[PATCH net v3 2/2] net: dsa: b53: restore the NSP IMP port 2 Gbps mode

The Northstar Plus IMP port uses the switch's proprietary 2 Gbps mode for
its internal link to the AMAC.  Its fixed-link description reports 1 Gbps,
so applying that speed clears PORT_OVERRIDE_SPEED_2000M and leaves the two
ends of the internal link configured at different speeds.

Force the 2 Gbps mode and flow control for BCM5301x and NSP CPU ports.  The
mismatch caused switch output queue discards on both copper and SerDes
ports, with one discard for every TCP retransmission.  With both ends
restored to their high-speed modes, iperf3 reports no retransmissions.

Fixes: b6a8a5477fe9 ("net: dsa: b53: fix resetting speed and pause on forced link")
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
---
 drivers/net/dsa/b53/b53_common.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/net/dsa/b53/b53_common.c b/drivers/net/dsa/b53/b53_common.c
index 0880310c9ce3..743fc04da0a4 100644
--- a/drivers/net/dsa/b53/b53_common.c
+++ b/drivers/net/dsa/b53/b53_common.c

@@ -1663,9 +1663,15 @@ static void b53_phylink_mac_link_up(struct phylink_config *config,
 	}
 
 	if (mode == MLO_AN_FIXED) {
-		/* Force flow control on BCM5301x's CPU port */
-		if (is5301x(dev) && dsa_is_cpu_port(ds, port))
-			tx_pause = rx_pause = true;
+		/* Force flow control and 2Gbits/sec on BCM5301x and NSP CPU
+		 * ports
+		 */
+		if ((is5301x(dev) || dev->chip_id == BCM58XX_DEVICE_ID) &&
+		    dsa_is_cpu_port(ds, port)) {
+			tx_pause = true;
+			rx_pause = true;
+			speed = 2000;
+		}
 
 		b53_force_port_config(dev, port, speed, duplex,
 				      tx_pause, rx_pause);

-- 
2.53.0