lib: nrf_modem: allow modem shutdown in flight mode

Allow the modem to be shut down when in flight mode (CFUN=4).

Signed-off-by: Eivind Jølsgard <eivind.jolsgard@nordicsemi.no>

doc/nrf/releases/release-notes-changelog.rst

@@ -333,6 +333,10 @@ Modem libraries
 
 * Added the :ref:`modem_battery_readme` library that obtains battery voltage information or notifications from a modem.
 
+* :ref:`nrf_modem_lib_readme`:
+
+  * Updated the :c:func:`nrf_modem_lib_shutdown` function to allow the modem to be in flight mode (``CFUN=4``) when shutting down the modem.
+
 * :ref:`pdn_readme` library:
 
   * Updated the library to allow a ``PDP_type``-only configuration in the :c:func:`pdn_ctx_configure` function.

lib/nrf_modem_lib/nrf_modem_lib.c

@@ -29,6 +29,7 @@ BUILD_ASSERT(IPC_IRQn == NRF_MODEM_IPC_IRQ, "NRF_MODEM_IPC_IRQ mismatch");
 
 #define AT_CFUN_READ "AT+CFUN?"
 #define AT_CFUN0_VAL 0
+#define AT_CFUN4_VAL 4
 
 /* The heap implementation in `nrf_modem_os.c` require some overhead
  * to allow allocating up to `NRF_MODEM_LIB_SHMEM_TX_SIZE` bytes.
@@ -163,9 +164,11 @@ int nrf_modem_lib_shutdown(void)
 		e->callback(e->context);
 	}
 
-	/* The application must set CFUN=0 before calling nrf_modem_shutdown. */
+	/* The application must disable both transmit and receive RF circuits, and deactivate
+	 *  LTE and GNSS services, before calling nrf_modem_shutdown.
+	 */
 	ret = nrf_modem_at_scanf(AT_CFUN_READ, "+CFUN: %hu", &mode);
-	if (ret == 1 && mode != AT_CFUN0_VAL) {
+	if (ret == 1 && (mode != AT_CFUN0_VAL && mode != AT_CFUN4_VAL)) {
 		LOG_WRN("Application should set minimal functional mode (CFUN=0) before "
 			"shutting down modem library");
 		nrf_modem_at_printf("AT+CFUN=0");