Re: [PATCH 4/4] serial: 8250: Add Airoha SoC UART and HSUART support

[Benjamin Larsson]

On 10/07/2026 09:13, Jiri Slaby wrote:
> > +static void airoha_set_termios(struct uart_port *port, struct 
> > ktermios *termios,
> > +                            const struct ktermios *old)
> > +{
> > +     const struct airoha_8250_clk_div_info *clk_div_info;
> > +     struct uart_8250_port *up = up_to_u8250p(port);
> > +     unsigned int xyd_x, nom, denom;
> > +     unsigned int baud;
> > +     int i;
> > +
> > +     serial8250_do_set_termios(port, termios, old);
> > +
> > +     baud = serial8250_get_baud_rate(port, termios, old);
> > +
> > +     /* Set DLAB to access the baud rate divider registers (BRDH, 
> > BRDL) */
> > +     serial_port_out(port, UART_LCR, up->lcr | UART_LCR_DLAB);
> > +
> > +     /* Set baud rate calculation defaults (BRDIV ([BRDH,BRDL]) to 1) */
> > +     serial_port_out(port, UART_AIROHA_BRDL, UART_BRDL_20M);
> > +     serial_port_out(port, UART_AIROHA_BRDH, UART_BRDH_20M);
> > +
> > +     /*
> > +      * Calculate XYD_x and XINCLKDR register by searching
> > +      * through a table of crystal_clock divisors.
> > +      */
> > +     for (i = 0 ; i < ARRAY_SIZE(airoha_clk_div_info) ; i++) {
> > +             clk_div_info = &airoha_clk_div_info[i];
> > +
> > +             denom = (XINDIV_CLOCK / 40) / clk_div_info->div;
> > +             nom = baud * (XYD_Y / 40);
>
> Are these "/ 40" to avoid overflow? Add a comment.

Yes, this is to keep it in 32bits.

> > +             xyd_x = ((nom / denom) << 4);
>
> * don't you want to round to closest instead of down?
> * I don't understand the purpose of the shift though.

IIRC this is the vendor calculation logic scaled to fit 32bits. With 
this rounding/calculation we hit the baud rate exactly for the baud 
rates that people actually use.

> > +             /* For the HSUART xyd_x needs to be scaled by a factor 
> > of 2 */
> > +             if (port->type == UART_PORT_AIROHA_HS)
> > +                     xyd_x = xyd_x >> 1;
>
> Do not use shifts for div/mul.
>
> > +             if (xyd_x < XYD_Y)
> > +                     break;
> > +     }
> > +
> > +     serial_port_out(port, UART_AIROHA_XINCLKDR, clk_div_info->mask);
> > +     serial_port_out(port, UART_AIROHA_XYD, (xyd_x << 16) | XYD_Y);
> > +
> > +     /* unset DLAB */
> > +     serial_port_out(port, UART_LCR, up->lcr);
> > +}
>
> thanks,
> --
> js
> suse labs

MvH
Benjamin Larsson