mac80211: update adjusting TBTT bit in beacon

This regression was introduced in "mac80211: cache mesh
beacon".

mesh_sync_offset_adjust_tbtt()  was assuming that the
beacon would be rebuilt in every single pre-tbtt
interrupt, but now the beacon update happens on the
workqueue, and it must be ready for immediate delivery to
the driver.

Save a pointer to the meshconf IE in the beacon_data (this
works because both the IE pointer and beacon buffer are
protected by the same rcu_{dereference,assign_pointer}())
for quick updates during pre-tbtt. This is faster and a
little prettier than iterating over the elements to find
the meshconf IE every time.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh.c

@@ -259,6 +259,9 @@ int mesh_add_meshconf_ie(struct ieee80211_sub_if_data *sdata,
 	*pos++ = WLAN_EID_MESH_CONFIG;
 	*pos++ = meshconf_len;
 
+	/* save a pointer for quick updates in pre-tbtt */
+	ifmsh->meshconf_offset = pos - skb->data;
+
 	/* Active path selection protocol ID */
 	*pos++ = ifmsh->mesh_pp_id;
 	/* Active path selection metric ID   */
@@ -723,6 +726,8 @@ ieee80211_mesh_build_beacon(struct ieee80211_if_mesh *ifmsh)
 
 	bcn->tail_len = skb->len;
 	memcpy(bcn->tail, skb->data, bcn->tail_len);
+	bcn->meshconf = (struct ieee80211_meshconf_ie *)
+					(bcn->tail + ifmsh->meshconf_offset);
 
 	dev_kfree_skb(skb);
 	rcu_assign_pointer(ifmsh->beacon, bcn);