rt2800: allow to reduce tx power on devices not exporting power limit

Some rt2800 devices don't have their calibrated max eirp tx power in
their calibration data. For those devices reduce tx power according to
difference between regulatory max channel power and requested tx power.

This patch is based on Helmut Schaa work.
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -2520,6 +2520,27 @@ static int rt2800_get_txpower_bw_comp(struct rt2x00_dev *rt2x00dev,
 	return comp_value;
 }
 
+static int rt2800_get_txpower_reg_delta(struct rt2x00_dev *rt2x00dev,
+					int power_level, int max_power)
+{
+	int delta;
+
+	if (test_bit(CAPABILITY_POWER_LIMIT, &rt2x00dev->cap_flags))
+		return 0;
+
+	/*
+	 * XXX: We don't know the maximum transmit power of our hardware since
+	 * the EEPROM doesn't expose it. We only know that we are calibrated
+	 * to 100% tx power.
+	 *
+	 * Hence, we assume the regulatory limit that cfg80211 calulated for
+	 * the current channel is our maximum and if we are requested to lower
+	 * the value we just reduce our tx power accordingly.
+	 */
+	delta = power_level - max_power;
+	return min(delta, 0);
+}
+
 static u8 rt2800_compensate_txpower(struct rt2x00_dev *rt2x00dev, int is_rate_b,
 				   enum ieee80211_band band, int power_level,
 				   u8 txpower, int delta)
@@ -2585,6 +2606,12 @@ static void rt2800_config_txpower(struct rt2x00_dev *rt2x00dev,
 	 */
 	delta += rt2800_get_gain_calibration_delta(rt2x00dev);
 
+	/*
+	 * Apply regulatory delta.
+	 */
+	delta += rt2800_get_txpower_reg_delta(rt2x00dev, power_level,
+					      chan->max_power);
+
 	/*
 	 * BBP_R1 controls TX power for all rates, it allow to set the following
 	 * gains -12, -6, 0, +6 dBm by setting values 2, 1, 0, 3 respectively.