stp: give auto-edge back to a port held by its own BPDU

stp_loop_hold_peer() takes operEdge away from the port that sent the
BPDU we got back, though that port heard nothing itself. When the hold
ran out the port stayed non-edge until its next link change, and every
later state change on it counted as a topology change. Restore edge when
the hold ends with the port still silent and auto-edge on, before the
transition is reported, so the recovery itself is not a change. Hellos
go out on edge ports too, so a loop that persists is caught again.

The console column that shows seconds since the last BPDU is now called
age[s]; it never was a receive count.

doc/stp.md

@@ -268,6 +268,15 @@ is reported with one of the roles Root, Designated, Alternate, Backup or
 Disabled; Disabled covers a port with STP turned off on it, one shut down by
 BPDU guard and one without link.
 
+The console's `age[s]` column (previously labelled `bpdu`) is the elapsed
+time since the last BPDU, capped at 255 seconds; it is not a receive count.
+Use the web counters or `/stpcnt.json` for full receive counts.
+
+An own BPDU received back on another port can temporarily hold its sending
+port in discarding, even if that sending port has received no BPDUs itself.
+When the hold expires, auto-edge is restored if the sending port has remained
+silent. A continuing loop keeps the hold active.
+
 BPDUs shorter than their type allows, judged by the 802.3 length field, are
 dropped: 7 bytes for a TCN, 38 for a Config BPDU and 39 for an RST BPDU,
 LLC header included.

rtl837x_stp.c

@@ -297,7 +297,7 @@ void stp_status(void) __banked
 	print_string("changes ");
 	print_short(stp_tc_count);
 	write_char('\n');
-	print_string("port state role edge tx bpdu\n");
+	print_string("port state role edge tx age[s]\n");
 	for (stp_i = 0; stp_i < STP_ENTITIES; stp_i++) {
 		if (!stp_ent_active(stp_i))
 			continue;
@@ -1297,6 +1297,13 @@ void stp_timers(void) __banked
 					print_string("STP: port learning ");
 					print_port_nl(stp_i);
 				} else {
+					/* A returned BPDU can hold its sending port even though
+					 * that port never received one. Once the hold expires,
+					 * recover auto-edge before the forwarding timer stops. */
+					if (stp_loop_held[stp_i] && (stp_pflags[stp_i] & STP_PF_AUTOEDGE)
+					    && !((stp_heard >> stp_i) & 1)
+					    && stp_bpdu_age[stp_i] > STP_EDGE_DELAY)
+						stp_pflags[stp_i] |= STP_PF_OPEREDGE;
 					stp_loop_held[stp_i] = 0;
 					stp_backup &= ~((uint16_t)1 << stp_i);
 					stp_state_set(stp_i, 0b11);