https://github.com/ivkos/hostap-force-ht40/commit/8526009738bd63af37ec4f7bff7c29755777dec8

--- hostapd-2.9/hostapd/config_file.c
+++ hostapd-2.9/hostapd/config_file.c
@@ -3419,6 +3419,8 @@
 				   line);
 			return 1;
 		}
+	} else if (os_strcmp(buf, "force_ht40") == 0) {
+		conf->force_ht40 = atoi(pos);
 	} else if (os_strcmp(buf, "require_ht") == 0) {
 		conf->require_ht = atoi(pos);
 	} else if (os_strcmp(buf, "obss_interval") == 0) {
--- hostapd-2.9/hostapd/hostapd.conf
+++ hostapd-2.9/hostapd/hostapd.conf
@@ -594,6 +594,12 @@
 # L-SIG TXOP protection support: [LSIG-TXOP-PROT] (disabled if not set)
 #ht_capab=[HT40-][SHORT-GI-20][SHORT-GI-40]
 
+# Skip checks for 40 MHz intolerance or overlapping stations and force
+# 40 MHz channel width
+# 0 = disabled (default)
+# 1 = enabled
+#force_ht40=0
+
 # Require stations to support HT PHY (reject association if they do not)
 #require_ht=1
 
--- hostapd-2.9/src/ap/ap_config.c
+++ hostapd-2.9/src/ap/ap_config.c
@@ -225,6 +225,8 @@
 
 	conf->ht_capab = HT_CAP_INFO_SMPS_DISABLED;
 
+	conf->force_ht40 = 0;
+
 	conf->ap_table_max_size = 255;
 	conf->ap_table_expiration_time = 60;
 	conf->track_sta_max_age = 180;
--- hostapd-2.9/src/ap/ap_config.h
+++ hostapd-2.9/src/ap/ap_config.h
@@ -932,6 +932,7 @@
 
 	int ht_op_mode_fixed;
 	u16 ht_capab;
+	int force_ht40;
 	int ieee80211n;
 	int secondary_channel;
 	int no_pri_sec_switch;
--- hostapd-2.9/src/ap/hw_features.c
+++ hostapd-2.9/src/ap/hw_features.c
@@ -310,7 +310,7 @@
 	wpa_scan_results_free(scan_res);
 
 	iface->secondary_ch = iface->conf->secondary_channel;
-	if (!oper40) {
+	if (!iface->conf->force_ht40 && !oper40) {
 		wpa_printf(MSG_INFO, "20/40 MHz operation not permitted on "
 			   "channel pri=%d sec=%d based on overlapping BSSes",
 			   iface->conf->channel,
--- hostapd-2.9/src/ap/ieee802_11_ht.c
+++ hostapd-2.9/src/ap/ieee802_11_ht.c
@@ -207,6 +207,9 @@
 	if (iface->current_mode->mode != HOSTAPD_MODE_IEEE80211G)
 		return 1;
 
+	if (iface->conf->force_ht40)
+		return 1;
+
 	pri_freq = hostapd_hw_get_freq(iface->bss[0], iface->conf->channel);
 
 	if (iface->conf->secondary_channel > 0)
@@ -299,7 +302,7 @@
 			       HOSTAPD_MODULE_IEEE80211,
 			       HOSTAPD_LEVEL_DEBUG,
 			       "20 MHz BSS width request bit is set in BSS coexistence information field");
-		is_ht40_allowed = 0;
+		is_ht40_allowed = iface->conf->force_ht40;
 	}
 
 	if (bc_ie->coex_param & WLAN_20_40_BSS_COEX_40MHZ_INTOL) {
@@ -309,7 +312,7 @@
 			       HOSTAPD_MODULE_IEEE80211,
 			       HOSTAPD_LEVEL_DEBUG,
 			       "40 MHz intolerant bit is set in BSS coexistence information field");
-		is_ht40_allowed = 0;
+		is_ht40_allowed = iface->conf->force_ht40;
 	}
 
 	/* 20/40 BSS Intolerant Channel Report element (zero or more times) */
@@ -341,7 +344,7 @@
 				       HOSTAPD_LEVEL_DEBUG,
 				       "20_40_INTOLERANT channel %d reported",
 				       chan);
-			is_ht40_allowed = 0;
+			is_ht40_allowed = iface->conf->force_ht40;
 		}
 
 		data += 2 + ielen;