feat(esp_modem): Make some CMUX params compile-time configurable

* bool flag to defragment CMUX payload (useful for devices that send longer messages)
* int flag to force a delay between creating virtual terminals (useful for chatty devices that send some requests)

components/esp_modem/Kconfig

@@ -0,0 +1,28 @@
+menu "esp-modem"
+
+    config ESP_MODEM_CMUX_DEFRAGMENT_PAYLOAD
+        bool "Defragment CMUX messages internally"
+        default y
+        help
+            If enabled (default), the esp-modem automatically defragments CMUX messages
+            to only pass the completed CMUX message to higher layers.
+            This is useful for messages in command mode (if they're received fragmented).
+            It's not a problem for messages in data mode as the upper layer (PPP protocol)
+            defines message boundaries.
+            Keep the default to true for most cases (as most devices use simply 1 byte CMUX
+            length, as the internal Rx buffer of size >= 256 bytes won't overflow)
+            Set to false if your devices uses 2 byte CMUX payload (e.g. A7672S).
+            The operation would work without an issue in data mode, but some replies
+            in command mode might come fragmented in rare cases so might need to retry
+            AT commands.
+
+    config ESP_MODEM_CMUX_DELAY_AFTER_DLCI_SETUP
+        int "Delay in ms to wait before creating another virtual terminal"
+        default 0
+        help
+            Some devices might need a pause before sending SABM command that creates
+            virtual terminal. This delay applies only to establishing a CMUX mode.
+            The typical reason for failing SABM request without a delay is that
+            some devices (SIM800) send MSC requests just after opening a new DLCI.
+
+endmenu

components/esp_modem/docs/README.md

@@ -69,10 +69,24 @@ after creating multiple virtual terminals, designating some of them solely to da
 
 ### DTE's
 
-Currently we support only UART, but modern modules support other communication interfaces, such as USB, SPI.
+Currently, we support only UART (and USB as a preview feature), but modern modules support other communication interfaces, such as USB, SPI.
 
 ### Other devices
 
 Adding a new device is a must-have requirement for the esp-modem component. Different modules support different commands,
 or some commands might have a different implementation. Adding a new device means to provide a new implementation
 as a class derived from `GenericModule`, where we could add new commands or modify the existing ones.
+
+## Configuration
+
+Modem abstraction is configurable both compile-time and run-time.
+
+### Component Kconfig
+
+Compile-time configuration is provided using menuconfig. Please check the description for the CMUX mode configuration options.
+
+### Runtime configuration
+
+Is defined using standard configuration structures for `DTE` and `DCE` objects separately. Please find documentation of
+* :cpp:class:`esp_modem_dte_config_t`
+* :cpp:class:`esp_modem_dce_config_t`

components/esp_modem/src/esp_modem_cmux.cpp

@@ -17,15 +17,18 @@
 #include <cxx_include/esp_modem_cmux.hpp>
 #include "cxx_include/esp_modem_dte.hpp"
 #include "esp_log.h"
+#include "sdkconfig.h"
 
 using namespace esp_modem;
 
+#ifdef CONFIG_ESP_MODEM_CMUX_DEFRAGMENT_PAYLOAD
 /**
  * @brief Define this to defragment partially received data of CMUX payload
  *        This is useful if upper layers expect the entire payload available
  *        for parsing.
  */
 #define DEFRAGMENT_CMUX_PAYLOAD
+#endif
 
 #define EA 0x01  /* Extension bit      */
 #define CR 0x02  /* Command / Response */
@@ -290,7 +293,7 @@ bool CMux::on_cmux_data(uint8_t *data, size_t actual_len)
         actual_len = term->read(data, data_to_read);
 #else
         data = buffer.get();
-        actual_len = term->read(data, buffer_size);
+        actual_len = term->read(data, buffer.size);
 #endif
     }
     ESP_LOG_BUFFER_HEXDUMP("CMUX Received", data, actual_len, ESP_LOG_VERBOSE);
@@ -390,7 +393,7 @@ bool CMux::init()
             }
         }
         if (i > 1) {    // wait for each virtual terminal to settle MSC (no need for control term, DLCI=0)
-            usleep(100'000);
+            usleep(CONFIG_ESP_MODEM_CMUX_DELAY_AFTER_DLCI_SETUP * 1'000);
         }
     }
     return true;