drbd: add packet_type 27 (return_code_only) to netlink api

In case we ever should add an other packet type,
we must not reuse 27, as that currently used for
"empty" return code only replies.
Document it as such.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_nl.c

@@ -2195,7 +2195,8 @@ static void drbd_connector_callback(struct cn_msg *req, struct netlink_skb_parms
 		goto fail;
 	}
 
-	if (nlp->packet_type >= P_nl_after_last_packet) {
+	if (nlp->packet_type >= P_nl_after_last_packet ||
+	    nlp->packet_type == P_return_code_only) {
 		retcode = ERR_PACKET_NR;
 		goto fail;
 	}
@@ -2219,7 +2220,7 @@ static void drbd_connector_callback(struct cn_msg *req, struct netlink_skb_parms
 	reply = (struct drbd_nl_cfg_reply *) cn_reply->data;
 
 	reply->packet_type =
-		cm->reply_body_size ? nlp->packet_type : P_nl_after_last_packet;
+		cm->reply_body_size ? nlp->packet_type : P_return_code_only;
 	reply->minor = nlp->drbd_minor;
 	reply->ret_code = NO_ERROR; /* Might by modified by cm->function. */
 	/* reply->tag_list; might be modified by cm->function. */
@@ -2525,6 +2526,7 @@ void drbd_nl_send_reply(struct cn_msg *req, int ret_code)
 	cn_reply->len = sizeof(struct drbd_nl_cfg_reply);
 	cn_reply->flags = 0;
 
+	reply->packet_type = P_return_code_only;
 	reply->minor = ((struct drbd_nl_cfg_req *)req->data)->drbd_minor;
 	reply->ret_code = ret_code;
 

include/linux/drbd_nl.h

@@ -146,9 +146,13 @@ NL_PACKET(new_c_uuid, 26,
        NL_BIT(		63,	T_MANDATORY,	clear_bm)
 )
 
+#ifdef NL_RESPONSE
+NL_RESPONSE(return_code_only, 27)
+#endif
+
 #undef NL_PACKET
 #undef NL_INTEGER
 #undef NL_INT64
 #undef NL_BIT
 #undef NL_STRING
-
+#undef NL_RESPONSE

include/linux/drbd_tag_magic.h

@@ -7,6 +7,7 @@
 /* declare packet_type enums */
 enum packet_types {
 #define NL_PACKET(name, number, fields) P_ ## name = number,
+#define NL_RESPONSE(name, number) P_ ## name = number,
 #define NL_INTEGER(pn, pr, member)
 #define NL_INT64(pn, pr, member)
 #define NL_BIT(pn, pr, member)