stp: actually promote ports out of blocking; calibrate timers

"stp on" put every port into blocking (stp_setup, port_timers = "10 s") but
nothing ever counted those timers down: stp_timers() only sent hello BPDUs.
On a network with no other (R)STP bridge - i.e. nobody sends us BPDUs - every
port therefore stayed blocking FOREVER and enabling STP took the whole
network down until "stp off".

- stp_timers(): count port_timers down; when a port's listen period expires
  with no better root heard, promote it to forwarding in MSTP_STATES (we are
  the designated bridge on that port).
- Calibrate the tick constants to the real stp_timers() rate (~64 Hz: main
  loop ~256 Hz / (STP_TICK_DIVIDER+1)): TIME_HELLO 0x200->0x80 is an actual
  2 s hello, port_timers 0xa00->0x280 an actual 10 s listen period. Measured
  before the fix, ports converged only after ~40 s.
- Move struct bridge into rtl837x_stp.h and export root_bridge/-_cost for
  the web UI status endpoint.

Verified on hardware: "stp on" -> ports report Blocking, after the 10 s
listen period all ports promote to Forwarding and LAN connectivity returns;
"stp off" restores forwarding immediately. We elect ourselves root (weRoot)
with no other bridge present.

(cherry picked from commit 8537a15ca254b2122272b20bec7a66426e86df4b)

rtl837x_stp.c

@@ -34,12 +34,7 @@ extern __xdata struct uip_eth_addr uip_ethaddr;
 
 extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE + 2];
 
-struct bridge {
-	uint8_t prio;
-	uint8_t ext;
-	uint8_t mac[6];
-};
-
+/* struct bridge now lives in rtl837x_stp.h (shared with the web UI). */
 __xdata struct bridge root_bridge;
 __xdata uint32_t root_bridge_cost;
 
@@ -211,6 +206,22 @@ void stp_timers(void) __banked
 			print_byte(i); write_char('\n');
 			stp_cnf_send(i);
 		}
+		/* Promote a port out of the initial blocking state once its listen
+		 * period expires. stp_setup() puts every port into blocking with
+		 * port_timers = 10 s, but nothing ever counted that down - so on a
+		 * network with no other RSTP bridge (nobody sends us BPDUs) every
+		 * port stayed blocking FOREVER and "stp on" killed the whole
+		 * network. If no better root was heard during the listen period we
+		 * are the designated bridge on that port: go to forwarding. */
+		if (port_timers[i]) {
+			if (!--port_timers[i]) {
+				reg_read_m(RTL837X_MSTP_STATES);
+				sfr_data[3 - (i >> 2)] |= (uint8_t)(0b11 << ((i << 1) & 0x7));
+				reg_write_m(RTL837X_MSTP_STATES);
+				print_string("STP: port forwarding ");
+				print_byte(i); write_char('\n');
+			}
+		}
 	}
 }
 
@@ -258,7 +269,7 @@ void stp_setup(void) __banked
 		uint8_t bit_mask = 0b01 << ( (i << 1) & 0x7);
 		sfr_data[3 - (i >> 2)] |= bit_mask;
 		port_hello[i] = TIME_HELLO;
-		port_timers[i] = 0xa00;	// 10 sec in blocking state
+		port_timers[i] = 0x280;	// 10 s in blocking state (at the ~64 Hz stp_timers rate)
 	}
 	sfr_data[1] |= 0x0f; // Do not block CPU-Port
 	reg_write_m(RTL837X_MSTP_STATES); // R5310-000d555f 

rtl837x_stp.h

@@ -7,6 +7,20 @@ void stp_setup(void) __banked;
 void stp_timers(void) __banked;
 void stp_off(void) __banked;
 
-#define TIME_HELLO 0x200 // 2 sec
+#define TIME_HELLO 0x80 // 2 sec (stp_timers runs at ~64 Hz: main loop ~256 Hz / STP_TICK_DIVIDER+1)
+
+/* Bridge identifier as carried in a BPDU (priority, extension, MAC). */
+struct bridge {
+	uint8_t prio;
+	uint8_t ext;
+	uint8_t mac[6];
+};
+
+/* Protocol state, exposed read-only for the web UI (page_impl.c send_stp())
+ * - the elected root bridge and our path cost to it. Owned by rtl837x_stp.c;
+ * stpEnabled is owned by rtlplayground.c. */
+extern __xdata uint8_t stpEnabled;
+extern __xdata struct bridge root_bridge;
+extern __xdata uint32_t root_bridge_cost;
 
 #endif