mac80211: Add DS Parameter Set into Probe Request on 2.4 GHz

IEEE Std 802.11k-2008 added DS Parameter Set information element into
Probe Request frames as an optional information on 2.4 GHz band (and
mandatory, if radio measurements are enabled). This allows APs to
filter out Probe Request frames that may be received from neighboring
overlapping channels and by doing so, reduce the number of unnecessary
frames in the air. Make mac80211 add this IE into Probe Request frames
whenever the channel is known (i.e., whenever hwscan is not used).

Signed-off-by: Jouni Malinen <j@w1.fi>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/ieee80211_i.h

@@ -1256,7 +1256,8 @@ void ieee80211_send_auth(struct ieee80211_sub_if_data *sdata,
 			 const u8 *key, u8 key_len, u8 key_idx);
 int ieee80211_build_preq_ies(struct ieee80211_local *local, u8 *buffer,
 			     const u8 *ie, size_t ie_len,
-			     enum ieee80211_band band, u32 rate_mask);
+			     enum ieee80211_band band, u32 rate_mask,
+			     u8 channel);
 void ieee80211_send_probe_req(struct ieee80211_sub_if_data *sdata, u8 *dst,
 			      const u8 *ssid, size_t ssid_len,
 			      const u8 *ie, size_t ie_len);

net/mac80211/scan.c

@@ -242,7 +242,8 @@ static bool ieee80211_prep_hw_scan(struct ieee80211_local *local)
 	local->hw_scan_req->n_channels = n_chans;
 
 	ielen = ieee80211_build_preq_ies(local, (u8 *)local->hw_scan_req->ie,
-					 req->ie, req->ie_len, band, (u32) -1);
+					 req->ie, req->ie_len, band, (u32) -1,
+					 0);
 	local->hw_scan_req->ie_len = ielen;
 
 	return true;

net/mac80211/util.c

@@ -895,7 +895,8 @@ void ieee80211_send_auth(struct ieee80211_sub_if_data *sdata,
 
 int ieee80211_build_preq_ies(struct ieee80211_local *local, u8 *buffer,
 			     const u8 *ie, size_t ie_len,
-			     enum ieee80211_band band, u32 rate_mask)
+			     enum ieee80211_band band, u32 rate_mask,
+			     u8 channel)
 {
 	struct ieee80211_supported_band *sband;
 	u8 *pos;
@@ -947,6 +948,12 @@ int ieee80211_build_preq_ies(struct ieee80211_local *local, u8 *buffer,
 		pos += ext_rates_len;
 	}
 
+	if (channel && sband->band == IEEE80211_BAND_2GHZ) {
+		*pos++ = WLAN_EID_DS_PARAMS;
+		*pos++ = 1;
+		*pos++ = channel;
+	}
+
 	/* insert custom IEs that go before HT */
 	if (ie && ie_len) {
 		static const u8 before_ht[] = {
@@ -1013,6 +1020,7 @@ void ieee80211_send_probe_req(struct ieee80211_sub_if_data *sdata, u8 *dst,
 	struct ieee80211_mgmt *mgmt;
 	size_t buf_len;
 	u8 *buf;
+	u8 chan;
 
 	/* FIXME: come up with a proper value */
 	buf = kmalloc(200 + ie_len, GFP_KERNEL);
@@ -1022,10 +1030,14 @@ void ieee80211_send_probe_req(struct ieee80211_sub_if_data *sdata, u8 *dst,
 		return;
 	}
 
+	chan = ieee80211_frequency_to_channel(
+		local->hw.conf.channel->center_freq);
+
 	buf_len = ieee80211_build_preq_ies(local, buf, ie, ie_len,
 					   local->hw.conf.channel->band,
 					   sdata->rc_rateidx_mask
-					   [local->hw.conf.channel->band]);
+					   [local->hw.conf.channel->band],
+					   chan);
 
 	skb = ieee80211_probereq_get(&local->hw, &sdata->vif,
 				     ssid, ssid_len,