applications: serial_lte_modem: Clarify CMUX MTU documentation

Add documentation regarding CONFIG_MODEM_CMUX_MTU.

Signed-off-by: Seppo Takalo <[email protected]>

Author: SeppoTakalo

applications/serial_lte_modem/doc/CMUX_AT_commands.rst

@@ -9,12 +9,13 @@ CMUX AT commands
 
 This page describes CMUX-related AT commands.
 
-The CMUX protocol enables multiplexing multiple data streams through a single serial link, setting up one channel per data stream.
+The GSM 0710 multiplexer protocol (CMUX) enables multiplexing multiple data streams through a single serial link, setting up one channel per data stream.
 For example, it can be used to exchange AT data and have a :ref:`Point-to-Point Protocol (PPP) <CONFIG_SLM_PPP>` link up at the same time on a single UART.
 
 .. note::
 
    To use the nRF91 Series SiP as a standalone modem in Zephyr, see :ref:`slm_as_zephyr_modem`.
+   To use the nRF91 Series SiP as a modem for a Linux device, see :ref:`slm_as_linux_modem`.
 
 CMUX is enabled in SLM by compiling it with the appropriate configuration files, depending on your use case.
 See the :ref:`slm_config_files` section for more information.
@@ -38,7 +39,7 @@ See the :ref:`slm_config_files` section for more information.
 
    * Only UIH frames are used.
    * The speed used is the configured baud rate of SLM's UART.
-   * The maximum frame size is ``2100`` (defined by :c:macro:`SLM_AT_MAX_RSP_LEN` found in :file:`slm_defines.h`).
+   * The maximum frame size is configured using the :kconfig:option:`CONFIG_MODEM_CMUX_MTU` Kconfig option.
 
 CMUX setup #XCMUX
 =================

applications/serial_lte_modem/doc/PPP_linux.rst

@@ -21,8 +21,8 @@ Prerequisites
 
 The Linux device needs to have the following packages installed:
 
-* pppd from the ppp package
-* ldattach from the util-linux package
+* ``pppd`` from the ppp package
+* ``ldattach`` from the util-linux package
 
 These should be available on all standard Linux distributions.
 
@@ -35,13 +35,19 @@ You can adjust the serial port baud rate using the devicetree overlay file.
 By default, the baud rate is set to 115200.
 If you change the baud rate, set the same rate in the :file:`scripts/slm_start_ppp.sh` and :file:`scripts/slm_stop_ppp.sh` scripts.
 
+.. note::
+   The standard ``ldattach`` utility sets MRU and MTU to 127 bytes.
+   This is hard-coded and cannot be changed.
+   If you change the SLM configuration, make sure that the :kconfig:option:`CONFIG_MODEM_CMUX_MTU` Kconfig option is set to 127 bytes.
+   This is already configured in the :file:`overlay-ppp-cmux-linux.conf` configuration overlay.
+
 Building and running
 ====================
 
 To build and program the SLM application to the nRF91 Series device, use the :file:`overlay-ppp-cmux-linux.conf` overlay file.
 
 Managing the connection
-=======================
+***********************
 
 The start and stop scripts are provided in the :file:`scripts` directory of the SLM application.
 The scripts assume that the nRF91 Series SiP is connected to the Linux device using the `/dev/ttyACM0` serial port.