iwlwifi: mvm: enable RX chain diversity if needed

In some situations (see comment) it makes sense to enable both
chains (if available) to get better throughput by having chain
diversity available.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

drivers/net/wireless/iwlwifi/mvm/phy-ctxt.c

@@ -156,6 +156,19 @@ static void iwl_mvm_phy_ctxt_cmd_data(struct iwl_mvm *mvm,
 	idle_cnt = chains_static;
 	active_cnt = chains_dynamic;
 
+	/* In scenarios where we only ever use a single-stream rates,
+	 * i.e. legacy 11b/g/a associations, single-stream APs or even
+	 * static SMPS, enable both chains to get diversity, improving
+	 * the case where we're far enough from the AP that attenuation
+	 * between the two antennas is sufficiently different to impact
+	 * performance.
+	 */
+	if (active_cnt == 1 && num_of_ant(mvm->fw->valid_rx_ant) > 1 &&
+	    !mvm->cfg->rx_with_siso_diversity) {
+		idle_cnt = 2;
+		active_cnt = 2;
+	}
+
 	cmd->rxchain_info = cpu_to_le32(mvm->fw->valid_rx_ant <<
 					PHY_RX_CHAIN_VALID_POS);
 	cmd->rxchain_info |= cpu_to_le32(idle_cnt << PHY_RX_CHAIN_CNT_POS);