feat(modem): Support esp-modem use without PPP

Closes https://github.com/espressif/esp-protocols/issues/851

components/esp_modem/Kconfig

@@ -103,4 +103,14 @@ menu "esp-modem"
             Set this to 'y' if you're making changes to the actual sources of
             the AT command definitions (typically in esp_modem_command_declare.inc)
 
+    config ESP_MODEM_USE_PPP_MODE
+        bool "Use PPP mode"
+        default y
+        select LWIP_PPP_SUPPORT
+        help
+            If enabled, the library can use PPP netif from lwip.
+            This is the default and most common setting.
+            But it's possible to disable it and use only AT commands,
+            in this case it's not required to enable LWIP_PPP_SUPPORT.
+
 endmenu

components/esp_modem/include/cxx_include/esp_modem_dce_factory.hpp

@@ -62,12 +62,16 @@ class Creator {
 public:
     Creator(std::shared_ptr<DTE> dte, esp_netif_t *esp_netif): dte(std::move(dte)), device(nullptr), netif(esp_netif)
     {
+#ifdef CONFIG_ESP_MODEM_USE_PPP_MODE
         ESP_MODEM_THROW_IF_FALSE(netif != nullptr, "Null netif");
+#endif
     }
 
     Creator(std::shared_ptr<DTE> dte, esp_netif_t *esp_netif, std::shared_ptr<T_Module> dev): dte(std::move(dte)), device(std::move(dev)), netif(esp_netif)
     {
+#ifdef CONFIG_ESP_MODEM_USE_PPP_MODE
         ESP_MODEM_THROW_IF_FALSE(netif != nullptr, "Null netif");
+#endif
     }
 
     explicit Creator(std::shared_ptr<DTE> dte): dte(std::move(dte)), device(nullptr), netif(nullptr)

components/esp_modem/src/esp_modem_netif.cpp

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2021-2024 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2021-2025 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -14,7 +14,6 @@
 #include "cxx_include/esp_modem_dte.hpp"
 #include "esp_netif_ppp.h"
 
-
 namespace esp_modem {
 
 void Netif::on_ppp_changed(void *arg, esp_event_base_t event_base,
@@ -39,6 +38,7 @@ esp_err_t Netif::esp_modem_dte_transmit(void *h, void *buffer, size_t len)
     return ESP_FAIL;
 }
 
+#ifdef CONFIG_ESP_MODEM_USE_PPP_MODE
 esp_err_t Netif::esp_modem_post_attach(esp_netif_t *esp_netif, void *args)
 {
     auto d = (ppp_netif_driver *) args;
@@ -62,6 +62,7 @@ esp_err_t Netif::esp_modem_post_attach(esp_netif_t *esp_netif, void *args)
 
     return ESP_OK;
 }
+#endif // CONFIG_ESP_MODEM_USE_PPP_MODE
 
 void Netif::receive(uint8_t *data, size_t len)
 {
@@ -73,12 +74,16 @@ Netif::Netif(std::shared_ptr<DTE> e, esp_netif_t *ppp_netif) :
 {
     driver.base.netif = ppp_netif;
     driver.ppp = this;
+#ifdef CONFIG_ESP_MODEM_USE_PPP_MODE
     driver.base.post_attach = esp_modem_post_attach;
     ESP_MODEM_THROW_IF_ERROR(esp_event_handler_register(NETIF_PPP_STATUS, ESP_EVENT_ANY_ID, &on_ppp_changed, (void *) this));
+#endif
     ESP_MODEM_THROW_IF_ERROR(esp_event_handler_register(IP_EVENT, IP_EVENT_PPP_GOT_IP, esp_netif_action_connected, ppp_netif));
     ESP_MODEM_THROW_IF_ERROR(
         esp_event_handler_register(IP_EVENT, IP_EVENT_PPP_LOST_IP, esp_netif_action_disconnected, ppp_netif));
+#ifdef CONFIG_ESP_MODEM_USE_PPP_MODE
     ESP_MODEM_THROW_IF_ERROR(esp_netif_attach(ppp_netif, &driver));
+#endif
 }
 
 void Netif::start()
@@ -120,7 +125,9 @@ Netif::~Netif()
         signal.clear(PPP_STARTED);
         signal.wait(PPP_EXIT, 30000);
     }
+#ifdef CONFIG_ESP_MODEM_USE_PPP_MODE
     esp_event_handler_unregister(NETIF_PPP_STATUS, ESP_EVENT_ANY_ID, &on_ppp_changed);
+#endif
     esp_event_handler_unregister(IP_EVENT, IP_EVENT_PPP_GOT_IP, esp_netif_action_connected);
     esp_event_handler_unregister(IP_EVENT, IP_EVENT_PPP_LOST_IP, esp_netif_action_disconnected);
 }