drivers: modem_cellular: added APP RDY signal handling

[petrosyan-van]

Some modems, such as the Quectel BG95/EG25-G, emit an APP RDY message when the application core has fully initialized and is ready to process AT commands. By listening for this signal, the state machine can proceed earlier than the configured startup_time_ms, improving connection setup time.

This patch:

• Adds a new MODEM_CELLULAR_EVENT_MODEM_READY event and a corresponding unsolicited chat match.
• Opens the UART and attaches the chat parser in the AWAIT_POWER_ON state, so that APP RDY can be captured immediately.
• Transitions to SET_BAUDRATE or RUN_INIT_SCRIPT as soon as APP RDY is seen, while keeping the existing timeout as a fallback for modems that don't emit this message.

Tested on a Quectel BG95-M3 on a custom board, where this change consistently advances the flow by several seconds, especially valuable for battery-powered applications where the modem is power-cycled frequently for short bursts of activity (e.g., uplink and shutdown). Over time, this can lead to noticeable energy savings :)

Modems that do not emit APP RDY will follow the same timeout-driven path as before. No behavioral regressions expected.

[rerickson1]

This is a nice change. I think it would be better to rename this to modem ready so it's better suited to use on other modems. For example, the HL7800 will emit a +KSUP message when it is ready. This even could be reused for that modem by simply matching on +KSUP received.

[bjarki-andreasen]

This is a nice change. I think it would be better to rename this to modem ready so it's better suited to use on other modems. For example, the HL7800 will emit a +KSUP message when it is ready. This even could be reused for that modem by simply matching on +KSUP received.

I agree, naming the unsol handler and event "modem ready", we can easily extend support for more modems by adding

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("APP RDY", "", modem_cellular_chat_on_modem_ready),
                          MODEM_CHAT_MATCH("+KSUP", "", modem_cellular_chat_on_modem_ready),
                          ...);

[petrosyan-van]

Thanks a lot for the feedback :)

I implemented the changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[bjarki-andreasen]

Nice!