rt2x00: Further L2 padding fixes.

Fix a couple of more bugs in the L2 padding code:
1. Compute the amount of L2 padding correctly (in 3 places).
2. Trim the skb correctly when the L2 padding has been applied.

Also introduce a central macro the compute the L2 padding size.
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

@@ -103,6 +103,12 @@
 #define GET_DURATION(__size, __rate)	(((__size) * 8 * 10) / (__rate))
 #define GET_DURATION_RES(__size, __rate)(((__size) * 8 * 10) % (__rate))
 
+/*
+ * Determine the number of L2 padding bytes required between the header and
+ * the payload.
+ */
+#define L2PAD_SIZE(__hdrlen)	(-(__hdrlen) & 3)
+
 /*
  * Determine the alignment requirement,
  * to make sure the 802.11 payload is padded to a 4-byte boundrary

drivers/net/wireless/rt2x00/rt2x00queue.c

@@ -181,7 +181,7 @@ void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int header_length)
 	unsigned int frame_length = skb->len;
 	unsigned int header_align = ALIGN_SIZE(skb, 0);
 	unsigned int payload_align = ALIGN_SIZE(skb, header_length);
-	unsigned int l2pad = 4 - (payload_align - header_align);
+	unsigned int l2pad = L2PAD_SIZE(header_length);
 
 	if (header_align == payload_align) {
 		/*
@@ -216,6 +216,7 @@ void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int header_length)
 		memmove(skb->data + header_length + l2pad,
 			skb->data + header_length + l2pad + payload_align,
 			frame_length - header_length);
+		skb_trim(skb, frame_length + l2pad);
 		skbdesc->flags |= SKBDESC_L2_PADDED;
 	}
 }
@@ -223,7 +224,7 @@ void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int header_length)
 void rt2x00queue_remove_l2pad(struct sk_buff *skb, unsigned int header_length)
 {
 	struct skb_frame_desc *skbdesc = get_skb_frame_desc(skb);
-	unsigned int l2pad = 4 - (header_length & 3);
+	unsigned int l2pad = L2PAD_SIZE(header_length);
 
 	if (!l2pad || (skbdesc->flags & SKBDESC_L2_PADDED))
 		return;
@@ -346,7 +347,8 @@ static void rt2x00queue_create_tx_descriptor(struct queue_entry *entry,
 	 * Header and alignment information.
 	 */
 	txdesc->header_length = ieee80211_get_hdrlen_from_skb(entry->skb);
-	txdesc->l2pad = ALIGN_SIZE(entry->skb, txdesc->header_length);
+	if (test_bit(DRIVER_REQUIRE_L2PAD, &rt2x00dev->flags))
+		txdesc->l2pad = L2PAD_SIZE(txdesc->header_length);
 
 	/*
 	 * Check whether this frame is to be acked.