Apache NuttX RTOS 16550 UART Driver: Print Baud Divisor. See https://lupyuen.github.io/articles/release#appendix-uart-clock-for-jh7110

uart_16550.c

// Output for Star64 JH7110: dlm=0x0, dll=0xd
// Output for Milk-V Duo S / Sophgo SG2000: dlm=0x0, dll=0xe

/****************************************************************************
 * Name: u16550_setup
 *
 * Description:
 *   Configure the UART baud, bits, parity, fifos, etc. This
 *   method is called the first time that the serial port is
 *   opened.
 *
 ****************************************************************************/

static int u16550_setup(FAR struct uart_dev_s *dev)
{
#ifndef CONFIG_16550_SUPRESS_CONFIG
  FAR struct u16550_s *priv = (FAR struct u16550_s *)dev->priv;
  uint16_t div;
  uint32_t lcr;
#if defined(CONFIG_SERIAL_IFLOWCONTROL) || defined(CONFIG_SERIAL_OFLOWCONTROL)
  uint32_t mcr;
#endif

  /* Clear fifos */

  u16550_serialout(priv, UART_FCR_OFFSET,
                   (UART_FCR_RXRST | UART_FCR_TXRST));

  /* Set trigger */

  u16550_serialout(priv, UART_FCR_OFFSET,
                   (UART_FCR_FIFOEN | UART_FCR_RXTRIGGER_8));

  /* Set up the IER */

  priv->ier = u16550_serialin(priv, UART_IER_OFFSET);

  /* Set up the LCR */

  lcr = 0;
  switch (priv->bits)
    {
      case 5 :
        lcr |= UART_LCR_WLS_5BIT;
        break;

      case 6 :
        lcr |= UART_LCR_WLS_6BIT;
        break;

      case 7 :
        lcr |= UART_LCR_WLS_7BIT;
        break;

      default:
      case 8 :
        lcr |= UART_LCR_WLS_8BIT;
        break;
    }

  if (priv->stopbits2)
    {
      lcr |= UART_LCR_STB;
    }

  if (priv->parity == 1)
    {
      lcr |= UART_LCR_PEN;
    }
  else if (priv->parity == 2)
    {
      lcr |= (UART_LCR_PEN | UART_LCR_EPS);
    }

#ifdef CONFIG_16550_WAIT_LCR
  /* Wait till UART is not busy before setting LCR */

  if (u16550_wait(priv) < 0)
    {
      _err("UART wait failed\n");
      return ERROR;
    }
#endif /* CONFIG_16550_WAIT_LCR */

  /* Enter DLAB=1 */

  u16550_serialout(priv, UART_LCR_OFFSET, (lcr | UART_LCR_DLAB));

uint32_t dlm = u16550_serialin(priv, UART_DLM_OFFSET);////
uint32_t dll = u16550_serialin(priv, UART_DLL_OFFSET);////
#ifdef TODO ////
  /* Set the BAUD divisor */

  div = u16550_divisor(priv);
  u16550_serialout(priv, UART_DLM_OFFSET, div >> 8);
  u16550_serialout(priv, UART_DLL_OFFSET, div & 0xff);
#endif //// TODO

#ifdef CONFIG_16550_WAIT_LCR
  /* Wait till UART is not busy before setting LCR */

  if (u16550_wait(priv) < 0)
    {
      _err("UART wait failed\n");
      return ERROR;
    }
#endif /* CONFIG_16550_WAIT_LCR */

  /* Clear DLAB */

  u16550_serialout(priv, UART_LCR_OFFSET, lcr);

  /* Configure the FIFOs */

  u16550_serialout(priv, UART_FCR_OFFSET,
                   (UART_FCR_RXTRIGGER_8 | UART_FCR_TXRST | UART_FCR_RXRST |
                    UART_FCR_FIFOEN));

  /* Set up the auto flow control */

#if defined(CONFIG_SERIAL_IFLOWCONTROL) || defined(CONFIG_SERIAL_OFLOWCONTROL)
  mcr = u16550_serialin(priv, UART_MCR_OFFSET);
  if (priv->flow)
    {
      mcr |= UART_MCR_AFCE;
    }
  else
    {
      mcr &= ~UART_MCR_AFCE;
    }

  mcr |= UART_MCR_RTS;

  u16550_serialout(priv, UART_MCR_OFFSET, mcr);
#endif /* defined(CONFIG_SERIAL_IFLOWCONTROL) || defined(CONFIG_SERIAL_OFLOWCONTROL) */

  /* Reconfigure DMA Rx timeout value */

#ifdef HAVE_16550_UART_DMA
  u16550_dmarxconfig(dev);
#endif

#endif
_info("dlm=0x%x, dll=0x%x\n", dlm, dll);////
  return OK;
}