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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>