rt2x00: Properly request tx headroom for alignment operations.

Current rt2x00 drivers may result in a "ieee80211_tx_status: headroom too
small" error message when a frame needs to be properly aligned before
transmitting it.
This is because the space needed to ensure proper alignment isn't
requested from mac80211.
Fix this by adding sufficient amount of alignment space to the amount
of headroom requested for TX frames.
Reported-by: David Ellingsworth <david@identd.dyndns.org>
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00.h

@@ -112,6 +112,12 @@
 #define ALIGN_SIZE(__skb, __header) \
 	(  ((unsigned long)((__skb)->data + (__header))) & 3 )
 
+/*
+ * Constants for extra TX headroom for alignment purposes.
+ */
+#define RT2X00_ALIGN_SIZE	4 /* Only whole frame needs alignment */
+#define RT2X00_L2PAD_SIZE	8 /* Both header & payload need alignment */
+
 /*
  * Standard timing and size defines.
  * These values should follow the ieee80211 specifications.

drivers/net/wireless/rt2x00/rt2x00dev.c

@@ -686,7 +686,17 @@ static int rt2x00lib_probe_hw(struct rt2x00_dev *rt2x00dev)
 	/*
 	 * Initialize extra TX headroom required.
 	 */
-	rt2x00dev->hw->extra_tx_headroom = rt2x00dev->ops->extra_tx_headroom;
+	rt2x00dev->hw->extra_tx_headroom =
+		max_t(unsigned int, IEEE80211_TX_STATUS_HEADROOM,
+		      rt2x00dev->ops->extra_tx_headroom);
+
+	/*
+	 * Take TX headroom required for alignment into account.
+	 */
+	if (test_bit(DRIVER_REQUIRE_L2PAD, &rt2x00dev->flags))
+		rt2x00dev->hw->extra_tx_headroom += RT2X00_L2PAD_SIZE;
+	else if (test_bit(DRIVER_REQUIRE_DMA, &rt2x00dev->flags))
+		rt2x00dev->hw->extra_tx_headroom += RT2X00_ALIGN_SIZE;
 
 	/*
 	 * Register HW.