band: generate HT chandef from frequency

For AP mode its convenient for IWD to choose an appropriate
channel definition rather than require the user provide very
low level parameters such as channel width, center1 frequency
etc. For now only HT is supported as VHT/HE etc. require
additional secondary channel frequencies.

The HT API tries to find an operating class using 40Mhz which
complies with any hardware restrictions. If an operating class is
found that is supported/not restricted it is marked as 'best' until
a better one is found. In this case 'better' is a larger channel
width. Since this is HT only 20mhz and 40mhz widths are checked.

src/band.c

@@ -1194,6 +1194,93 @@ int oci_from_chandef(const struct band_chandef *own, uint8_t oci[static 3])
 	return -ENOENT;
 }
 
+/* Find an HT chandef for the frequency */
+int band_freq_to_ht_chandef(uint32_t freq, const struct band_freq_attrs *attr,
+				struct band_chandef *chandef)
+{
+	enum band_freq band;
+	enum band_chandef_width width;
+	unsigned int i;
+	const struct operating_class_info *best = NULL;
+
+	if (attr->disabled || !attr->supported)
+		return -EINVAL;
+
+	if (!band_freq_to_channel(freq, &band))
+		return -EINVAL;
+
+	for (i = 0; i < L_ARRAY_SIZE(e4_operating_classes); i++) {
+		const struct operating_class_info *info =
+						&e4_operating_classes[i];
+		enum band_chandef_width w;
+
+		if (e4_has_frequency(info, freq) < 0)
+			continue;
+
+		/* Any restrictions for this channel width? */
+		switch (info->channel_spacing) {
+		case 20:
+			w = BAND_CHANDEF_WIDTH_20;
+			break;
+		case 40:
+			w = BAND_CHANDEF_WIDTH_40;
+
+			/* 6GHz remove the upper/lower 40mhz channel concept */
+			if (band == BAND_FREQ_6_GHZ)
+				break;
+
+			if (info->flags & PRIMARY_CHANNEL_UPPER &&
+						attr->no_ht40_plus)
+				continue;
+
+			if (info->flags & PRIMARY_CHANNEL_LOWER &&
+						attr->no_ht40_minus)
+				continue;
+
+			break;
+		default:
+			continue;
+		}
+
+		if (!best || best->channel_spacing < info->channel_spacing) {
+			best = info;
+			width = w;
+		}
+	}
+
+	if (!best)
+		return -ENOENT;
+
+	chandef->frequency = freq;
+	chandef->channel_width = width;
+
+	/*
+	 * Choose a secondary channel frequency:
+	 * - 20mhz no secondary
+	 * - 40mhz we can base the selection off the channel flags, either
+	 *   higher or lower.
+	 */
+	switch (width) {
+	case BAND_CHANDEF_WIDTH_20:
+		return 0;
+	case BAND_CHANDEF_WIDTH_40:
+		if (band == BAND_FREQ_6_GHZ)
+			return 0;
+
+		if (best->flags & PRIMARY_CHANNEL_UPPER)
+			chandef->center1_frequency = freq - 10;
+		else
+			chandef->center1_frequency = freq + 10;
+
+		return 0;
+	default:
+		/* Should never happen */
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 uint8_t band_freq_to_channel(uint32_t freq, enum band_freq *out_band)
 {
 	uint32_t channel = 0;

src/band.h

@@ -101,6 +101,8 @@ int band_estimate_nonht_rate(const struct band *band,
 				const uint8_t *supported_rates,
 				const uint8_t *ext_supported_rates,
 				int32_t rssi, uint64_t *out_data_rate);
+int band_freq_to_ht_chandef(uint32_t freq, const struct band_freq_attrs *attr,
+				struct band_chandef *chandef);
 
 int oci_to_frequency(uint32_t operating_class, uint32_t channel);