* Caller has higher version number, leave it
+			   up to them to decide whether to continue.
+			   They can recall with a lower number if
+			   they're happy to be compatible with us */
+			return NETBACK_ACCEL_VERSION;
+		} else {
+			/* We have a more recent version than caller.
+			   Currently reject, but may in future be able
+			   to be backwardly compatible */
+			return -EPROTO;
+		}
+	}
+
+	new_accelerator = 
 		kmalloc(sizeof(struct netback_accelerator), GFP_KERNEL);
-	unsigned eth_name_len, flags;
-
 	if (!new_accelerator) {
 		DPRINTK("%s: failed to allocate memory for accelerator\n",
 			__FUNCTION__);
-		return;
-	}
+		return -ENOMEM;
+	}
+
 
 	new_accelerator->id = id;
 	
@@ -124,7 +141,7 @@ void netback_connect_accelerator(int id,
 		DPRINTK("%s: failed to allocate memory for eth_name string\n",
 			__FUNCTION__);
 		kfree(new_accelerator);
-		return;
+		return -ENOMEM;
 	}
 	strlcpy(new_accelerator->eth_name, eth_name, eth_name_len);
 	
@@ -139,6 +156,8 @@ void netback_connect_accelerator(int id,
 	/* tell existing backends about new plugin */
 	xenbus_for_each_backend(new_accelerator, 
 				netback_accelerator_tell_backend);
+
+	return 0;
 
 }
 EXPORT_SYMBOL_GPL(netback_connect_accelerator);
diff -r 330deb0c6935 drivers/xen/netback/common.h
--- a/drivers/xen/netback/common.h	Wed Oct 03 13:30:20 2007 +0100
+++ b/drivers/xen/netback/common.h	Wed Oct 03 13:36:10 2007 +0100
@@ -149,9 +149,16 @@ struct backend_info {
 	struct netback_accelerator *accelerator;
 };
 
-/* Connect an accelerator plugin module to netback */
-extern void netback_connect_accelerator(int id, const char *eth_name, 
-					struct netback_accel_hooks *hooks);
+#define NETBACK_ACCEL_VERSION 0x00010000
+
+/* 
+ * Connect an accelerator plugin module to netback.  Returns zero on
+ * success, < 0 on error, > 0 (with highest version number supported)
+ * if version mismatch.
+ */
+extern int netback_connect_accelerator(unsigned version,
+				       int id, const char *eth_name, 
+				       struct netback_accel_hooks *hooks);
 /* Disconnect a previously connected accelerator plugin module */
 extern void netback_disconnect_accelerator(int id, const char *eth_name);