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+How to use radiotap headers
+===========================
+
+Pointer to the radiotap include file
+------------------------------------
+
+Radiotap headers are variable-length and extensible, you can get most of the
+information you need to know on them from:
+
+./include/net/ieee80211_radiotap.h
+
+This document gives an overview and warns on some corner cases.
+
+
+Structure of the header
+-----------------------
+
+There is a fixed portion at the start which contains a u32 bitmap that defines
+if the possible argument associated with that bit is present or not.  So if b0
+of the it_present member of ieee80211_radiotap_header is set, it means that
+the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
+argument area.
+
+   < 8-byte ieee80211_radiotap_header >
+   [ <possible argument bitmap extensions ... > ]
+   [ <argument> ... ]
+
+At the moment there are only 13 possible argument indexes defined, but in case
+we run out of space in the u32 it_present member, it is defined that b31 set
+indicates that there is another u32 bitmap following (shown as "possible
+argument bitmap extensions..." above), and the start of the arguments is moved
+forward 4 bytes each time.
+
+Note also that the it_len member __le16 is set to the total number of bytes
+covered by the ieee80211_radiotap_header and any arguments following.
+
+
+Requirements for arguments
+--------------------------
+
+After the fixed part of the header, the arguments follow for each argument
+index whose matching bit is set in the it_present member of
+ieee80211_radiotap_header.
+
+ - the arguments are all stored little-endian!
+
+ - the argument payload for a given argument index has a fixed size.  So
+   IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
+   present.  See the comments in ./include/net/ieee80211_radiotap.h for a nice
+   breakdown of all the argument sizes
+
+ - the arguments must be aligned to a boundary of the argument size using
+   padding.  So a u16 argument must start on the next u16 boundary if it isn't
+   already on one, a u32 must start on the next u32 boundary and so on.
+
+ - "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
+   the first byte of the radiotap header.  The absolute alignment of that first
+   byte isn't defined.  So even if the whole radiotap header is starting at, eg,
+   address 0x00000003, still the first byte of the radiotap header is treated as
+   0 for alignment purposes.
+
+ - the above point that there may be no absolute alignment for multibyte
+   entities in the fixed radiotap header or the argument region means that you
+   have to take special evasive action when trying to access these multibyte
+   entities.  Some arches like Blackfin cannot deal with an attempt to
+   dereference, eg, a u16 pointer that is pointing to an odd address.  Instead
+   you have to use a kernel API get_unaligned() to dereference the pointer,
+   which will do it bytewise on the arches that require that.
+
+ - The arguments for a given argument index can be a compound of multiple types
+   together.  For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
+   consisting of two u16s of total length 4.  When this happens, the padding
+   rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
+
+
+Example valid radiotap header
+-----------------------------
+
+	0x00, 0x00, // <-- radiotap version + pad byte
+	0x0b, 0x00, // <- radiotap header length
+	0x04, 0x0c, 0x00, 0x00, // <-- bitmap
+	0x6c, // <-- rate (in 500kHz units)
+	0x0c, //<-- tx power
+	0x01 //<-- antenna
+
+
+Using the Radiotap Parser
+-------------------------
+
+If you are having to parse a radiotap struct, you can radically simplify the
+job by using the radiotap parser that lives in net/wireless/radiotap.c and has
+its prototypes available in include/net/cfg80211.h.  You use it like this:
+
+#include <net/cfg80211.h>
+
+/* buf points to the start of the radiotap header part */
+
+int MyFunction(u8 * buf, int buflen)
+{
+	int pkt_rate_100kHz = 0, antenna = 0, pwr = 0;
+	struct ieee80211_radiotap_iterator iterator;
+	int ret = ieee80211_radiotap_iterator_init(&iterator, buf, buflen);
+
+	while (!ret) {
+
+		ret = ieee80211_radiotap_iterator_next(&iterator);
+
+		if (ret)
+			continue;
+
+		/* see if this argument is something we can use */
+
+		switch (iterator.this_arg_index) {
+		/*
+		 * You must take care when dereferencing iterator.this_arg
+		 * for multibyte types... the pointer is not aligned.  Use
+		 * get_unaligned((type *)iterator.this_arg) to dereference
+		 * iterator.this_arg for type "type" safely on all arches.
+		 */
+		case IEEE80211_RADIOTAP_RATE:
+			/* radiotap "rate" u8 is in
+			 * 500kbps units, eg, 0x02=1Mbps
+			 */
+			pkt_rate_100kHz = (*iterator.this_arg) * 5;
+			break;
+
+		case IEEE80211_RADIOTAP_ANTENNA:
+			/* radiotap uses 0 for 1st ant */
+			antenna = *iterator.this_arg);
+			break;
+
+		case IEEE80211_RADIOTAP_DBM_TX_POWER:
+			pwr = *iterator.this_arg;
+			break;
+
+		default:
+			break;
+		}
+	}  /* while more rt headers */
+
+	if (ret != -ENOENT)
+		return TXRX_DROP;
+
+	/* discard the radiotap header part */
+	buf += iterator.max_length;
+	buflen -= iterator.max_length;
+
+	...
+
+}
+
+Andy Green <andy@warmcat.com>