iwlwifi: use only one chain for scan in PS

When doing scan in power saving mode, choose only 1 valid RX chain instead of
turning all chains on.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-scan.c

@@ -581,6 +581,7 @@ static void iwl_bg_request_scan(struct work_struct *data)
 	u8 rate;
 	bool is_active = false;
 	int  chan_mod;
+	u8 active_chains;
 
 	conf = ieee80211_get_hw_conf(priv->hw);
 
@@ -734,9 +735,22 @@ static void iwl_bg_request_scan(struct work_struct *data)
 	rate_flags |= iwl_ant_idx_to_flags(priv->scan_tx_ant[band]);
 	scan->tx_cmd.rate_n_flags = iwl_hw_set_rate_n_flags(rate, rate_flags);
 
+	/* In power save mode use one chain, otherwise use all chains */
+	if (test_bit(STATUS_POWER_PMI, &priv->status)) {
+		/* rx_ant has been set to all valid chains previously */
+		active_chains = rx_ant &
+				((u8)(priv->chain_noise_data.active_chains));
+		if (!active_chains)
+			active_chains = rx_ant;
+
+		IWL_DEBUG_SCAN(priv, "chain_noise_data.active_chains: %u\n",
+				priv->chain_noise_data.active_chains);
+
+		rx_ant = first_antenna(active_chains);
+	}
 	/* MIMO is not used here, but value is required */
 	rx_chain |= ANT_ABC << RXON_RX_CHAIN_VALID_POS;
-	rx_chain |= ANT_ABC << RXON_RX_CHAIN_FORCE_MIMO_SEL_POS;
+	rx_chain |= rx_ant << RXON_RX_CHAIN_FORCE_MIMO_SEL_POS;
 	rx_chain |= rx_ant << RXON_RX_CHAIN_FORCE_SEL_POS;
 	rx_chain |= 0x1 << RXON_RX_CHAIN_DRIVER_FORCE_POS;
 	scan->rx_chain = cpu_to_le16(rx_chain);