drivers: modem_cellular: added APP RDY signal handling

Some modems (e.g. Quectel BG95/EG25-G, several SIMCom parts) print
“APP RDY” when the application core is ready.  Parse this line and
proceed from AWAIT_POWER_ON immediately, while keeping the existing
startup timeout as a fall-back.

* add MODEM_CELLULAR_EVENT_MODEM_READY + chat match
* open UART and attach chat in AWAIT_POWER_ON
* jump to SET_BAUDRATE / RUN_INIT_SCRIPT on event

Signed-off-by: Van Petrosyan <van.petrosyan@sensirion.com>

Author: petrosyan-van

drivers/modem/modem_cellular.c

@@ -81,6 +81,7 @@ enum modem_cellular_event {
 	MODEM_CELLULAR_EVENT_BUS_OPENED,
 	MODEM_CELLULAR_EVENT_BUS_CLOSED,
 	MODEM_CELLULAR_EVENT_PPP_DEAD,
+	MODEM_CELLULAR_EVENT_MODEM_READY,
 };
 
 struct modem_cellular_data {
@@ -243,6 +244,8 @@ static const char *modem_cellular_event_str(enum modem_cellular_event event)
 		return "bus closed";
 	case MODEM_CELLULAR_EVENT_PPP_DEAD:
 		return "ppp dead";
+	case MODEM_CELLULAR_EVENT_MODEM_READY:
+		return "modem ready";
 	}
 
 	return "";
@@ -355,6 +358,15 @@ static void modem_cellular_chat_callback_handler(struct modem_chat *chat,
 	}
 }
 
+static void modem_cellular_chat_on_modem_ready(struct modem_chat *chat, char **argv, uint16_t argc,
+					void *user_data)
+{
+	struct modem_cellular_data *data = user_data;
+
+	modem_cellular_delegate_event(data, MODEM_CELLULAR_EVENT_MODEM_READY);
+}
+
+
 static void modem_cellular_chat_on_imei(struct modem_chat *chat, char **argv, uint16_t argc,
 					void *user_data)
 {
@@ -514,7 +526,8 @@ MODEM_CHAT_MATCH_DEFINE(cgmr_match __maybe_unused, "", "", modem_cellular_chat_o
 MODEM_CHAT_MATCHES_DEFINE(unsol_matches,
 			  MODEM_CHAT_MATCH("+CREG: ", ",", modem_cellular_chat_on_cxreg),
 			  MODEM_CHAT_MATCH("+CEREG: ", ",", modem_cellular_chat_on_cxreg),
-			  MODEM_CHAT_MATCH("+CGREG: ", ",", modem_cellular_chat_on_cxreg));
+			  MODEM_CHAT_MATCH("+CGREG: ", ",", modem_cellular_chat_on_cxreg),
+			  MODEM_CHAT_MATCH("APP RDY", "", modem_cellular_chat_on_modem_ready));
 
 MODEM_CHAT_MATCHES_DEFINE(abort_matches, MODEM_CHAT_MATCH("ERROR", "", NULL));
 
@@ -772,7 +785,9 @@ static int modem_cellular_on_await_power_on_state_enter(struct modem_cellular_da
 		(const struct modem_cellular_config *)data->dev->config;
 
 	modem_cellular_start_timer(data, K_MSEC(config->startup_time_ms));
-	return 0;
+	modem_pipe_attach(data->uart_pipe, modem_cellular_bus_pipe_handler, data);
+	modem_chat_attach(&data->chat, data->uart_pipe);
+	return modem_pipe_open_async(data->uart_pipe);
 }
 
 static void modem_cellular_await_power_on_event_handler(struct modem_cellular_data *data,
@@ -782,6 +797,9 @@ static void modem_cellular_await_power_on_event_handler(struct modem_cellular_da
 		(const struct modem_cellular_config *)data->dev->config;
 
 	switch (evt) {
+	case MODEM_CELLULAR_EVENT_MODEM_READY:
+		/* disable the timer and fall through, as we are ready to proceed */
+		modem_cellular_stop_timer(data);
 	case MODEM_CELLULAR_EVENT_TIMEOUT:
 		if (config->set_baudrate_chat_script != NULL) {
 			modem_cellular_enter_state(data, MODEM_CELLULAR_STATE_SET_BAUDRATE);