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Diffstat (limited to 'Documentation/networking/radiotap-headers.txt')
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diff --git a/Documentation/networking/radiotap-headers.txt b/Documentation/networking/radiotap-headers.txt deleted file mode 100644 index 953331c7984f..000000000000 --- a/Documentation/networking/radiotap-headers.txt +++ /dev/null @@ -1,152 +0,0 @@ -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> |