realtek: eth: rtl838x: ignore STP state on CPU transmit

The switch applies the egress spanning-tree port state to frames the
CPU sends to a port, so a port that is not forwarding drops them:
kernel STP cannot announce a designated port during the forward
delay, user-space RSTP cannot send proposals from a discarding port,
and LLDP stays silent on blocked ports.

Set BP_FLTR2 in the TX CPU tag. It bypasses the egress spanning-tree
port state and egress VLAN filtering, as IGNORE_STP does when
transmitting on RTL93xx. Skipping the VLAN filter does not widen
where frames go: the CPU names each frame's single destination port
(AS_DPM).

Signed-off-by: Paul Tobias <openwrt-commits@tobias.pt>
Link: https://github.com/openwrt/openwrt/pull/25442
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

target/linux/realtek/files-6.18/drivers/net/ethernet/realtek/rtl838x_eth.c

@@ -32,7 +32,8 @@
 static void rteth_838x_create_tx_header(struct rteth_frag *frag, unsigned int port, int prio)
 {
 	/* cpu_tag[0] is reserved on the RTL83XX SoCs */
-	frag->cpu_tag[1] = 0x0400;  /* BIT 10: RTL8380_CPU_TAG */
+	frag->cpu_tag[1] = FIELD_PREP(RTETH_838X_TAG1_PROTO_MASK, RTETH_838X_TAG1_PROTO) |
+			   RTETH_838X_TAG1_BP_FLTR2;
 	frag->cpu_tag[2] = 0x0200;  /* Set only AS_DPM, to enable DPM settings below */
 	frag->cpu_tag[3] = 0x0000;
 	frag->cpu_tag[4] = BIT(port) >> 16;

target/linux/realtek/files-6.18/drivers/net/ethernet/realtek/rtl838x_eth.h

@@ -27,6 +27,10 @@
 #define RTETH_838X_QM_PKT2CPU_INTPRI_CNT	3
 #define RTETH_838X_RMA_CTRL_0			(0x4300)
 #define RTETH_838X_RMA_CTRL_1			(0x4304)
+#define RTETH_838X_TAG1_PROTO_MASK		GENMASK(15, 8)
+#define RTETH_838X_TAG1_PROTO			0x04
+/* TX CPU tag: bypass the egress spanning-tree port state and egress VLAN filtering */
+#define RTETH_838X_TAG1_BP_FLTR2		BIT(4)
 
 #define RTETH_839X_CPU_PORT			52
 #define RTETH_839X_DMA_IF_CTRL			(0x786c)