stp: contain BPDUs to the CPU while STP runs

With STP enabled the switch is a participating bridge, so BPDUs must be
consumed, not relayed - yet the reserved group 01:80:C2:00:00:00 was
flooded across the VLAN just like any multicast, leaking every BPDU to
all ports (the same defect class as the LACPDU flood addressed in the
LACP branch, PR #299).

On stp on, write a CPU-only static L2 multicast entry for the BPDU group
per VLAN: BPDUs can arrive VLAN-tagged and classify into the tag's VID,
so cover every VLAN present in the VLAN table plus every port's PVID for
the untagged case.

On stp off the same entries are retargeted to all ports + CPU, restoring
the previous flood behaviour: an unmanaged switch is expected to be
transparent to BPDUs so the surrounding spanning tree can span through
it, and dropping them instead would partition that topology.

Note: with STP enabled the ports start out blocking, which also stops
egress of CPU-originated LACPDUs, so an active LACP aggregate drops
until the ports reach forwarding - a pre-existing interaction, not
changed here.

rtl837x_stp.c

@@ -11,11 +11,18 @@
 #include "rtl837x_sfr.h"
 #include "rtl837x_regs.h"
 #include "rtl837x_stp.h"
+#include "rtl837x_port.h"	/* port_pvid_get(), port_l2mc_set() */
 #include "uip.h"
 #include "machine.h"
 
 extern __code struct machine machine;
 extern __xdata uint8_t sfr_data[4];
+extern __xdata struct machine_runtime machine_detected;	/* owned by rtl837x_port.c */
+
+/* Scratch for stp_fdb_update(), in xdata: plain locals would overflow the
+ * near-full internal-RAM overlay (OSEG), cf. rtl837x_lacp.c. */
+__xdata uint16_t stp_fdb_vid;
+__xdata uint8_t  stp_fdb_i;
 
 extern __xdata struct uip_eth_addr uip_ethaddr;
 
@@ -202,6 +209,39 @@ void stp_timers(void) __banked
 }
 
 
+/*
+ * Steer BPDUs (01:80:C2:00:00:00) while STP runs: one static CPU-only L2
+ * multicast entry per PVID in use, so BPDUs reach the CPU without being
+ * flooded to other ports (a bridge running STP must consume BPDUs, not
+ * relay them - relaying poisons the neighbours' view of the topology).
+ *
+ * With STP off the same entries are retargeted to all ports + CPU, which
+ * restores the previous flood behaviour ("BPDU transparency"): the
+ * surrounding spanning tree can keep spanning *through* this switch, which
+ * unmanaged setups rely on. Same per-PVID/IVL rules as the LACP steering -
+ * see port_l2mc_set() and rtl837x_lacp.c. NOTE: changing a port's PVID
+ * while STP runs needs `stp off`/`on` to refresh the entries.
+ */
+static void stp_fdb_update(__xdata uint16_t pmask)
+{
+	/* Unlike LACPDUs (always untagged, so per-PVID entries suffice), BPDUs
+	 * can arrive VLAN-tagged and then classify into the tag's VID - cover
+	 * every VLAN that exists in the VLAN table, plus every port's PVID for
+	 * the untagged case. A duplicate VID just overwrites the same slot. */
+	for (stp_fdb_vid = 1; stp_fdb_vid < 4095; stp_fdb_vid++) {
+		if (vlan_get(stp_fdb_vid) < 0)
+			continue;
+		if (!(sfr_data[0] & 0x02))	/* bit 1: VLAN table entry valid */
+			continue;
+		port_l2mc_set(0x00, stp_fdb_vid, pmask);
+	}
+	for (stp_fdb_i = machine.min_port; stp_fdb_i <= machine.max_port; stp_fdb_i++) {
+		stp_fdb_vid = port_pvid_get(stp_fdb_i);
+		port_l2mc_set(0x00, stp_fdb_vid, pmask);
+	}
+}
+
+
 void stp_setup(void) __banked
 {
 	print_string("Enabling STP: ");
@@ -222,6 +262,9 @@ void stp_setup(void) __banked
 	root_bridge.prio = 0x80; // This corresponds to 32768
 	root_bridge.ext	= 0x00;
 	memcpy(root_bridge.mac, uip_ethaddr.addr, 6);
+
+	/* Take BPDUs to the CPU only - we are a participating bridge now. */
+	stp_fdb_update(PMASK_CPU);
 }
 
 
@@ -236,4 +279,7 @@ void stp_off(void) __banked
 	}
 	sfr_data[1] |= 0x0f; // Do not block CPU-Port
 	reg_write_m(RTL837X_MSTP_STATES);
+
+	/* Restore BPDU transparency: flood them again like an unmanaged switch. */
+	stp_fdb_update(PMASK_CPU | (machine_detected.isRTL8373 ? PMASK_9 : PMASK_6));
 }