cfg80211: Advertise maximum associated STAs in AP mode

This allows drivers to advertise the maximum number of associated
stations they support in AP mode (including P2P GO). User space
applications can use this for cleaner way of handling the limit (e.g.,
hostapd rejecting IEEE 802.11 authentication without manual
configuration of the limit) or to figure out what type of use cases can
be executed with multiple devices before trying and failing.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -2883,6 +2883,11 @@ struct wiphy_vendor_command {
  * @n_vendor_commands: number of vendor commands
  * @vendor_events: array of vendor events supported by the hardware
  * @n_vendor_events: number of vendor events
+ *
+ * @max_ap_assoc_sta: maximum number of associated stations supported in AP mode
+ *	(including P2P GO) or 0 to indicate no such limit is advertised. The
+ *	driver is allowed to advertise a theoretical limit that it can reach in
+ *	some cases, but may not always reach.
  */
 struct wiphy {
 	/* assign these fields before you register the wiphy */
@@ -2998,6 +3003,8 @@ struct wiphy {
 	const struct nl80211_vendor_cmd_info *vendor_events;
 	int n_vendor_commands, n_vendor_events;
 
+	u16 max_ap_assoc_sta;
+
 	char priv[0] __aligned(NETDEV_ALIGN);
 };
 

include/uapi/linux/nl80211.h

@@ -1568,6 +1568,13 @@ enum nl80211_commands {
  * @NL80211_ATTR_MAC_HINT: MAC address recommendation as initial BSS
  * @NL80211_ATTR_WIPHY_FREQ_HINT: frequency of the recommended initial BSS
  *
+ * @NL80211_ATTR_MAX_AP_ASSOC_STA: Device attribute that indicates how many
+ *	associated stations are supported in AP mode (including P2P GO); u32.
+ *	Since drivers may not have a fixed limit on the maximum number (e.g.,
+ *	other concurrent operations may affect this), drivers are allowed to
+ *	advertise values that cannot always be met. In such cases, an attempt
+ *	to add a new station entry with @NL80211_CMD_NEW_STATION may fail.
+ *
  * @NL80211_ATTR_MAX: highest attribute number currently defined
  * @__NL80211_ATTR_AFTER_LAST: internal use
  */
@@ -1899,6 +1906,8 @@ enum nl80211_attrs {
 	NL80211_ATTR_MAC_HINT,
 	NL80211_ATTR_WIPHY_FREQ_HINT,
 
+	NL80211_ATTR_MAX_AP_ASSOC_STA,
+
 	/* add attributes here, update the policy in nl80211.c */
 
 	__NL80211_ATTR_AFTER_LAST,

net/wireless/nl80211.c

@@ -1588,6 +1588,12 @@ static int nl80211_send_wiphy(struct cfg80211_registered_device *dev,
 		    (nla_put_flag(msg, NL80211_ATTR_SUPPORT_5_MHZ) ||
 		     nla_put_flag(msg, NL80211_ATTR_SUPPORT_10_MHZ)))
 			goto nla_put_failure;
+
+		if (dev->wiphy.max_ap_assoc_sta &&
+		    nla_put_u32(msg, NL80211_ATTR_MAX_AP_ASSOC_STA,
+				dev->wiphy.max_ap_assoc_sta))
+			goto nla_put_failure;
+
 		state->split_start++;
 		break;
 	case 11: