wifi: mac80211: support antenna control in injection

Support antenna control for injection by parsing the antenna
radiotap field (which may be presented multiple times) and
telling the driver about the resulting antenna bitmap. Of
course there's no guarantee the driver will actually honour
this, just like any other injection control.

If misconfigured, i.e. the injected HT/VHT MCS needs more
chains than antennas are configured, the bitmap is reset to
zero, indicating no selection.

For now this is only set up for two anntenas so we keep more
free bits, but that can be trivially extended if any driver
implements support for it that can deal with hardware with
more antennas.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230920211508.f71001aa4da9.I00ccb762a806ea62bc3d728fa3a0d29f4f285eeb@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/ieee80211.h

@@ -1705,6 +1705,8 @@ struct ieee80211_mcs_info {
 #define		IEEE80211_HT_MCS_TX_MAX_STREAMS	4
 #define IEEE80211_HT_MCS_TX_UNEQUAL_MODULATION	0x10
 
+#define IEEE80211_HT_MCS_CHAINS(mcs) ((mcs) == 32 ? 1 : (1 + ((mcs) >> 3)))
+
 /*
  * 802.11n D5.0 20.3.5 / 20.6 says:
  * - indices 0 to 7 and 32 are single spatial stream

include/net/mac80211.h

@@ -1178,7 +1178,11 @@ struct ieee80211_tx_info {
 					u8 use_cts_prot:1;
 					u8 short_preamble:1;
 					u8 skip_table:1;
-					/* 2 bytes free */
+
+					/* for injection only (bitmap) */
+					u8 antennas:2;
+
+					/* 14 bits free */
 				};
 				/* only needed before rate control */
 				unsigned long jiffies;

net/mac80211/tx.c

@@ -2162,6 +2162,11 @@ bool ieee80211_parse_tx_radiotap(struct sk_buff *skb,
 			rate_found = true;
 			break;
 
+		case IEEE80211_RADIOTAP_ANTENNA:
+			/* this can appear multiple times, keep a bitmap */
+			info->control.antennas |= BIT(*iterator.this_arg);
+			break;
+
 		case IEEE80211_RADIOTAP_DATA_RETRIES:
 			rate_retries = *iterator.this_arg;
 			break;
@@ -2256,8 +2261,17 @@ bool ieee80211_parse_tx_radiotap(struct sk_buff *skb,
 		}
 
 		if (rate_flags & IEEE80211_TX_RC_MCS) {
+			/* reset antennas if not enough */
+			if (IEEE80211_HT_MCS_CHAINS(rate) >
+					hweight8(info->control.antennas))
+				info->control.antennas = 0;
+
 			info->control.rates[0].idx = rate;
 		} else if (rate_flags & IEEE80211_TX_RC_VHT_MCS) {
+			/* reset antennas if not enough */
+			if (vht_nss > hweight8(info->control.antennas))
+				info->control.antennas = 0;
+
 			ieee80211_rate_set_vht(info->control.rates, vht_mcs,
 					       vht_nss);
 		} else if (sband) {