fix(modem): Refine mode switch data->command

* netif.stop() moved after setting the transition callback
* send PPP escape sequence if enabled before waiting for transition to
complete
* add newline character before sync() command (after the "+++")

Closes https://github.com/espressif/esp-protocols/issues/692

components/esp_modem/Kconfig

@@ -76,4 +76,13 @@ menu "esp-modem"
         help
             If enabled, APIs to add URC handler are available
 
+    config ESP_MODEM_PPP_ESCAPE_BEFORE_EXIT
+        bool "Send escape sequence when switching PPP -> CMD"
+        default n
+        help
+            If enabled, the library sends a PPP escape ("+++" command)
+            to switch to command mode. This make switching from PPP to CMD
+            mode more robust for some devices (e.g. Quectel), but might cause
+            trouble for other devices (e.g. SIMCOM).
+
 endmenu

components/esp_modem/include/cxx_include/esp_modem_dce_module.hpp

@@ -77,7 +77,9 @@ public:
                 if (set_command_mode() == command_result::OK) {
                     return true;
                 }
-                Task::Delay(1000); // Mandatory 1s pause after escape
+                // send a newline to delimit the escape from the upcoming sync command
+                uint8_t delim = '\n';
+                dte->write(&delim, 1);
                 if (sync() == command_result::OK) {
                     return true;
                 }

components/esp_modem/src/esp_modem_dce.cpp

@@ -18,7 +18,6 @@ namespace transitions {
 
 static bool exit_data(DTE &dte, ModuleIf &device, Netif &netif)
 {
-    netif.stop();
     auto signal = std::make_shared<SignalGroup>();
     std::weak_ptr<SignalGroup> weak_signal = signal;
     dte.set_read_cb([&netif, weak_signal](uint8_t *data, size_t len) -> bool {
@@ -32,7 +31,7 @@ static bool exit_data(DTE &dte, ModuleIf &device, Netif &netif)
         if (memchr(data, '\n', len))
         {
             ESP_LOG_BUFFER_HEXDUMP("esp-modem: debug_data (CMD)", data, len, ESP_LOG_DEBUG);
-            const auto pass = std::list<std::string_view>({"NO CARRIER", "DISCONNECTED"});
+            const auto pass = std::list<std::string_view>({"NO CARRIER", "DISCONNECTED", "OK"});
             std::string_view response((char *) data, len);
             for (auto &it : pass)
                 if (response.find(it) != std::string::npos) {
@@ -44,8 +43,14 @@ static bool exit_data(DTE &dte, ModuleIf &device, Netif &netif)
         }
         return false;
     });
+    netif.stop();
     netif.wait_until_ppp_exits();
-    if (!signal->wait(1, 2000)) {
+#ifdef ESP_MODEM_PPP_ESCAPE_BEFORE_EXIT
+    std::array<uint8_t, 3> ppp_escape = {'+', '+', '+'};
+    dte.write(ppp_escape.data(), ppp_escape.size());
+#endif
+    if (!signal->wait(1, 2000)) { // wait for any of the disconnection messages
+        // if no reply -> set device to command mode
         dte.set_read_cb(nullptr);
         if (!device.set_mode(modem_mode::COMMAND_MODE)) {
             return false;

components/esp_modem/src/esp_modem_netif_linux.cpp

@@ -52,7 +52,6 @@ void Netif::start()
 
 void Netif::stop()
 {
-    ppp_dte->set_read_cb(nullptr);
     signal.clear(PPP_STARTED);
 }