linux-kernel - Re: [External] Re: [PATCH v5 4/4] serial: 8250_dw: fix PSLVERR on RX

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Message-ID: <CAEEQ3wmc_QkhwvWP_yR8j5JbxeOLUDrAAot5VyL7+jjDydbD9A@mail.gmail.com>
Date: Thu, 8 May 2025 19:51:19 +0800
From: yunhui cui <cuiyunhui@...edance.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: arnd@...db.de, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	benjamin.larsson@...exis.eu, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	heikki.krogerus@...ux.intel.com, Jiri Slaby <jirislaby@...nel.org>, 
	jkeeping@...usicbrands.com, john.ogness@...utronix.de, 
	LKML <linux-kernel@...r.kernel.org>, linux-serial <linux-serial@...r.kernel.org>, 
	markus.mayer@...aro.org, matt.porter@...aro.org, namcao@...utronix.de, 
	paulmck@...nel.org, pmladek@...e.com, schnelle@...ux.ibm.com, 
	sunilvl@...tanamicro.com, tim.kryger@...aro.org
Subject: Re: [External] Re: [PATCH v5 4/4] serial: 8250_dw: fix PSLVERR on RX_TIMEOUT

 Hi Ilpo,

On Tue, May 6, 2025 at 8:25 PM Ilpo Järvinen
<ilpo.jarvinen@...ux.intel.com> wrote:
>
> I think shortlog should include mention to "dummy read" to make it a
> bit more specific.
>
> On Tue, 6 May 2025, Yunhui Cui wrote:
>
> > In the case of RX_TIMEOUT, to avoid PSLVERR, disable the FIFO
>
> As with patch 2, please don't assume it is know to the reader how PSLVERR
> is triggered.
>
> > before reading UART_RX when UART_LSR_DR is not set.
>
> IMO, it would be better to explain the problem better first, something
> along these lines:
>
> DW UART can fire RX_TIMEOUT interrupt without data and remain in that
> state forever. dw8250_handle_irq() detects this condition by checking if
> UART_LSR_DR is not asserted when RX_TIMEOUT occurred, and if detected,
> performs a dummy read to kick DW UART out of this state.
>
> Performing dummy read from UART_RX is problematic because with ... it lead
> to ...
>
> And only then explain the solution (disable FIFO for while performing of
> the dummy read).

Okay, thank you. It will be updated in the next version.

>
> >
> > Fixes: 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt")
> > Signed-off-by: Yunhui Cui <cuiyunhui@...edance.com>
> > ---
> >  drivers/tty/serial/8250/8250_dw.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> > index f41c4a9ed58b..ffa8cb10b39c 100644
> > --- a/drivers/tty/serial/8250/8250_dw.c
> > +++ b/drivers/tty/serial/8250/8250_dw.c
> > @@ -288,9 +288,17 @@ static int dw8250_handle_irq(struct uart_port *p)
> >               uart_port_lock_irqsave(p, &flags);
> >               status = serial_lsr_in(up);
> >
> > -             if (!(status & (UART_LSR_DR | UART_LSR_BI)))
> > +             if (!(status & (UART_LSR_DR | UART_LSR_BI))) {
> > +                     /* To avoid PSLVERR, disable the FIFO first. */
> > +                     if (up->fcr & UART_FCR_ENABLE_FIFO)
> > +                             serial_out(up, UART_FCR, 0);
> > +
> >                       (void) p->serial_in(p, UART_RX);
> >
> > +                     if (up->fcr & UART_FCR_ENABLE_FIFO)
> > +                             serial_out(up, UART_FCR, up->fcr);
> > +             }
> > +
> >               uart_port_unlock_irqrestore(p, flags);
> >       }
> >
> >
>
> --
>  i.
>

Thanks,
Yunhui