wireless: implement basic rate helper function

This adds a helper function that, given a bitmap of basic
rates and a bitrate returns the response rate for this rate.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/wireless.h

@@ -340,6 +340,22 @@ ieee80211_get_channel(struct wiphy *wiphy, int freq)
 	return __ieee80211_get_channel(wiphy, freq);
 }
 
+/**
+ * ieee80211_get_response_rate - get basic rate for a given rate
+ *
+ * @sband: the band to look for rates in
+ * @basic_rates: bitmap of basic rates
+ * @bitrate: the bitrate for which to find the basic rate
+ *
+ * This function returns the basic rate corresponding to a given
+ * bitrate, that is the next lower bitrate contained in the basic
+ * rate map, which is, for this function, given as a bitmap of
+ * indices of rates in the band's bitrate table.
+ */
+struct ieee80211_rate *
+ieee80211_get_response_rate(struct ieee80211_supported_band *sband,
+			    u64 basic_rates, int bitrate);
+
 /**
  * regulatory_hint - driver hint to the wireless core a regulatory domain
  * @wiphy: the wireless device giving the hint (used only for reporting

net/wireless/util.c

@@ -7,6 +7,25 @@
 #include <asm/bitops.h>
 #include "core.h"
 
+struct ieee80211_rate *
+ieee80211_get_response_rate(struct ieee80211_supported_band *sband,
+			    u64 basic_rates, int bitrate)
+{
+	struct ieee80211_rate *result = &sband->bitrates[0];
+	int i;
+
+	for (i = 0; i < sband->n_bitrates; i++) {
+		if (!(basic_rates & BIT(i)))
+			continue;
+		if (sband->bitrates[i].bitrate > bitrate)
+			continue;
+		result = &sband->bitrates[i];
+	}
+
+	return result;
+}
+EXPORT_SYMBOL(ieee80211_get_response_rate);
+
 int ieee80211_channel_to_frequency(int chan)
 {
 	if (chan < 14)