[RFC AP: ACS: select primary channel for 5G-band based on interference factor

[Марков Михаил Александрович]

Make ACS algorithm select the best channel within a bandwidth as primary
channel instead of just picking the first one.

The reason i had to add acs_adjust_sec_chan is because even if i use VHT80
ieee80211n_allowed_ht40_channel_pair -> allowed_ht40_channel_pair
check fails and i get fallback to 20 MHz.

Signed-off-by: Markov Mikhail <markov.mikhail at itmh.ru>
---
 src/ap/acs.c       | 42 ++++++++++++++++++++++++++++++++++--------
 src/ap/ap_config.h |  1 +
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/src/ap/acs.c b/src/ap/acs.c
index aa2ceb0d1..1ac37271d 100644
--- a/src/ap/acs.c
+++ b/src/ap/acs.c
@@ -642,9 +642,10 @@ acs_find_ideal_chan_mode(struct hostapd_iface *iface,
 			 int n_chans, u32 bw,
 			 struct hostapd_channel_data **rand_chan,
 			 struct hostapd_channel_data **ideal_chan,
+			 struct hostapd_channel_data **first_chan,
 			 long double *ideal_factor)
 {
-	struct hostapd_channel_data *chan, *adj_chan = NULL;
+	struct hostapd_channel_data *chan, *adj_chan = NULL, *pri_chan;
 	long double factor;
 	int i, j;
 	unsigned int k;
@@ -653,7 +654,7 @@ acs_find_ideal_chan_mode(struct hostapd_iface *iface,
 		double total_weight;
 		struct acs_bias *bias, tmp_bias;
 
-		chan = &mode->channels[i];
+		pri_chan = chan = &mode->channels[i];
 
 		/* Since in the current ACS implementation the first channel is
 		 * always a primary channel, skip channels not available as
@@ -727,6 +728,9 @@ acs_find_ideal_chan_mode(struct hostapd_iface *iface,
 			if (acs_usable_chan(adj_chan)) {
 				factor += adj_chan->interference_factor;
 				total_weight += 1;
+
+				if (adj_chan->interference_factor < pri_chan->interference_factor)
+					pri_chan = adj_chan;
 			}
 		}
 
@@ -805,7 +809,8 @@ acs_find_ideal_chan_mode(struct hostapd_iface *iface,
 		if (acs_usable_chan(chan) &&
 		    (!*ideal_chan || factor < *ideal_factor)) {
 			*ideal_factor = factor;
-			*ideal_chan = chan;
+			*ideal_chan = pri_chan;
+			*first_chan = chan;
 		}
 
 		/* This channel would at least be usable */
@@ -825,7 +830,7 @@ static struct hostapd_channel_data *
 acs_find_ideal_chan(struct hostapd_iface *iface)
 {
 	struct hostapd_channel_data *ideal_chan = NULL,
-		*rand_chan = NULL;
+		*rand_chan = NULL, *first_chan = NULL;
 	long double ideal_factor = 0;
 	int i;
 	int n_chans = 1;
@@ -866,14 +871,16 @@ acs_find_ideal_chan(struct hostapd_iface *iface)
 		mode = &iface->hw_features[i];
 		if (!hostapd_hw_skip_mode(iface, mode))
 			acs_find_ideal_chan_mode(iface, mode, n_chans, bw,
-						 &rand_chan, &ideal_chan,
+						 &rand_chan, &ideal_chan, &first_chan,
 						 &ideal_factor);
 	}
 
 	if (ideal_chan) {
 		wpa_printf(MSG_DEBUG, "ACS: Ideal channel is %d (%d MHz) with total interference factor of %Lg",
 			   ideal_chan->chan, ideal_chan->freq, ideal_factor);
-		return ideal_chan;
+		iface->conf->bw_first_chan = first_chan->chan;
+		/* Primary channel selection belongs only to 5G band. */
+		return is_24ghz_mode(iface->current_mode->mode)? first_chan : ideal_chan;
 	}
 
 	return rand_chan;
@@ -905,7 +912,24 @@ static void acs_adjust_center_freq(struct hostapd_iface *iface)
 	}
 
 	hostapd_set_oper_centr_freq_seg0_idx(iface->conf,
-					     iface->conf->channel + offset);
+						iface->conf->bw_first_chan + offset);
+}
+
+
+static void acs_adjust_sec_chan(struct hostapd_config *conf)
+{
+	const int ht40_plus_channels[] = { 36, 44, 52, 60, 100, 108, 116, 124, 132, 140,
+								149, 157, 165, 184, 192 };
+	int i, sec_chan = -1;
+
+	for (i = 0; i < ARRAY_SIZE(ht40_plus_channels); i++) {
+		if (conf->channel == ht40_plus_channels[i]) {
+		    sec_chan = 1;
+		    break;
+		}
+	}
+
+	conf->secondary_channel = sec_chan;
 }
 
 
@@ -961,8 +985,10 @@ static void acs_study(struct hostapd_iface *iface)
 	iface->conf->channel = ideal_chan->chan;
 	iface->freq = ideal_chan->freq;
 
-	if (iface->conf->ieee80211ac || iface->conf->ieee80211ax)
+	if (iface->conf->ieee80211ac || iface->conf->ieee80211ax) {
 		acs_adjust_center_freq(iface);
+		acs_adjust_sec_chan(iface->conf);
+	}
 
 	err = 0;
 fail:
diff --git a/src/ap/ap_config.h b/src/ap/ap_config.h
index bada04c3e..18d662c6e 100644
--- a/src/ap/ap_config.h
+++ b/src/ap/ap_config.h
@@ -909,6 +909,7 @@ struct hostapd_config {
 	int fragm_threshold;
 	u8 op_class;
 	u8 channel;
+	u8 bw_first_chan;
 	int enable_edmg;
 	u8 edmg_channel;
 	u8 acs;
-- 
2.28.0