[PATCH] cfg80211: Radiotap parser

Generic code to walk through the fields in a radiotap header, accounting
for nasties like extended "field present" bitfields and alignment rules

Signed-off-by: Andy Green <andy@warmcat.com>
Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

Documentation/networking/radiotap-headers.txt

@@ -84,4 +84,69 @@ Example valid radiotap header
 	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>