mac80211: unset peer's HT 40 MHz support if not usable

When an interface is configured to a 20 MHz channel
and the device as well as the peer are 40 MHz capable
the HT capabilities of the peer are not restricted to
20 MHz, even though they're supposed to be restricted
to the currently possible capabilities.

Unset the 40 MHz HT capability bits in this case.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/ht.c

@@ -120,6 +120,21 @@ void ieee80211_ht_cap_ie_to_sta_ht_cap(struct ieee80211_sub_if_data *sdata,
 		   IEEE80211_HT_CAP_SGI_20 |
 		   IEEE80211_HT_CAP_SGI_40 |
 		   IEEE80211_HT_CAP_DSSSCCK40));
+
+	/* Unset 40 MHz if we're not using a 40 MHz channel */
+	switch (sdata->vif.bss_conf.chandef.width) {
+	case NL80211_CHAN_WIDTH_20_NOHT:
+	case NL80211_CHAN_WIDTH_20:
+		ht_cap->cap &= ~IEEE80211_HT_CAP_SGI_40;
+		ht_cap->cap &= ~IEEE80211_HT_CAP_SUP_WIDTH_20_40;
+		break;
+	case NL80211_CHAN_WIDTH_40:
+	case NL80211_CHAN_WIDTH_80:
+	case NL80211_CHAN_WIDTH_80P80:
+	case NL80211_CHAN_WIDTH_160:
+		break;
+	}
+
 	/*
 	 * The STBC bits are asymmetric -- if we don't have
 	 * TX then mask out the peer's RX and vice versa.