Arduino HardwareSerial::flush() should not flush input !

[bitli]

Hardware:

ESP32 using PlatformIO Espressif1.9 and Arduino framework

Description:

I have to toggle GPIO around UART rx and tx activities, porting code from AVR Arduino. This does not work due to a bug in the HardwareSerial.cpp of the ESP32 Arduino framework

On Arduino the Serial.flush() flushes ONLY the TX buffer (to flush the RX, you just read it to end). This how it is documented and implemented (this is also true of SoftSerial).

Code:

On ESP32, the code of HardwareSerial.cpp is

void HardwareSerial::flush()
{
    uartFlush(_uart);
}

and in esp32-hal-uart.c, the code of uartFlush() flushes the output AND ERRONEOUSLY THE INPUT. This cause data loss on the input when we flush output.

void uartFlush(uart_t* uart)
{
    if(uart == NULL) {
        return;
    }

    UART_MUTEX_LOCK();
    while(uart->dev->status.txfifo_cnt || uart->dev->status.st_utx_out);

    //Due to hardware issue, we can not use fifo_rst to reset uart fifo.
    //See description about UART_TXFIFO_RST and UART_RXFIFO_RST in <<esp32_technical_reference_manual>> v2.6 or later.

    // we read the data out and make `fifo_len == 0 && rd_addr == wr_addr`.
    while(uart->dev->status.rxfifo_cnt != 0 || (uart->dev->mem_rx_status.wr_addr != uart->dev->mem_rx_status.rd_addr)) {
        READ_PERI_REG(UART_FIFO_REG(uart->num));
    }

    UART_MUTEX_UNLOCK();
}

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