linux-kernel - Re: [External] Re: [PATCH v5 1/4] serial: 8250: fix panic due to PSLVERR

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Message-ID: <CAEEQ3wkVubuObJ8AoMmRb++54JybnHk8jC8k72=Qyq6vQoDwHg@mail.gmail.com>
Date: Mon, 12 May 2025 11:06:19 +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 1/4] serial: 8250: fix panic due to PSLVERR

Hi Ilpo,

On Tue, May 6, 2025 at 8:35 PM Ilpo Järvinen
<ilpo.jarvinen@...ux.intel.com> wrote:
>
> On Tue, 6 May 2025, Yunhui Cui wrote:
>
> > When the PSLVERR_RESP_EN parameter is set to 1, the device generates
> > an error response if an attempt is made to read an empty RBR (Receive
> > Buffer Register) while the FIFO is enabled.
> >
> > In serial8250_do_startup(), calling serial_port_out(port, UART_LCR,
> > UART_LCR_WLEN8) triggers dw8250_check_lcr(), which invokes
> > dw8250_force_idle() and serial8250_clear_and_reinit_fifos(). The latter
> > function enables the FIFO via serial_out(p, UART_FCR, p->fcr).
> > Execution proceeds to the dont_test_tx_en label:
>
> Where is dont_test_tx_en label?

Oh, I'll rebase the latest code of linux-next.

>
> > ...
> > serial_port_in(port, UART_RX);
> > This satisfies the PSLVERR trigger condition.
> >
> > Because another CPU(e.g., using printk()) is accessing the UART (UART
>
> Because -> When ?
>
> missing space before (
>
> > is busy), the current CPU fails the check (value & ~UART_LCR_SPAR) ==
> > (lcr & ~UART_LCR_SPAR)
>
> Add:
>
> ... in dw8250_check_lcr()

Okay, it will be updated in the next version.

>
> >, causing it to enter dw8250_force_idle().
> >
> > Put serial_port_out(port, UART_LCR, UART_LCR_WLEN8) under the port->lock
> > to fix this issue.
> >
> > Panic backtrace:
> > [    0.442336] Oops - unknown exception [#1]
> > [    0.442343] epc : dw8250_serial_in32+0x1e/0x4a
> > [    0.442351]  ra : serial8250_do_startup+0x2c8/0x88e
> > ...
> > [    0.442416] console_on_rootfs+0x26/0x70
> >
> > Fixes: c49436b657d0 ("serial: 8250_dw: Improve unwritable LCR workaround")
> > Link: https://lore.kernel.org/all/84cydt5peu.fsf@jogness.linutronix.de/T/
> > Signed-off-by: Yunhui Cui <cuiyunhui@...edance.com>
> > ---
> >  drivers/tty/serial/8250/8250_port.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index 3f256e96c722..a913135d5217 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -2380,9 +2380,10 @@ int serial8250_do_startup(struct uart_port *port)
> >       /*
> >        * Now, initialize the UART
> >        */
> > -     serial_port_out(port, UART_LCR, UART_LCR_WLEN8);
> >
> >       uart_port_lock_irqsave(port, &flags);
> > +     serial_port_out(port, UART_LCR, UART_LCR_WLEN8);
> > +
> >       if (up->port.flags & UPF_FOURPORT) {
> >               if (!up->port.irq)
> >                       up->port.mctrl |= TIOCM_OUT1;
> >
>
> --
>  i.
>

Thanks,
Yunhui