Improve BSS selection with default noise floor values

When noise floor measurements are not available, compute SNR
using default values for the noise floor. This helps steer us
towards 5 GHz BSSes in high signal strength environments.

In more detail...

Existing code prefers a 5 GHz BSS when the 5 GHz BSS's signal
strength is "close" to that of the 2.4 GHz BSS, or when both SNRs
are large. However, the mwifiex driver does not provide noise
floor measurements, so we can't compute SNRs.

Because mwifiex doesn't provide NF measurements, the "large SNR"
code wasn't effective. By using default values for the noise floor,
we can again compute SNRs, and decide that the SNR is high enough
that we shouldn't worry about the exact difference in SNR.

The default noise floor values (one for 2.4 GHz, and one for 5 GHz)
were chosen by measurement in a noisy environment, so they should be
conservative.

Note that while this patch is motivated by mwifiex, it affects
ath9k as well. Although ath9k provides noise floor measurements
in general, it will sometimes fail to provide a measurement for
one or more specific channels.

As a result of this patch, we'll always compare BSSes based on SNR
(either measured or estimated), rather than sometimes comparing
based on signal strength. ("Always" assumes that the
WPA_SCAN_LEVEL_DBM flag is set. It is for mwifiex and ath9k.)

While there:
- fix a whitespace issue (spaces -> tab)
- clean up existing comments
- update dump_scan_res to indicate whether the noise floor is
  measured, or default

Signed-hostap: mukesh agrawal <quiche@chromium.org>

wpa_supplicant/scan.c

@@ -1594,11 +1594,12 @@ struct wpabuf * wpa_scan_get_vendor_ie_multi(const struct wpa_scan_res *res,
  */
 #define GREAT_SNR 30
 
+#define IS_5GHZ(n) (n > 4000)
+
 /* Compare function for sorting scan results. Return >0 if @b is considered
  * better. */
 static int wpa_scan_result_compar(const void *a, const void *b)
 {
-#define IS_5GHZ(n) (n > 4000)
 #define MIN(a,b) a < b ? a : b
 	struct wpa_scan_res **_wa = (void *) a;
 	struct wpa_scan_res **_wb = (void *) b;
@@ -1626,18 +1627,18 @@ static int wpa_scan_result_compar(const void *a, const void *b)
 	    (wb->caps & IEEE80211_CAP_PRIVACY) == 0)
 		return -1;
 
-	if ((wa->flags & wb->flags & WPA_SCAN_LEVEL_DBM) &&
-	    !((wa->flags | wb->flags) & WPA_SCAN_NOISE_INVALID)) {
+	if (wa->flags & wb->flags & WPA_SCAN_LEVEL_DBM) {
 		snr_a = MIN(wa->level - wa->noise, GREAT_SNR);
 		snr_b = MIN(wb->level - wb->noise, GREAT_SNR);
 	} else {
-		/* Not suitable information to calculate SNR, so use level */
+		/* Level is not in dBm, so we can't calculate
+		 * SNR. Just use raw level (units unknown). */
 		snr_a = wa->level;
 		snr_b = wb->level;
 	}
 
-	/* best/max rate preferred if SNR close enough */
-        if ((snr_a && snr_b && abs(snr_b - snr_a) < 5) ||
+	/* if SNR is close, decide by max rate or frequency band */
+	if ((snr_a && snr_b && abs(snr_b - snr_a) < 5) ||
 	    (wa->qual && wb->qual && abs(wb->qual - wa->qual) < 10)) {
 		maxrate_a = wpa_scan_get_max_rate(wa);
 		maxrate_b = wpa_scan_get_max_rate(wb);
@@ -1647,8 +1648,6 @@ static int wpa_scan_result_compar(const void *a, const void *b)
 			return IS_5GHZ(wa->freq) ? -1 : 1;
 	}
 
-	/* use freq for channel preference */
-
 	/* all things being equal, use SNR; if SNRs are
 	 * identical, use quality values since some drivers may only report
 	 * that value and leave the signal level zero */
@@ -1656,7 +1655,6 @@ static int wpa_scan_result_compar(const void *a, const void *b)
 		return wb->qual - wa->qual;
 	return snr_b - snr_a;
 #undef MIN
-#undef IS_5GHZ
 }
 
 
@@ -1721,15 +1719,15 @@ static void dump_scan_res(struct wpa_scan_results *scan_res)
 	for (i = 0; i < scan_res->num; i++) {
 		struct wpa_scan_res *r = scan_res->res[i];
 		u8 *pos;
-		if ((r->flags & (WPA_SCAN_LEVEL_DBM | WPA_SCAN_NOISE_INVALID))
-		    == WPA_SCAN_LEVEL_DBM) {
+		if (r->flags & WPA_SCAN_LEVEL_DBM) {
 			int snr = r->level - r->noise;
+			int noise_valid = !(r->flags & WPA_SCAN_NOISE_INVALID);
+
 			wpa_printf(MSG_EXCESSIVE, MACSTR " freq=%d qual=%d "
-				   "noise=%d level=%d snr=%d%s flags=0x%x "
-				   "age=%u",
+				   "noise=%d%s level=%d snr=%d%s flags=0x%x age=%u",
 				   MAC2STR(r->bssid), r->freq, r->qual,
-				   r->noise, r->level, snr,
-				   snr >= GREAT_SNR ? "*" : "", r->flags,
+				   r->noise, noise_valid ? "" : "~", r->level,
+				   snr, snr >= GREAT_SNR ? "*" : "", r->flags,
 				   r->age);
 		} else {
 			wpa_printf(MSG_EXCESSIVE, MACSTR " freq=%d qual=%d "
@@ -1802,6 +1800,14 @@ static void filter_scan_res(struct wpa_supplicant *wpa_s,
 }
 
 
+/*
+ * Noise floor values to use when we have signal strength
+ * measurements, but no noise floor measurments. These values were
+ * measured in an office environment with many APs.
+ */
+#define DEFAULT_NOISE_FLOOR_2GHZ (-89)
+#define DEFAULT_NOISE_FLOOR_5GHZ (-92)
+
 /**
  * wpa_supplicant_get_scan_results - Get scan results
  * @wpa_s: Pointer to wpa_supplicant data
@@ -1835,6 +1841,17 @@ wpa_supplicant_get_scan_results(struct wpa_supplicant *wpa_s,
 	}
 	filter_scan_res(wpa_s, scan_res);
 
+	for (i = 0; i < scan_res->num; i++) {
+		struct wpa_scan_res *scan_res_item = scan_res->res[i];
+
+		if (scan_res_item->flags & WPA_SCAN_NOISE_INVALID) {
+			scan_res_item->noise =
+				IS_5GHZ(scan_res_item->freq) ?
+				DEFAULT_NOISE_FLOOR_5GHZ :
+				DEFAULT_NOISE_FLOOR_2GHZ;
+		}
+	}
+
 #ifdef CONFIG_WPS
 	if (wpas_wps_searching(wpa_s)) {
 		wpa_dbg(wpa_s, MSG_DEBUG, "WPS: Order scan results with WPS "