Why does HAL_UART_Receive_IT return HAL_OK before the interrupt fires?

Question

STM

STM32F103. HAL. Interrupts are configured.
The problem is that the first time this function is called, it returns HAL_OK.
And that's basically what she should do.

Implementation of HAL_UART_Receive_IT (file stm32f1xx_hal_uart.с)

/**
  * @brief  Receives an amount of data in non blocking mode. 
  * @param  huart: pointer to a UART_HandleTypeDef structure that contains
  *                the configuration information for the specified UART module.
  * @param  pData: Pointer to data buffer
  * @param  Size: Amount of data to be received
  * @retval HAL status
  */
HAL_StatusTypeDef HAL_UART_Receive_IT(UART_HandleTypeDef *huart, uint8_t *pData, uint16_t Size)
{
  /* Check that a Rx process is not already ongoing */
  if(huart->RxState == HAL_UART_STATE_READY)
  {
    if((pData == NULL) || (Size == 0U))
    {
      return HAL_ERROR;
    }

    /* Process Locked */
    __HAL_LOCK(huart);

    huart->pRxBuffPtr = pData;
    huart->RxXferSize = Size;
    huart->RxXferCount = Size;

    huart->ErrorCode = HAL_UART_ERROR_NONE;
    huart->RxState = HAL_UART_STATE_BUSY_RX;
    
    /* Process Unlocked */
    __HAL_UNLOCK(huart);

    /* Enable the UART Parity Error Interrupt */
    __HAL_UART_ENABLE_IT(huart, UART_IT_PE);

    /* Enable the UART Error Interrupt: (Frame error, noise error, overrun error) */
    __HAL_UART_ENABLE_IT(huart, UART_IT_ERR);

    /* Enable the UART Data Register not empty Interrupt */
    __HAL_UART_ENABLE_IT(huart, UART_IT_RXNE);

    return HAL_OK;
  }
  else
  {
    return HAL_BUSY;
  }
}

In this case, huart.RxState is set to READY only in the interrupt handler.

Function called in the handler that works with RxState

/**
  * @brief  Receives an amount of data in non blocking mode 
  * @param  huart: pointer to a UART_HandleTypeDef structure that contains
  *                the configuration information for the specified UART module.
  * @retval HAL status
  */
static HAL_StatusTypeDef UART_Receive_IT(UART_HandleTypeDef *huart)
{
  uint16_t* tmp;
  
  /* Check that a Rx process is ongoing */
  if(huart->RxState == HAL_UART_STATE_BUSY_RX) 
  {
    if(huart->Init.WordLength == UART_WORDLENGTH_9B)
    {
      tmp = (uint16_t*) huart->pRxBuffPtr;
      if(huart->Init.Parity == UART_PARITY_NONE)
      {
        *tmp = (uint16_t)(huart->Instance->DR & (uint16_t)0x01FF);
        huart->pRxBuffPtr += 2U;
      }
      else
      {
        *tmp = (uint16_t)(huart->Instance->DR & (uint16_t)0x00FF);
        huart->pRxBuffPtr += 1U;
      }
    }
    else
    {
      if(huart->Init.Parity == UART_PARITY_NONE)
      {
        *huart->pRxBuffPtr++ = (uint8_t)(huart->Instance->DR & (uint8_t)0x00FF);
      }
      else
      {
        *huart->pRxBuffPtr++ = (uint8_t)(huart->Instance->DR & (uint8_t)0x007F);
      }
    }

    if(--huart->RxXferCount == 0U)
    {
      /* Disable the IRDA Data Register not empty Interrupt */
      __HAL_UART_DISABLE_IT(huart, UART_IT_RXNE);

      /* Disable the UART Parity Error Interrupt */
      __HAL_UART_DISABLE_IT(huart, UART_IT_PE);
        /* Disable the UART Error Interrupt: (Frame error, noise error, overrun error) */
        __HAL_UART_DISABLE_IT(huart, UART_IT_ERR);

      /* Rx process is completed, restore huart->RxState to Ready */
      huart->RxState = HAL_UART_STATE_READY;

      HAL_UART_RxCpltCallback(huart);

      return HAL_OK;
    }
    return HAL_OK;
  }
  else
  {
    return HAL_BUSY;
  }
}

So the question is: How to behave? To cause in idle this function at start of the program? Or do I not understand something?
I forced RxState to BUSY, but then the program does not even enter the interrupt.

Answer 1

Everything is logical. The function set up an interrupt transfer, put the data in the data register, and exited with a OK return. Did she have any problems with that? If the function were to wait for the interrupt to fire, then it would block the robot of the entire program for this period, and work in the same way as HAL_UART_Receive