ieee802154: change needed headroom/tailroom

This patch cleanups needed_headroom, needed_tailroom and hard_header_len
fields for wpan and lowpan interfaces.

For wpan interfaces the worst case mac header len should be part of
needed_headroom, currently this is set as hard_header_len, but
hard_header_len should be set to the minimum header length which xmit
call assumes and this is the minimum frame length of 802.15.4.
The hard_header_len value will check inside send callbacl of AF_PACKET
raw sockets.

For lowpan interfaces, if fragmentation isn't needed the skb will
call dev_hard_header for 802154 layer and queue it afterwards. This
happens without new skb allocation, so we need the same headroom and
tailroom lengths like 802154 inside 802154 6lowpan layer. At least we
assume as minimum header length an ipv6 header size.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/linux/ieee802154.h

@@ -31,6 +31,17 @@
 #define IEEE802154_ACK_PSDU_LEN		5
 #define IEEE802154_MIN_PSDU_LEN		9
 #define IEEE802154_FCS_LEN		2
+#define IEEE802154_MAX_AUTH_TAG_LEN	16
+
+/*  General MAC frame format:
+ *  2 bytes: Frame Control
+ *  1 byte:  Sequence Number
+ * 20 bytes: Addressing fields
+ * 14 bytes: Auxiliary Security Header
+ */
+#define IEEE802154_MAX_HEADER_LEN	(2 + 1 + 20 + 14)
+#define IEEE802154_MIN_HEADER_LEN	(IEEE802154_ACK_PSDU_LEN - \
+					 IEEE802154_FCS_LEN)
 
 #define IEEE802154_PAN_ID_BROADCAST	0xffff
 #define IEEE802154_ADDR_SHORT_BROADCAST	0xffff