mac80211: Set lower memory limit for non-VHT devices

Small devices can run out of memory from queueing too many packets. If
VHT is not supported by the PHY, having more than 4 MBytes of total
queue in the TXQ intermediate queues is not needed, and so we can safely
limit the memory usage in these cases and avoid OOM.
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -1433,6 +1433,8 @@ int ieee80211_txq_setup_flows(struct ieee80211_local *local)
 	struct fq *fq = &local->fq;
 	int ret;
 	int i;
+	bool supp_vht = false;
+	enum nl80211_band band;
 
 	if (!local->ops->wake_tx_queue)
 		return 0;
@@ -1441,6 +1443,23 @@ int ieee80211_txq_setup_flows(struct ieee80211_local *local)
 	if (ret)
 		return ret;
 
+	/*
+	 * If the hardware doesn't support VHT, it is safe to limit the maximum
+	 * queue size. 4 Mbytes is 64 max-size aggregates in 802.11n.
+	 */
+	for (band = 0; band < NUM_NL80211_BANDS; band++) {
+		struct ieee80211_supported_band *sband;
+
+		sband = local->hw.wiphy->bands[band];
+		if (!sband)
+			continue;
+
+		supp_vht = supp_vht || sband->vht_cap.vht_supported;
+	}
+
+	if (!supp_vht)
+		fq->memory_limit = 4 << 20; /* 4 Mbytes */
+
 	codel_params_init(&local->cparams);
 	local->cparams.interval = MS2TIME(100);
 	local->cparams.target = MS2TIME(20);