app: Implement AT+CGDATA command to attach PPP

AT+CGDATA per 3GPP TS 27.007 section 10.1.12.

Supported forms:
  AT+CGDATA           - Start PPP with default CID
  AT+CGDATA="PPP"     - Start PPP with default CID (L2P must be "PPP")
  AT+CGDATA="PPP",<cid> - Start PPP with specified CID
  AT+CGDATA=?         - Report supported L2P values

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

app/src/sm_ppp.c

@@ -325,8 +325,11 @@ static int ppp_start(void)
 	struct ppp_context *const ctx = net_if_l2_data(ppp_iface);
 
 	if (!configure_ppp_link_ip_addresses(ctx)) {
-		ret = -EADDRNOTAVAIL;
-		goto error;
+		return -EADDRNOTAVAIL;
+	}
+
+	if (!ppp_pipe) {
+		return -EINVAL;
 	}
 
 	ppp_state = PPP_STATE_STARTING;
@@ -715,6 +718,77 @@ static int handle_at_ppp(enum at_parser_cmd_type cmd_type, struct at_parser *par
 	return -SILENT_AT_COMMAND_RET;
 }
 
+SM_AT_CMD_CUSTOM(cgdata, "AT+CGDATA", handle_at_cgdata);
+static int handle_at_cgdata(enum at_parser_cmd_type cmd_type, struct at_parser *parser,
+			    uint32_t param_count)
+{
+	/* AT+CGDATA per 3GPP TS 27.007 section 10.1.12.
+	 *
+	 * Supported forms:
+	 *   AT+CGDATA           - Start PPP with default CID
+	 *   AT+CGDATA="PPP"     - Start PPP with default CID (L2P must be "PPP")
+	 *   AT+CGDATA="PPP",<cid> - Start PPP with specified CID
+	 *   AT+CGDATA=?         - Report supported L2P values
+	 */
+	int ret;
+
+	if (cmd_type == AT_PARSER_CMD_TYPE_TEST) {
+		rsp_send("\r\n+CGDATA: (\"PPP\")\r\n");
+		return 0;
+	}
+
+	if (cmd_type != AT_PARSER_CMD_TYPE_SET || param_count > 3) {
+		return -EINVAL;
+	}
+
+	/* Default CID (0 = default PDP context). */
+	unsigned int cid = 0;
+
+	if (param_count >= 2) {
+		/* Parse and validate the L2P string parameter. */
+		char l2p[8];
+		size_t l2p_len = sizeof(l2p);
+
+		ret = util_string_get(parser, 1, l2p, &l2p_len);
+		if (ret || strcasecmp(l2p, "PPP") != 0) {
+			return -EINVAL;
+		}
+	}
+
+	if (param_count == 3) {
+		ret = at_parser_num_get(parser, 2, &cid);
+		if (ret) {
+			return -EINVAL;
+		}
+	}
+
+	if (ppp_state != PPP_STATE_STOPPED) {
+		LOG_ERR("PPP already running");
+		return -EALREADY;
+	}
+
+	if (ppp_pipe) {
+		sm_ppp_detach();
+	}
+
+	struct sm_at_host_ctx *ctx = sm_at_host_get_current();
+	struct modem_pipe *pipe = ctx ? sm_at_host_get_pipe(ctx) : NULL;
+
+	if (!ctx || !pipe) {
+		LOG_ERR("No pipe available for PPP.");
+		return -ENODEV;
+	}
+	ppp_pipe = pipe;
+	sm_ppp_keep_pipe_attached = false;
+	ppp_pdn_cid = cid;
+	rsp_send("\r\nCONNECT\r\n");
+	sm_at_host_release(ctx);
+	modem_ppp_attach(&ppp_module, ppp_pipe);
+	sm_ppp_set_auto_start(true);
+	delegate_ppp_event(PPP_START, PPP_REASON_CMD);
+	return -SILENT_AT_COMMAND_RET;
+}
+
 static void ppp_data_passing_thread(void*, void*, void*)
 {
 	struct zsock_pollfd fds[PPP_FDS_COUNT];

doc/app/at_ppp.rst

@@ -12,6 +12,86 @@ This page describes AT commands related to the Point-to-Point Protocol (PPP).
 PPP is enabled in |SM| by compiling it with the appropriate configuration files, depending on your use case (with or without CMUX).
 See the :ref:`sm_config_files` section for more information.
 
+Enter data state +CGDATA
+========================
+
+The ``AT+CGDATA`` command enters data state and starts PPP, as specified in `3GPP TS 27.007`_ (section 10.1.12).
+
+Set command
+-----------
+
+The set command starts PPP on the current channel, optionally specifying the layer 2 protocol and PDN connection.
+
+Syntax
+~~~~~~
+
+::
+
+   AT+CGDATA[=<L2P>[,<cid>]]
+
+* The ``<L2P>`` parameter is a string specifying the layer 2 protocol to use.
+  The only supported value is ``"PPP"``.
+  If omitted, ``"PPP"`` is assumed.
+
+* The ``<cid>`` parameter is an integer indicating the PDN connection to use.
+  Its default value is ``0``, which represents the default PDN connection.
+
+Response syntax
+~~~~~~~~~~~~~~~
+
+::
+
+   CONNECT
+
+Examples
+~~~~~~~~
+
+With default parameters:
+
+::
+
+   AT+CGDATA
+
+   CONNECT
+
+Request PPP to use specified PDN connection:
+
+::
+
+   AT+CGDATA="PPP",1
+
+   CONNECT
+
+Test command
+------------
+
+The test command lists the supported layer 2 protocols.
+
+Syntax
+~~~~~~
+
+::
+
+   AT+CGDATA=?
+
+Response syntax
+~~~~~~~~~~~~~~~
+
+::
+
+   +CGDATA: (<L2P_values>)
+
+Example
+~~~~~~~
+
+::
+
+   AT+CGDATA=?
+
+   +CGDATA: ("PPP")
+
+   OK
+
 Control PPP #XPPP
 =================