[PATCH wireless-next v2 3/7] wifi: rt2800: make MCS 16 fall back to MCS 8 on devices with 3 TX chains

[Ho Jie Feng <hjf3108@gmail.com>]

MCS 15 has worse reliability than MCS 16, since 16 and above
are 3 stream rates, and 15 is the top of the 2 stream rates.
Falling back from MCS 16 to MCS 15 does not make sense, so
make the fallback to MCS 8 on all devices with 3 TX chains,
like the Ralink driver does.

The retry chain is now MCS 16 to MCS 8, skipping 7 MCS rates
in between. Reflect that in the retry calculation code. This
also fixes the retry count on RT3883, which already used this
fallback.

Test results:
   before after
TX 28.2   29.1
RX 70.4   70.2

Signed-off-by: Ho Jie Feng <hjf3108@gmail.com>
---
 drivers/net/wireless/ralink/rt2x00/rt2800lib.c | 10 +++++++++-
 drivers/net/wireless/ralink/rt2x00/rt2x00.h    |  5 +++++
 drivers/net/wireless/ralink/rt2x00/rt2x00dev.c | 14 ++++++++++++--
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/drivers/net/wireless/ralink/rt2x00/rt2800lib.c b/drivers/net/wireless/ralink/rt2x00/rt2800lib.c
index e7b6edf485099..5e1ee016624c6 100644
--- a/drivers/net/wireless/ralink/rt2x00/rt2800lib.c
+++ b/drivers/net/wireless/ralink/rt2x00/rt2800lib.c
@@ -1116,6 +1116,14 @@ void rt2800_txdone_entry(struct queue_entry *entry, u32 status, __le32 *txwi,
 		 */
 		__set_bit(TXDONE_SUCCESS, &txdesc.flags);
 		txdesc.retry = ((mcs > real_mcs) ? mcs - real_mcs : 0);
+
+		/*
+		 * On devices with 3 TX chains MCS16 falls back to MCS8, so a
+		 * frame that was sent below MCS16 after a three stream rate
+		 * skipped MCS9 to MCS15.
+		 */
+		if (rt2x00_is_3t(rt2x00dev) && mcs >= 16 && real_mcs < 16)
+			txdesc.retry -= 7;
 	} else {
 		/*
 		 * Transmission failed. The number of retries is
@@ -6303,7 +6311,7 @@ static int rt2800_init_registers(struct rt2x00_dev *rt2x00dev)
 	reg = rt2x00_rt(rt2x00dev, RT5592) ? 0x00000082 : 0x00000002;
 	rt2800_register_write(rt2x00dev, TXOP_HLDR_ET, reg);
 
-	if (rt2x00_rt(rt2x00dev, RT3883)) {
+	if (rt2x00_is_3t(rt2x00dev)) {
 		rt2800_register_write(rt2x00dev, TX_FBK_CFG_3S_0, 0x12111008);
 		rt2800_register_write(rt2x00dev, TX_FBK_CFG_3S_1, 0x16151413);
 	}
diff --git a/drivers/net/wireless/ralink/rt2x00/rt2x00.h b/drivers/net/wireless/ralink/rt2x00/rt2x00.h
index a80508db32d9c..f82dda0266f9b 100644
--- a/drivers/net/wireless/ralink/rt2x00/rt2x00.h
+++ b/drivers/net/wireless/ralink/rt2x00/rt2x00.h
@@ -1193,6 +1193,11 @@ static inline bool rt2x00_is_soc(struct rt2x00_dev *rt2x00dev)
 	return rt2x00_intf(rt2x00dev, RT2X00_CHIP_INTF_SOC);
 }
 
+static inline bool rt2x00_is_3t(struct rt2x00_dev *rt2x00dev)
+{
+	return rt2x00dev->default_ant.tx_chain_num == 3;
+}
+
 /* Helpers for capability flags */
 
 static inline bool
diff --git a/drivers/net/wireless/ralink/rt2x00/rt2x00dev.c b/drivers/net/wireless/ralink/rt2x00/rt2x00dev.c
index 4d94b7062f440..db33c1215de97 100644
--- a/drivers/net/wireless/ralink/rt2x00/rt2x00dev.c
+++ b/drivers/net/wireless/ralink/rt2x00/rt2x00dev.c
@@ -329,10 +329,10 @@ static void rt2x00lib_fill_tx_status(struct rt2x00_dev *rt2x00dev,
 	 * lowest rate was used.
 	 */
 	for (i = 0; i < retry_rates && i < IEEE80211_TX_MAX_RATES; i++) {
-		tx_info->status.rates[i].idx = rate_idx - i;
+		tx_info->status.rates[i].idx = rate_idx;
 		tx_info->status.rates[i].flags = rate_flags;
 
-		if (rate_idx - i == 0) {
+		if (rate_idx == 0) {
 			/*
 			 * The lowest rate (index 0) was used until the
 			 * number of max retries was reached.
@@ -340,8 +340,18 @@ static void rt2x00lib_fill_tx_status(struct rt2x00_dev *rt2x00dev,
 			tx_info->status.rates[i].count = retry_rates - i;
 			i++;
 			break;
+		} else if (rate_idx == 16) {
+			/*
+			 * TX_FBK_CFG_3S_0 makes MCS16 fall back to MCS8 on
+			 * devices with 3 TX chains, the decrement below shifts
+			 * it back to 8.
+			 */
+			if (rt2x00_is_3t(rt2x00dev) &&
+			    (rate_flags & IEEE80211_TX_RC_MCS))
+				rate_idx = 9;
 		}
 		tx_info->status.rates[i].count = 1;
+		rate_idx--;
 	}
 	if (i < (IEEE80211_TX_MAX_RATES - 1))
 		tx_info->status.rates[i].idx = -1; /* terminate */
-- 
2.53.0