mac80211: handle power constraint/country IE better

Currently, mac80211 uses the power constraint IE, and reduces
the regulatory max TX power by it. This can cause issues if
the AP is advertising a large power constraint value matching
a high TX power in its country IE, for example in this case:

...
Country: US  Environment: Indoor/Outdoor
    ...
    Channels [157 - 157] @ 30 dBm
    ...
Power constraint: 13 dB
...

What happened here is that our local regulatory TX power is
15 dBm, and gets reduced by 13 dB so we end up with only
2 dBm effective TX power, which is way too low.

Instead, handle the country IE/power constraint IE combined
and restrict our TX power to the max of the regulatory power
and the maximum power advertised by the AP, in this case
17 dBm (= 30 dBm - 13 dB).

Also print a message when this happens to let the user know
and help us debug issues with it.

Reported-by: Carl A. Cook <CACook@quantum-equities.com>
Tested-by: Carl A. Cook <CACook@quantum-equities.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 3 insertions, 5 deletions

diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index bd75293..416e85e 100644
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -150,13 +150,11 @@ int ieee80211_hw_config(struct ieee80211_local *local, u32 changed)
 
 	if (test_bit(SCAN_SW_SCANNING, &local->scanning) ||
 	    test_bit(SCAN_ONCHANNEL_SCANNING, &local->scanning) ||
-	    test_bit(SCAN_HW_SCANNING, &local->scanning))
+	    test_bit(SCAN_HW_SCANNING, &local->scanning) ||
+	    !local->ap_power_level)
 		power = chan->max_power;
 	else
-		power = local->power_constr_level ?
-			min(chan->max_power,
-				(chan->max_reg_power  - local->power_constr_level)) :
-			chan->max_power;
+		power = min(chan->max_power, local->ap_power_level);
 
 	if (local->user_power_level >= 0)
 		power = min(power, local->user_power_level);