applications: slm: Fix HWFC problems on nRF9151DK

applications/serial_lte_modem/Kconfig

@@ -201,6 +201,12 @@ config SLM_MODEM_PIPE_TIMEOUT
 
 endif
 
+config SLM_SKIP_READY_MSG
+	bool "Skip ready message"
+	help
+	  Skip sending the ready message when the application is powered on.
+
+
 module = SLM
 module-str = serial modem
 source "${ZEPHYR_BASE}/subsys/logging/Kconfig.template.log_config"

applications/serial_lte_modem/overlay-cmux.conf

@@ -5,6 +5,7 @@
 #
 
 CONFIG_SLM_CMUX=y
+CONFIG_SLM_SKIP_READY_MSG=y
 
 # Zephyr modem subsystem
 CONFIG_MODEM_MODULES=y

applications/serial_lte_modem/src/slm_at_host.c

@@ -910,9 +910,12 @@ int slm_at_host_init(void)
 		return err;
 	}
 
-	err = slm_at_send_str(SLM_SYNC_STR);
-	if (err) {
-		return err;
+	if (!IS_ENABLED(CONFIG_SLM_SKIP_READY_MSG)) {
+		/* Send Ready string to indicate that AT host is ready */
+		err = slm_at_send_str(SLM_SYNC_STR);
+		if (err) {
+			return err;
+		}
 	}
 
 	/* This is here and not earlier because in case of firmware

applications/serial_lte_modem/src/slm_cmux.c

@@ -114,6 +114,10 @@ static void init_dlci(size_t dlci_idx, uint16_t recv_buf_size,
 
 static int cmux_write_at_channel(const uint8_t *data, size_t len)
 {
+	if (cmux.dlcis[cmux.at_channel].instance.state != MODEM_CMUX_DLCI_STATE_OPEN) {
+		return 0;
+	}
+
 	int ret = modem_pipe_transmit(cmux.dlcis[cmux.at_channel].pipe, data, len);
 
 	if (ret != len) {