[PATCH net] net: dsa: b53: skip EAP mode setup on Northstar SoCs when using port 5 or 7

From: Rafał Miłecki <rafal@milecki.pl>

Northstar devices have 3 CPU ports. This design was meant for dual IMP
setups when WAN traffic goes to port 5 (IMP1) and LAN traffic goes to
port 8 (IMP0). While all 3 ports support Broadcom header some features
are not available for ports 5 and 7. Trying to set EAP mode when using
those ports breaks standalone ports.

Fixes: 4227ea91e265 ("net: dsa: b53: prevent standalone from trying to forward to other ports")
Cc: Semih Baskan <redacted>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
---
This fixes a regression. A very similar fix was carried by OpenWrt for
about half a year now.

A recent discussion in
[PATCH net 0/2] net: dsa: b53: fix 8021q uppers on standalone ports
allowed understanding the issue a bit better so I decided to send an
upstream PATCH finally.

 drivers/net/dsa/b53/b53_common.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/net/dsa/b53/b53_common.c b/drivers/net/dsa/b53/b53_common.c
index 3f5b9592794d..4b4a9258ecd4 100644
--- a/drivers/net/dsa/b53/b53_common.c
+++ b/drivers/net/dsa/b53/b53_common.c

@@ -332,6 +332,21 @@ static void b53_set_eap_mode(struct b53_device *dev, int port, int mode)
 {
 	u64 eap_conf;
 
+	/*
+	 * Northstar devices (SoCs with BCM53011 / BCM53012) have 3 Ethernet
+	 * controllers connected to 3 switch ports: 5 (IMP1), 7 and 8 (IMP0).
+	 * Each of those ports can be used as CPU one as all support Broadcom
+	 * header but ports 5 and 7 have some limitations.
+	 *
+	 * Setting EAP_MODE_SIMPLIFIED with port 5/7 used as CPU port breaks
+	 * standalone ports, see:
+	 * https://lore.kernel.org/netdev/ce4d9b7b-aaf6-4796-94fb-8c3d6a1dcd4d@gmail.com/
+	 */
+	if (is5301x(dev) && !dsa_is_cpu_port(dev->ds, B53_CPU_PORT)) {
+		dev_warn_once(dev->dev, "skipping EAP setup because of not using CPU port 8\n");
+		return;
+	}
+
 	if (is5325(dev) || is5365(dev) || dev->chip_id == BCM5389_DEVICE_ID)
 		return;
 

-- 
2.51.0