net: ppp: Properly terminate LCP state at modem side when closing down

Takes the modem state machine by calling lcp_close instead of lcp_down
Using this method the LCP layer sends a TERMINATE_REQ to the modem and the
network interface is only taken down when the LCP layer has properly
finished.
Moved the ppp_mgmt_raise_carrier_off_event and net_if_carrier_down
to lcp.c to avoid breaking the interface.
Tested on a real modem.

Fixes: #41627

Signed-off-by: Sjors Hettinga <[email protected]>

Author: ssharks

subsys/net/l2/ppp/lcp.c

@@ -206,6 +206,11 @@ static void lcp_finished(struct ppp_fsm *fsm)
 					       lcp.fsm);
 
 	ppp_link_terminated(ctx);
+
+	/* take the remainder down */
+	ppp_mgmt_raise_carrier_off_event(ctx->iface);
+
+	net_if_carrier_down(ctx->iface);
 }
 
 #if defined(CONFIG_NET_L2_PPP_OPTION_MRU)

subsys/net/l2/ppp/ppp_l2.c

@@ -190,10 +190,10 @@ static int ppp_send(struct net_if *iface, struct net_pkt *pkt)
 	return ret;
 }
 
-static void ppp_lower_down(struct ppp_context *ctx)
+static void ppp_close(struct ppp_context *ctx)
 {
 	if (ppp_lcp) {
-		ppp_lcp->lower_down(ctx);
+		ppp_lcp->close(ctx, "Shutdown");
 	}
 }
 
@@ -230,7 +230,7 @@ static int ppp_enable(struct net_if *iface, bool state)
 	ctx->is_enabled = state;
 
 	if (!state) {
-		ppp_lower_down(ctx);
+		ppp_close(ctx);
 
 		if (ppp->stop) {
 			ppp->stop(net_if_get_device(iface));
@@ -284,11 +284,15 @@ static void carrier_on_off(struct k_work *work)
 		ppp_mgmt_raise_carrier_on_event(ctx->iface);
 		net_if_up(ctx->iface);
 	} else {
-		ppp_lower_down(ctx);
-		ppp_change_phase(ctx, PPP_DEAD);
+		if (ppp_lcp) {
+			ppp_lcp->close(ctx, "Shutdown");
+			/* signaling for the carrier off event is done from the LCP callback */
+		} else {
+			ppp_change_phase(ctx, PPP_DEAD);
 
-		ppp_mgmt_raise_carrier_off_event(ctx->iface);
-		net_if_carrier_down(ctx->iface);
+			ppp_mgmt_raise_carrier_off_event(ctx->iface);
+			net_if_carrier_down(ctx->iface);
+		}
 	}
 }