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>

1 files changed, 49 insertions, 11 deletions

diff --git a/net/mac80211/mlme.c b/net/mac80211/mlme.c
index 0b812a88f95f..a7b92f5f7161 100644
--- a/net/mac80211/mlme.c
+++ b/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);
 }