esp_system: introduce intermediary function to call after system init

This MR uses an intermediary function `start_app` to call after system
initialization instead of `app_main`.

In RTOS builds, freertos provides `start_app` and calls `app_main`.
In non-RTOS builds, user provides `start_app` directly.

components/esp_system/CMakeLists.txt

@@ -8,9 +8,12 @@ idf_component_register(SRCS "panic.c" "system_api.c" "startup.c"
 
 add_subdirectory(port)
 
-# Rely on user code to define app_main
-target_link_libraries(${COMPONENT_LIB} INTERFACE "-u app_main")
+# After system initialization, `start_app` (and its other cores variant) is called. 
+# This is provided by the user or from another component. Since we can't establish 
+# dependency on what we don't know, force linker to not drop the symbol regardless
+# of link line order.
+target_link_libraries(${COMPONENT_LIB} INTERFACE "-u start_app")
 
 if (NOT CONFIG_ESP_SYSTEM_SINGLE_CORE_MODE)
-    target_link_libraries(${COMPONENT_LIB} INTERFACE "-u app_mainX")
+    target_link_libraries(${COMPONENT_LIB} INTERFACE "-u start_app_other_cores")
 endif()

components/esp_system/startup.c

@@ -77,23 +77,23 @@
 #define STRINGIFY2(s) #s
 
 // App entry point for core 0
-extern void app_main(void);
+extern void start_app(void);
 
 // Entry point for core 0 from hardware init (port layer)
 void start_cpu0(void) __attribute__((weak, alias("start_cpu0_default"))) __attribute__((noreturn));
 
 #if !CONFIG_ESP_SYSTEM_SINGLE_CORE_MODE
 // Entry point for core [1..X] from hardware init (port layer)
-void start_cpuX(void) __attribute__((weak, alias("start_cpuX_default"))) __attribute__((noreturn));
+void start_cpu_other_cores(void) __attribute__((weak, alias("start_cpu_other_cores_default"))) __attribute__((noreturn));
 
 // App entry point for core [1..X]
-void app_mainX(void) __attribute__((weak, alias("app_mainX_default"))) __attribute__((noreturn));
+void start_app_other_cores(void) __attribute__((weak, alias("start_app_other_cores_default"))) __attribute__((noreturn));
 
 static volatile bool s_system_inited[SOC_CPU_CORES_NUM] = { false };
 
 sys_startup_fn_t g_startup_fn[SOC_CPU_CORES_NUM] = { [0] = start_cpu0,
 #if SOC_CPU_CORES_NUM > 1
-    [1 ... SOC_CPU_CORES_NUM - 1] = start_cpuX
+    [1 ... SOC_CPU_CORES_NUM - 1] = start_cpu_other_cores
 #endif
 };
 
@@ -143,14 +143,14 @@ static void IRAM_ATTR do_system_init_fn(void)
 }
 
 #if !CONFIG_ESP_SYSTEM_SINGLE_CORE_MODE
-static void IRAM_ATTR app_mainX_default(void)
+static void IRAM_ATTR start_app_other_cores_default(void)
 {
     while (1) {
         ets_delay_us(UINT32_MAX);
     }
 }
 
-static void IRAM_ATTR start_cpuX_default(void)
+static void IRAM_ATTR start_cpu_other_cores_default(void)
 {
     do_system_init_fn();
 
@@ -158,7 +158,7 @@ static void IRAM_ATTR start_cpuX_default(void)
         ets_delay_us(100);
     }
 
-    app_mainX();
+    start_app_other_cores();
 }
 #endif
 
@@ -331,7 +331,7 @@ void IRAM_ATTR start_cpu0_default(void)
     s_system_full_inited = true;
 #endif
 
-    app_main();
+    start_app();
     while (1);
 }
 

components/freertos/CMakeLists.txt

@@ -63,4 +63,10 @@ set_source_files_properties(
     _ESP_FREERTOS_INTERNAL
     )
 
-target_link_libraries(${COMPONENT_LIB} INTERFACE "-Wl,--wrap=app_main")
+# The freertos component provides the `start_app` and `start_app_other_cores` 
+# if it is included in the build. It then calls `app_main` 
+# from the main task created, which must be provided by the user. 
+# Like for `start_app` and `start_app_other_cores`,
+# we can't establish dependency on what we don't yet know, so we force the
+# linker to not drop this symbol.
+target_link_libraries(${COMPONENT_LIB} INTERFACE "-u app_main")

components/freertos/xtensa/port.c

@@ -138,6 +138,8 @@ extern void _frxt_tick_timer_init(void);
 /* Defined in xtensa_context.S */
 extern void _xt_coproc_init(void);
 
+extern void app_main(void);
+
 static const char* TAG = "cpu_start"; // [refactor-todo]: might be appropriate to change in the future, but
 								// for now maintain the same log output
 
@@ -460,15 +462,6 @@ void  __attribute__((weak)) vApplicationStackOverflowHook( TaskHandle_t xTask, c
 	esp_system_abort(buf);
 }
 
-// `esp_system` calls the app entry point app_main for core 0 and app_mainX for
-// the rest of the cores which the app normally provides for non-os builds.
-// If `freertos` is included in the build, wrap the call to app_main and provide
-// our own definition of app_mainX so that we can do our own initializations for each
-// core and start the scheduler.
-//
-// We now simply execute the real app_main in the context of the main task that
-// we also start.
-extern void __real_app_main(void);
 
 static void main_task(void* args)
 {
@@ -515,7 +508,7 @@ static void main_task(void* args)
 	}
 #endif
 
-	__real_app_main();
+	app_main();
 	vTaskDelete(NULL);
 }
 
@@ -530,7 +523,7 @@ static void main_task(void* args)
 
 
 #if !CONFIG_FREERTOS_UNICORE
-void app_mainX(void)
+void start_app_other_cores(void)
 {
 	// For now, we only support up to two core: 0 and 1.
 	if (xPortGetCoreID() >= 2) {
@@ -562,7 +555,7 @@ void app_mainX(void)
 }
 #endif
 
-void __wrap_app_main(void)
+void start_app(void)
 {
 #if CONFIG_ESP_INT_WDT
 	esp_int_wdt_init();