linux-kernel - Re: [PATCH v2 2/2] serial: sh-sci: Fix deadlock during RSCI FIFO overrun error

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Message-ID: <CAMuHMdUXimjkB_Cz=ac43RhOGF2Lf1ypLK1sLwk=ONXf7kdhQQ@mail.gmail.com>
Date: Thu, 6 Nov 2025 17:51:18 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Biju <biju.das.au@...il.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	Biju Das <biju.das.jz@...renesas.com>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, 
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>, 
	Claudiu Beznea <claudiu.beznea.uj@...renesas.com>, Nam Cao <namcao@...utronix.de>, 
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org, 
	linux-renesas-soc@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH v2 2/2] serial: sh-sci: Fix deadlock during RSCI FIFO
 overrun error

Hi Biju,

On Wed, 29 Oct 2025 at 09:21, Biju <biju.das.au@...il.com> wrote:
> From: Biju Das <biju.das.jz@...renesas.com>
>
> On RSCI IP, a deadlock occurs during a FIFO overrun error, as it uses a
> different register to clear the FIFO overrun error status.
>
> Cc: stable@...nel.org
> Fixes: 0666e3fe95ab ("serial: sh-sci: Add support for RZ/T2H SCI")
> Signed-off-by: Biju Das <biju.das.jz@...renesas.com>

Thanks for your patch!

> --- a/drivers/tty/serial/rsci.c
> +++ b/drivers/tty/serial/rsci.c
> @@ -414,6 +414,7 @@ static const struct sci_port_params_bits rsci_port_param_bits = {
>         .rxtx_enable = CCR0_RE | CCR0_TE,
>         .te_clear = CCR0_TE | CCR0_TEIE,
>         .poll_sent_bits = CSR_TDRE | CSR_TEND,
> +       .overrun_clr = CFCLR_ORERC,
>  };
>
>  static const struct sci_port_params rsci_port_params = {
> diff --git a/drivers/tty/serial/sh-sci-common.h b/drivers/tty/serial/sh-sci-common.h
> index e3c028df14f1..bcdb41ddc15d 100644
> --- a/drivers/tty/serial/sh-sci-common.h
> +++ b/drivers/tty/serial/sh-sci-common.h
> @@ -51,6 +51,7 @@ struct sci_port_params_bits {
>         unsigned int rxtx_enable;
>         unsigned int te_clear;
>         unsigned int poll_sent_bits;
> +       unsigned int overrun_clr;

I don't really see a need to add this field, as there are two alternatives...

>  };
>
>  struct sci_common_regs {
> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> index 62bb62b82cbe..b33894d0273b 100644
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -1024,8 +1024,12 @@ static int sci_handle_fifo_overrun(struct uart_port *port)
>
>         status = s->ops->read_reg(port, s->params->overrun_reg);
>         if (status & s->params->overrun_mask) {
> -               status &= ~s->params->overrun_mask;
> -               s->ops->write_reg(port, s->params->overrun_reg, status);
> +               if (s->type == SCI_PORT_RSCI) {
> +                       s->ops->clear_SCxSR(port, s->params->param_bits->overrun_clr);

1. You can just use CFCLR_ORERC directly here,
2. All of the CFCLR_*C clearing bits match the corresponding CSR_*
   status bits, so you could do without the former, and thus use
   params->overrun_mask here, too.

> +               } else {
> +                       status &= ~s->params->overrun_mask;
> +                       s->ops->write_reg(port, s->params->overrun_reg, status);
> +               }
>
>                 port->icount.overrun++;
>

BTW, how is this related to "[PATCH] tty: serial: sh-sci: fix RSCI
FIFO overrun handling" [1], which is already applied (and which I
still don't fully understand the big picture behind. I'll reply there)?

[1] https://lore.kernel.org/20250923154707.1089900-1-cosmin-gabriel.tanislav.xa@renesas.com
    commit ef8fef45c74b5a00 ("tty: serial: sh-sci: fix RSCI FIFO
    overrun handling") in v6.18-rc3.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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