mac80211: support DTPC IE (from Cisco Client eXtensions)

Linux already supports 802.11h, where the access point can tell the
client to reduce its transmission power. However, 802.11h is only
defined for 5 GHz, where the need for this is much smaller than on
2.4 GHz.

Cisco has their own solution, called DTPC (Dynamic Transmit Power
Control). Cisco APs on a controller sometimes but not always send
802.11h; they always send DTPC, even on 2.4 GHz. This patch adds support
for parsing and honoring the DTPC IE in addition to the 802.11h
element (they do not always contain the same limits, so both must
be honored); the format is not documented, but very simple.

Tested (on top of wireless.git and on 3.16.1) against a Cisco Aironet
1142 joined to a Cisco 2504 WLC, by setting various transmit power
levels for the given access points and observing the results.
The Wireshark 802.11 dissector agrees with the interpretation of the
element, except for negative numbers, which seem to never happen
anyway.

Signed-off-by: Steinar H. Gunderson <sgunderson@bigfoot.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>

include/linux/ieee80211.h

@@ -1824,7 +1824,8 @@ enum ieee80211_eid {
 	WLAN_EID_DMG_TSPEC = 146,
 	WLAN_EID_DMG_AT = 147,
 	WLAN_EID_DMG_CAP = 148,
-	/* 149-150 reserved for Cisco */
+	/* 149 reserved for Cisco */
+	WLAN_EID_CISCO_VENDOR_SPECIFIC = 150,
 	WLAN_EID_DMG_OPERATION = 151,
 	WLAN_EID_DMG_BSS_PARAM_CHANGE = 152,
 	WLAN_EID_DMG_BEAM_REFINEMENT = 153,

net/mac80211/ieee80211_i.h

@@ -1369,6 +1369,7 @@ struct ieee802_11_elems {
 	const struct ieee80211_wide_bw_chansw_ie *wide_bw_chansw_ie;
 	const u8 *country_elem;
 	const u8 *pwr_constr_elem;
+	const u8 *cisco_dtpc_elem;
 	const struct ieee80211_timeout_interval_ie *timeout_int;
 	const u8 *opmode_notif;
 	const struct ieee80211_sec_chan_offs_ie *sec_chan_offs;

net/mac80211/mlme.c

@@ -1230,14 +1230,30 @@ ieee80211_find_80211h_pwr_constr(struct ieee80211_sub_if_data *sdata,
 	return have_chan_pwr;
 }
 
+static void ieee80211_find_cisco_dtpc(struct ieee80211_sub_if_data *sdata,
+				      struct ieee80211_channel *channel,
+				      const u8 *cisco_dtpc_ie,
+				      int *pwr_level)
+{
+	/* From practical testing, the first data byte of the DTPC element
+	 * seems to contain the requested dBm level, and the CLI on Cisco
+	 * APs clearly state the range is -127 to 127 dBm, which indicates
+	 * a signed byte, although it seemingly never actually goes negative.
+	 * The other byte seems to always be zero.
+	 */
+	*pwr_level = (__s8)cisco_dtpc_ie[4];
+}
+
 static u32 ieee80211_handle_pwr_constr(struct ieee80211_sub_if_data *sdata,
 				       struct ieee80211_channel *channel,
 				       struct ieee80211_mgmt *mgmt,
 				       const u8 *country_ie, u8 country_ie_len,
-				       const u8 *pwr_constr_ie)
+				       const u8 *pwr_constr_ie,
+				       const u8 *cisco_dtpc_ie)
 {
-	bool has_80211h_pwr = false;
-	int chan_pwr, pwr_reduction_80211h;
+	bool has_80211h_pwr = false, has_cisco_pwr = false;
+	int chan_pwr = 0, pwr_reduction_80211h = 0;
+	int pwr_level_cisco, pwr_level_80211h;
 	int new_ap_level;
 
 	if (country_ie && pwr_constr_ie &&
@@ -1246,16 +1262,35 @@ static u32 ieee80211_handle_pwr_constr(struct ieee80211_sub_if_data *sdata,
 		has_80211h_pwr = ieee80211_find_80211h_pwr_constr(
 			sdata, channel, country_ie, country_ie_len,
 			pwr_constr_ie, &chan_pwr, &pwr_reduction_80211h);
-		new_ap_level = max_t(int, 0, chan_pwr - pwr_reduction_80211h);
+		pwr_level_80211h =
+			max_t(int, 0, chan_pwr - pwr_reduction_80211h);
+	}
+
+	if (cisco_dtpc_ie) {
+		ieee80211_find_cisco_dtpc(
+			sdata, channel, cisco_dtpc_ie, &pwr_level_cisco);
+		has_cisco_pwr = true;
 	}
 
-	if (!has_80211h_pwr)
+	if (!has_80211h_pwr && !has_cisco_pwr)
 		return 0;
 
-	sdata_info(sdata,
-		   "Limiting TX power to %d (%d - %d) dBm as advertised by %pM\n",
-		   new_ap_level, chan_pwr, pwr_reduction_80211h,
-		   sdata->u.mgd.bssid);
+	/* If we have both 802.11h and Cisco DTPC, apply both limits
+	 * by picking the smallest of the two power levels advertised.
+	 */
+	if (has_80211h_pwr &&
+	    (!has_cisco_pwr || pwr_level_80211h <= pwr_level_cisco)) {
+		sdata_info(sdata,
+			   "Limiting TX power to %d (%d - %d) dBm as advertised by %pM\n",
+			   pwr_level_80211h, chan_pwr, pwr_reduction_80211h,
+			   sdata->u.mgd.bssid);
+		new_ap_level = pwr_level_80211h;
+	} else {  /* has_cisco_pwr is always true here. */
+		sdata_info(sdata,
+			   "Limiting TX power to %d dBm as advertised by %pM\n",
+			   pwr_level_cisco, sdata->u.mgd.bssid);
+		new_ap_level = pwr_level_cisco;
+	}
 
 	if (sdata->ap_power_level == new_ap_level)
 		return 0;
@@ -2923,7 +2958,9 @@ static void ieee80211_rx_mgmt_probe_resp(struct ieee80211_sub_if_data *sdata,
 /*
  * This is the canonical list of information elements we care about,
  * the filter code also gives us all changes to the Microsoft OUI
- * (00:50:F2) vendor IE which is used for WMM which we need to track.
+ * (00:50:F2) vendor IE which is used for WMM which we need to track,
+ * as well as the DTPC IE (part of the Cisco OUI) used for signaling
+ * changes to requested client power.
  *
  * We implement beacon filtering in software since that means we can
  * avoid processing the frame here and in cfg80211, and userspace
@@ -3232,7 +3269,8 @@ static void ieee80211_rx_mgmt_beacon(struct ieee80211_sub_if_data *sdata,
 	changed |= ieee80211_handle_pwr_constr(sdata, chan, mgmt,
 					       elems.country_elem,
 					       elems.country_elem_len,
-					       elems.pwr_constr_elem);
+					       elems.pwr_constr_elem,
+					       elems.cisco_dtpc_elem);
 
 	ieee80211_bss_info_change_notify(sdata, changed);
 }

net/mac80211/util.c

@@ -1015,6 +1015,31 @@ u32 ieee802_11_parse_elems_crc(const u8 *start, size_t len, bool action,
 			}
 			elems->pwr_constr_elem = pos;
 			break;
+		case WLAN_EID_CISCO_VENDOR_SPECIFIC:
+			/* Lots of different options exist, but we only care
+			 * about the Dynamic Transmit Power Control element.
+			 * First check for the Cisco OUI, then for the DTPC
+			 * tag (0x00).
+			 */
+			if (elen < 4) {
+				elem_parse_failed = true;
+				break;
+			}
+
+			if (pos[0] != 0x00 || pos[1] != 0x40 ||
+			    pos[2] != 0x96 || pos[3] != 0x00)
+				break;
+
+			if (elen != 6) {
+				elem_parse_failed = true;
+				break;
+			}
+
+			if (calc_crc)
+				crc = crc32_be(crc, pos - 2, elen + 2);
+
+			elems->cisco_dtpc_elem = pos;
+			break;
 		case WLAN_EID_TIMEOUT_INTERVAL:
 			if (elen >= sizeof(struct ieee80211_timeout_interval_ie))
 				elems->timeout_int = (void *)pos;