app: Add partial implementation of standard AT+CMUX command

Only following commands are supported:
AT+CMUX=0
AT+CMUX=0,0
AT+CMUX?
AT+CMUX=?

Baud rate switch is not supported.

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

Author: SeppoTakalo

app/src/sm_cmux.c

@@ -264,8 +264,8 @@ static int cmux_start(void)
 	return 0;
 }
 
-SM_AT_CMD_CUSTOM(xcmux, "AT#XCMUX", handle_at_cmux);
-static int handle_at_cmux(enum at_parser_cmd_type cmd_type, struct at_parser *parser,
+SM_AT_CMD_CUSTOM(xcmux, "AT#XCMUX", handle_at_xcmux);
+static int handle_at_xcmux(enum at_parser_cmd_type cmd_type, struct at_parser *parser,
 			  uint32_t param_count)
 {
 	unsigned int at_dlci;
@@ -320,8 +320,8 @@ static int handle_at_cmux(enum at_parser_cmd_type cmd_type, struct at_parser *pa
 	return ret;
 }
 
-SM_AT_CMD_CUSTOM(xcmuxcld, "AT#XCMUXCLD", handle_at_cmuxcld);
-static int handle_at_cmuxcld(enum at_parser_cmd_type cmd_type, struct at_parser *parser,
+SM_AT_CMD_CUSTOM(xcmuxcld, "AT#XCMUXCLD", handle_at_xcmuxcld);
+static int handle_at_xcmuxcld(enum at_parser_cmd_type cmd_type, struct at_parser *parser,
 			     uint32_t param_count)
 {
 	if (cmd_type != AT_PARSER_CMD_TYPE_SET || param_count != 1) {
@@ -339,3 +339,67 @@ static int handle_at_cmuxcld(enum at_parser_cmd_type cmd_type, struct at_parser
 
 	return -SILENT_AT_COMMAND_RET;
 }
+
+SM_AT_CMD_CUSTOM(atcmux, "AT+CMUX", handle_at_cmux);
+static int handle_at_cmux(enum at_parser_cmd_type cmd_type, struct at_parser *parser,
+			  uint32_t param_count)
+{
+	/* AT+CMUX follows the 3GPP TS 27.010 specification.
+	 *
+	 * Only following commands are supported:
+	 * AT+CMUX=0       (basic mode)
+	 * AT+CMUX=0,0     (basic mode, subset 0)
+	 * AT+CMUX?        (read current configuration)
+	 * AT+CMUX=?       (list supported parameter ranges)
+	 * All other parameter combinations are rejected with an error response.
+	 */
+	unsigned int mode;
+	unsigned int subset;
+	int ret;
+
+	switch (cmd_type) {
+	case AT_PARSER_CMD_TYPE_TEST:
+		/* Report supported ranges: only mode 0, subset 0 */
+		rsp_send("\r\n+CMUX: (0),(0)\r\n");
+		return 0;
+
+	case AT_PARSER_CMD_TYPE_READ:
+		/* Report current (and only) configuration */
+		rsp_send("\r\n+CMUX: 0,0\r\n");
+		return 0;
+
+	case AT_PARSER_CMD_TYPE_SET:
+		if (param_count < 2 || param_count > 3) {
+			return -EINVAL;
+		}
+
+		ret = at_parser_num_get(parser, 1, &mode);
+		if (ret || mode != 0) {
+			return -EINVAL;
+		}
+
+		if (param_count == 3) {
+			ret = at_parser_num_get(parser, 2, &subset);
+			if (ret || subset != 0) {
+				return -EINVAL;
+			}
+		}
+
+		if (sm_cmux_is_started()) {
+			return -EALREADY;
+		}
+
+		/* Respond before starting CMUX. */
+		rsp_send_ok();
+		ret = cmux_start();
+		if (ret) {
+			LOG_ERR("Failed to start CMUX. (%d)", ret);
+		} else {
+			ret = -SILENT_AT_COMMAND_RET;
+		}
+		return ret;
+
+	default:
+		return -EINVAL;
+	}
+}

doc/app/at_cmux.rst

@@ -23,14 +23,102 @@ See the :ref:`sm_config_files` section for more information.
 
 .. note::
 
-   |SM| does not have an equivalent to the ``AT+CMUX`` command described in `3GPP TS 27.007`_.
-   Here is how |SM|'s implementation of CMUX relates to the standard command's parameters:
-
    * Only basic mode (mode 0) is supported.
    * Only UIH frames are used.
    * The speed used is the configured baud rate of |SM|'s UART.
    * No system parameters (N1, T1, T2, T3, k, m) are configurable.
 
+CMUX setup +CMUX
+================
+
+The ``AT+CMUX`` command starts the CMUX multiplexer.
+It is defined in `3GPP TS 27.007`_ (section 5.7) and the underlying protocol is specified in `3GPP TS 27.010`_.
+
+Only basic mode (``<mode>=0``) with subset ``0`` is supported.
+All other parameter values are rejected.
+
+Set command
+-----------
+
+The set command allows you to start the CMUX multiplexer.
+
+An ``OK`` response is sent before CMUX is started, after which only CMUX framing is accepted on the serial link.
+
+Syntax
+~~~~~~
+
+::
+
+   AT+CMUX=<mode>[,<subset>]
+
+* The ``<mode>`` parameter selects the operation mode. Only ``0`` (basic mode) is supported.
+* The ``<subset>`` parameter selects the subset of mode 0. Only ``0`` is supported. Optional; defaults to ``0``.
+
+Read command
+------------
+
+The read command returns the current multiplexer configuration.
+
+Syntax
+~~~~~~
+
+::
+
+   AT+CMUX?
+
+Response syntax
+~~~~~~~~~~~~~~~
+
+::
+
+   +CMUX: <mode>,<subset>
+
+* The ``<mode>`` parameter is always ``0`` (basic mode).
+* The ``<subset>`` parameter is always ``0``.
+
+Test command
+------------
+
+The test command returns the supported parameter ranges.
+
+Syntax
+~~~~~~
+
+::
+
+   AT+CMUX=?
+
+Response syntax
+~~~~~~~~~~~~~~~
+
+::
+
+   +CMUX: (0),(0)
+
+Example
+-------
+
+::
+
+   AT+CMUX=?
+
+   +CMUX: (0),(0)
+
+   OK
+   AT+CMUX?
+
+   +CMUX: 0,0
+
+   OK
+   AT+CMUX=0
+
+   OK
+   // CMUX is now started. Open the CMUX channels to continue communication.
+   AT+CMUX=0,0
+
+   OK
+   // Equivalent to AT+CMUX=0.
+
 CMUX setup #XCMUX
 =================