/** @brief  Clears the specified UART pending flag.
  * @param  __HANDLE__ specifies the UART Handle.
  *         UART Handle selects the USARTx or UARTy peripheral
  *         (USART,UART availability and x,y values depending on device).
  * @param  __FLAG__ specifies the flag to check.
  *          This parameter can be any combination of the following values:
  *            @arg UART_FLAG_CTS:  CTS Change flag (not available for UART4 and UART5).
  *            @arg UART_FLAG_LBD:  LIN Break detection flag.
  *            @arg UART_FLAG_TC:   Transmission Complete flag.
  *            @arg UART_FLAG_RXNE: Receive data register not empty flag.
  *
  * @note   PE (Parity error), FE (Framing error), NE (Noise error), ORE (Overrun
  *          error) and IDLE (Idle line detected) flags are cleared by software
  *          sequence: a read operation to USART_SR register followed by a read
  *          operation to USART_DR register.
  * @note   RXNE flag can be also cleared by a read to the USART_DR register.
  * @note   TC flag can be also cleared by software sequence: a read operation to
  *          USART_SR register followed by a write operation to USART_DR register.
  * @note   TXE flag is cleared only by a write to the USART_DR register.
  *
  * @retval None
  */
#define __HAL_UART_CLEAR_FLAG(__HANDLE__, __FLAG__) ((__HANDLE__)->Instance->SR = ~(__FLAG__))

/** @brief  Clears the UART PE pending flag.
  * @param  __HANDLE__ specifies the UART Handle.
  *         UART Handle selects the USARTx or UARTy peripheral
  *         (USART,UART availability and x,y values depending on device).
  * @retval None
  */
#define __HAL_UART_CLEAR_PEFLAG(__HANDLE__)     \
  do{                                           \
    __IO uint32_t tmpreg = 0x00U;               \
    tmpreg = (__HANDLE__)->Instance->SR;        \
    tmpreg = (__HANDLE__)->Instance->DR;        \
    UNUSED(tmpreg);                             \
  } while(0U)

/** @brief  Clears the UART IDLE pending flag.
  * @param  __HANDLE__ specifies the UART Handle.
  *         UART Handle selects the USARTx or UARTy peripheral
  *         (USART,UART availability and x,y values depending on device).
  * @retval None
  */
#define __HAL_UART_CLEAR_IDLEFLAG(__HANDLE__) __HAL_UART_CLEAR_PEFLAG(__HANDLE__)

以上是两个宏的实现。__HAL_UART_CLEAR_FLAG如上所述，不同于读改写的清零方式，这是直接清零。__HAL_UART_CLEAR_IDLEFLAG是读改写，要先读取SR，再读取DR才能清除对应的标志位。