mac80211: Add define for TX headroom reserved by mac80211 itself.

Add a definition of the amount of TX headroom reserved by mac80211 itself
for its own purposes. Also add BUILD_BUG_ON to validate the value.
This define can then be used by drivers to request additional TX headroom
in the most efficient manner.
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -1737,6 +1737,12 @@ static inline void ieee80211_rx_ni(struct ieee80211_hw *hw,
 	local_bh_enable();
 }
 
+/*
+ * The TX headroom reserved by mac80211 for its own tx_status functions.
+ * This is enough for the radiotap header.
+ */
+#define IEEE80211_TX_STATUS_HEADROOM	13
+
 /**
  * ieee80211_tx_status - transmit status callback
  *

net/mac80211/main.c

@@ -515,6 +515,8 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 	 * and we need some headroom for passing the frame to monitor
 	 * interfaces, but never both at the same time.
 	 */
+	BUILD_BUG_ON(IEEE80211_TX_STATUS_HEADROOM !=
+			sizeof(struct ieee80211_tx_status_rtap_hdr));
 	local->tx_headroom = max_t(unsigned int , local->hw.extra_tx_headroom,
 				   sizeof(struct ieee80211_tx_status_rtap_hdr));