text: // Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #include <stddef.h>
  #include <stdlib.h>
  #include <string.h>
  #include <assert.h>
  #include "esp_err.h"
  #include "esp_ipc.h"
  #include "esp_attr.h"
  
  #include "freertos/FreeRTOS.h"
  #include "freertos/task.h"
  #include "freertos/semphr.h"
  
  static TaskHandle_t s_ipc_task_handle[portNUM_PROCESSORS];
  static SemaphoreHandle_t s_ipc_mutex[portNUM_PROCESSORS];    // This mutex is used as a global lock for esp_ipc_* APIs
  static SemaphoreHandle_t s_ipc_sem[portNUM_PROCESSORS];      // Two semaphores used to wake each of ipc tasks
  static SemaphoreHandle_t s_ipc_ack[portNUM_PROCESSORS];      // Semaphore used to acknowledge that task was woken up,
                                                               //   or function has finished running
  static volatile esp_ipc_func_t s_func[portNUM_PROCESSORS];   // Function which should be called by high priority task
  static void * volatile s_func_arg[portNUM_PROCESSORS];       // Argument to pass into s_func
  typedef enum {
      IPC_WAIT_FOR_START,
      IPC_WAIT_FOR_END
  } esp_ipc_wait_t;
  
  static volatile esp_ipc_wait_t s_ipc_wait[portNUM_PROCESSORS];// This variable tells high priority task when it should give
                                                               //   s_ipc_ack semaphore: before s_func is called, or
                                                               //   after it returns
  
  static void IRAM_ATTR ipc_task(void* arg)
  {
      const uint32_t cpuid = (uint32_t) arg;
      assert(cpuid == xPortGetCoreID());
      while (true) {
          // Wait for IPC to be initiated.
          // This will be indicated by giving the semaphore corresponding to
          // this CPU.
          if (xSemaphoreTake(s_ipc_sem[cpuid], portMAX_DELAY) != pdTRUE) {
              // TODO: when can this happen?
              abort();
          }
  
          esp_ipc_func_t func = s_func[cpuid];
          void* arg = s_func_arg[cpuid];
  
          if (s_ipc_wait[cpuid] == IPC_WAIT_FOR_START) {
              xSemaphoreGive(s_ipc_ack[cpuid]);
          }
          (*func)(arg);
          if (s_ipc_wait[cpuid] == IPC_WAIT_FOR_END) {
              xSemaphoreGive(s_ipc_ack[cpuid]);
          }
      }
      // TODO: currently this is unreachable code. Introduce esp_ipc_uninit
      // function which will signal to both tasks that they can shut down.
      // Not critical at this point, we don't have a use case for stopping
      // IPC yet.
      // Also need to delete the semaphore here.
      vTaskDelete(NULL);
  }
  
  /*
   * Initialize inter-processor call module. This function is called automatically
   * on CPU start and should not be called from the application.
   *
   * This function start two tasks, one on each CPU. These tasks are started
   * with high priority. These tasks are normally inactive, waiting until one of
   * the esp_ipc_call_* functions to be used. One of these tasks will be
   * woken up to execute the callback provided to esp_ipc_call_nonblocking or
   * esp_ipc_call_blocking.
   */
  static void esp_ipc_init(void) __attribute__((constructor));
  
  static void esp_ipc_init(void)
  {
      char task_name[15];
      for (int i = 0; i < portNUM_PROCESSORS; ++i) {
          snprintf(task_name, sizeof(task_name), "ipc%d", i);
          s_ipc_mutex[i] = xSemaphoreCreateMutex();
          s_ipc_ack[i] = xSemaphoreCreateBinary();
          s_ipc_sem[i] = xSemaphoreCreateBinary();
          portBASE_TYPE res = xTaskCreatePinnedToCore(ipc_task, task_name, CONFIG_ESP_IPC_TASK_STACK_SIZE, (void*) i,
                                                      configMAX_PRIORITIES - 1, &s_ipc_task_handle[i], i);
          assert(res == pdTRUE);
      }
  }
  
  static esp_err_t esp_ipc_call_and_wait(uint32_t cpu_id, esp_ipc_func_t func, void* arg, esp_ipc_wait_t wait_for)
  {
      if (cpu_id >= portNUM_PROCESSORS) {
          return ESP_ERR_INVALID_ARG;
      }
      if (xTaskGetSchedulerState() != taskSCHEDULER_RUNNING) {
          return ESP_ERR_INVALID_STATE;
      }
  
  #ifdef CONFIG_ESP_IPC_USES_CALLERS_PRIORITY
      TaskHandle_t task_handler = xTaskGetCurrentTaskHandle();
      UBaseType_t priority_of_current_task = uxTaskPriorityGet(task_handler);
      UBaseType_t priority_of_running_ipc_task = uxTaskPriorityGet(s_ipc_task_handle[cpu_id]);
      if (priority_of_running_ipc_task < priority_of_current_task) {
          vTaskPrioritySet(s_ipc_task_handle[cpu_id], priority_of_current_task);
      }
  
      xSemaphoreTake(s_ipc_mutex[cpu_id], portMAX_DELAY);
      vTaskPrioritySet(s_ipc_task_handle[cpu_id], priority_of_current_task);
  #else
      xSemaphoreTake(s_ipc_mutex[0], portMAX_DELAY);
  #endif
  
      s_func[cpu_id] = func;
      s_func_arg[cpu_id] = arg;
      s_ipc_wait[cpu_id] = wait_for;
      xSemaphoreGive(s_ipc_sem[cpu_id]);
      xSemaphoreTake(s_ipc_ack[cpu_id], portMAX_DELAY);
  #ifdef CONFIG_ESP_IPC_USES_CALLERS_PRIORITY
      xSemaphoreGive(s_ipc_mutex[cpu_id]);
  #else
      xSemaphoreGive(s_ipc_mutex[0]);
  #endif
      return ESP_OK;
  }
  
  esp_err_t esp_ipc_call(uint32_t cpu_id, esp_ipc_func_t func, void* arg)
  {
      return esp_ipc_call_and_wait(cpu_id, func, arg, IPC_WAIT_FOR_START);
  }
  
  esp_err_t esp_ipc_call_blocking(uint32_t cpu_id, esp_ipc_func_t func, void* arg)
  {
      return esp_ipc_call_and_wait(cpu_id, func, arg, IPC_WAIT_FOR_END);
  }
  