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

commit d24deb25 upstream.

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>
[bwh: Adjust context for 2.6.32]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/net/mac80211.h

@@ -1700,6 +1700,12 @@ void ieee80211_rx(struct ieee80211_hw *hw, struct sk_buff *skb);
  */
 void ieee80211_rx_irqsafe(struct ieee80211_hw *hw, struct sk_buff *skb);
 
+/*
+ * 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

@@ -859,6 +859,8 @@ 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));