Re: [PATCH 3/4] serial: sh-sci: Skip TDxE status clear for PORT_SCI

[Geert Uytterhoeven]

Hi Biju,

On Mon, 19 Oct 2026 at 19:01, Biju <biju.das.au@xxxxxxxxx> wrote:
> From: Au Dang <au.dang.jx@xxxxxxxxxxx>
>
> On PORT_SCI, the TDxE (Transmit Data Empty) flag in SCxSR is cleared
> automatically by hardware when data is written to TDR. Clearing it
> again from software via clear_SCxSR() generates a spurious TXI
> interrupt, which causes the last character to be transmitted twice.
>
> Skip the clear_SCxSR() call for PORT_SCI in sci_transmit_chars() and
> sci_poll_put_char(), as the hardware already handles it after each TDR
> write.
>
> Signed-off-by: Au Dang <au.dang.jx@xxxxxxxxxxx>
> Signed-off-by: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>

Thanks for your patch!

> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -913,7 +913,8 @@ static void sci_poll_put_char(struct uart_port *port, unsigned char c)
>         } while (!(status & SCxSR_TDxE(port)));
>
>         sci_serial_out(port, SCxTDR, c);
> -       s->ops->clear_SCxSR(port, SCxSR_TDxE_CLEAR(port) & ~SCxSR_TEND(port));
> +       if (s->type != PORT_SCI)
> +               s->ops->clear_SCxSR(port, SCxSR_TDxE_CLEAR(port) & ~SCxSR_TEND(port));

Is this issue present on both classic SCI and RZ RSCI?
s->ops->clear_SCxSR() points to either sci_clear_SCxSR() or
rsci_clear_CFC(). The former already has an internal check for
PORT_SCI, the latter has not. So probably it should be handled there.

>  }
>  #endif /* CONFIG_CONSOLE_POLL || CONFIG_SERIAL_SH_SCI_CONSOLE ||
>           CONFIG_SERIAL_SH_SCI_EARLYCON */
> @@ -1049,7 +1050,8 @@ static void sci_transmit_chars(struct uart_port *port)
>                 port->icount.tx++;
>         } while (--count > 0);
>
> -       s->ops->clear_SCxSR(port, SCxSR_TDxE_CLEAR(port));
> +       if (s->type != PORT_SCI)
> +               s->ops->clear_SCxSR(port, SCxSR_TDxE_CLEAR(port));
>
>         if (kfifo_len(&tport->xmit_fifo) < WAKEUP_CHARS)
>                 uart_write_wakeup(port);

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds