mac80211: make station hash table max_size configurable

Allow debug builds to configure the station hash table maximum
size in order to run with hash collisions in limited scenarios
such as hwsim testing. The default remains 0 which effectively
means no limit.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/Kconfig

@@ -305,3 +305,13 @@ config MAC80211_DEBUG_COUNTERS
 	  and show them in debugfs.
 
 	  If unsure, say N.
+
+config MAC80211_STA_HASH_MAX_SIZE
+	int "Station hash table maximum size" if MAC80211_DEBUG_MENU
+	default 0
+	---help---
+	  Setting this option to a low value (e.g. 4) allows testing the
+	  hash table with collisions relatively deterministically (just
+	  connect more stations than the number selected here.)
+
+	  If unsure, leave the default of 0.

net/mac80211/sta_info.c

@@ -70,6 +70,7 @@ static const struct rhashtable_params sta_rht_params = {
 	.key_offset = offsetof(struct sta_info, sta.addr),
 	.key_len = ETH_ALEN,
 	.hashfn = sta_addr_hash,
+	.max_size = CONFIG_MAC80211_STA_HASH_MAX_SIZE,
 };
 
 /* Caller must hold local->sta_mtx */