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 if there is no 802.11h element;
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@xxxxxxxxxxx>
---
include/linux/ieee80211.h | 3 ++-
net/mac80211/ieee80211_i.h | 1 +
net/mac80211/mlme.c | 27 +++++++++++++++++++++++++++
net/mac80211/util.c | 19 +++++++++++++++++++
4 files changed, 49 insertions(+), 1 deletion(-)
diff --git a/include/linux/ieee80211.h b/include/linux/ieee80211.h
index 63ab3873..d0ec287 100644
--- a/include/linux/ieee80211.h
+++ b/include/linux/ieee80211.h
@@ -1806,7 +1806,8 @@ 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,
diff --git a/net/mac80211/ieee80211_i.h b/net/mac80211/ieee80211_i.h
index ef7a089..e995556 100644
--- a/net/mac80211/ieee80211_i.h
+++ b/net/mac80211/ieee80211_i.h
@@ -1367,6 +1367,7 @@ 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;
diff --git a/net/mac80211/mlme.c b/net/mac80211/mlme.c
index 31a8afa..39167c1 100644
--- a/net/mac80211/mlme.c
+++ b/net/mac80211/mlme.c
@@ -1227,6 +1227,30 @@ static u32 ieee80211_handle_pwr_constr(struct ieee80211_sub_if_data *sdata,
return 0;
}
+static u32 ieee80211_handle_cisco_dtpc(struct ieee80211_sub_if_data *sdata,
+ struct ieee80211_channel *channel,
+ const u8 *cisco_dtpc_elem)
+{
+ /* 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.
+ */
+ __s8 new_ap_level = (__s8)cisco_dtpc_elem[4];
+
+ if (sdata->ap_power_level == new_ap_level)
+ return 0;
+
+ sdata_info(sdata,
+ "Limiting TX power to %d dBm as advertised by %pM\n",
+ new_ap_level, sdata->u.mgd.bssid);
+ sdata->ap_power_level = new_ap_level;
+ if (__ieee80211_recalc_txpower(sdata))
+ return BSS_CHANGED_TXPOWER;
+ return 0;
+}
+
/* powersave */
static void ieee80211_enable_ps(struct ieee80211_local *local,
struct ieee80211_sub_if_data *sdata)
@@ -3197,6 +3221,9 @@ static void ieee80211_rx_mgmt_beacon(struct ieee80211_sub_if_data *sdata,
elems.country_elem,
elems.country_elem_len,
elems.pwr_constr_elem);
+ else if (elems.cisco_dtpc_elem)
+ changed |= ieee80211_handle_cisco_dtpc(sdata, chan,
+ elems.cisco_dtpc_elem);
ieee80211_bss_info_change_notify(sdata, changed);
}
diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index 725af7a..a253487 100644
--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -1014,6 +1014,25 @@ 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;
+ }
+ elems->cisco_dtpc_elem = pos;
+ break;
case WLAN_EID_TIMEOUT_INTERVAL:
if (elen >= sizeof(struct ieee80211_timeout_interval_ie))
elems->timeout_int = (void *)pos;
--
1.7.10.4
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