realtek: use assisted learning on CPU port

L2 learning on the CPU port is currently not consistently configured and
relies on the default configuration of the device. On RTL83xx, it is
disabled for packets transmitted with a TX header, as hardware learning
corrupts the forwarding table otherwise. As a result, unneeded flooding
of traffic for the CPU port can already happen on some devices now. It
is also likely that similar issues exist on RTL93xx, which doesn't have
a field to disable learning in the TX header.

To address this, disable hardware learning for the CPU port globally on
all devices. Instead, enable assisted learning to let DSA write FDB
entries to the switch.

For now, this does not sync local/bridge entries to the switch. However,
support for that was added in Linux 5.14, so the next switch to a newer
kernel version is going to fix this.

Signed-off-by: Jan Hoffmann <[email protected]>

target/linux/realtek/files-5.10/drivers/net/dsa/rtl83xx/dsa.c

@@ -162,6 +162,16 @@ static void rtl83xx_setup_bpdu_traps(struct rtl838x_switch_priv *priv)
 		priv->r->set_receive_management_action(i, BPDU, COPY2CPU);
 }
 
+static void rtl83xx_port_set_salrn(struct rtl838x_switch_priv *priv,
+				   int port, bool enable)
+{
+	int shift = SALRN_PORT_SHIFT(port);
+	int val = enable ? SALRN_MODE_HARDWARE : SALRN_MODE_DISABLED;
+
+	sw_w32_mask(SALRN_MODE_MASK << shift, val << shift,
+		    priv->r->l2_port_new_salrn(port));
+}
+
 static int rtl83xx_setup(struct dsa_switch *ds)
 {
 	int i;
@@ -205,6 +215,9 @@ static int rtl83xx_setup(struct dsa_switch *ds)
 
 	priv->r->l2_learning_setup();
 
+	rtl83xx_port_set_salrn(priv, priv->cpu_port, false);
+	ds->assisted_learning_on_cpu_port = true;
+
 	/*
 	 *  Make sure all frames sent to the switch's MAC are trapped to the CPU-port
 	 *  0: FWD, 1: DROP, 2: TRAP2CPU
@@ -263,6 +276,9 @@ static int rtl93xx_setup(struct dsa_switch *ds)
 
 	priv->r->l2_learning_setup();
 
+	rtl83xx_port_set_salrn(priv, priv->cpu_port, false);
+	ds->assisted_learning_on_cpu_port = true;
+
 	rtl83xx_enable_phy_polling(priv);
 
 	priv->r->pie_init(priv);

target/linux/realtek/files-5.10/drivers/net/dsa/rtl83xx/rtl838x.h

@@ -245,6 +245,12 @@
 #define RTL839X_L2_PORT_NEW_SALRN(p)		(0x38F0 + (((p >> 4) << 2)))
 #define RTL930X_L2_PORT_SALRN(p)		(0x8FEC + (((p >> 4) << 2)))
 #define RTL931X_L2_PORT_NEW_SALRN(p)		(0xC820 + (((p >> 4) << 2)))
+
+#define SALRN_PORT_SHIFT(p)			((p % 16) * 2)
+#define SALRN_MODE_MASK				0x3
+#define SALRN_MODE_HARDWARE			0
+#define SALRN_MODE_DISABLED			2
+
 #define RTL838X_L2_PORT_NEW_SA_FWD(p)		(0x3294 + (((p >> 4) << 2)))
 #define RTL839X_L2_PORT_NEW_SA_FWD(p)		(0x3900 + (((p >> 4) << 2)))
 #define RTL930X_L2_PORT_NEW_SA_FWD(p)		(0x8FF4 + (((p / 10) << 2)))