Re: [External] Re: [PATCH v7 1/4] serial: 8250: fix panic due to PSLVERR

yunhui cui
2025-06-09T07:02:59Z

Hi,

On Fri, Jun 6, 2025 at 6:40 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Wed, May 28, 2025 at 02:26:06PM +0800, 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 serial_port_in(port, UART_RX).
> > This satisfies the PSLVERR trigger condition.
> >
> > When another CPU (e.g., using printk()) is accessing the UART (UART
> > is busy), the current CPU fails the check (value & ~UART_LCR_SPAR) ==
> > (lcr & ~UART_LCR_SPAR) in dw8250_check_lcr(), 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@bytedance.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 6d7b8c4667c9c..07fe818dffa34 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -2376,9 +2376,10 @@ 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;
> > --
> > 2.39.5
> >
> >
>
> Hi,
>
> This is the friendly patch-bot of Greg Kroah-Hartman. You have sent him
> a patch that has triggered this response. He used to manually respond
> to these common problems, but in order to save his sanity (he kept
> writing the same thing over and over, yet to different people), I was
> created. Hopefully you will not take offence and will fix the problem
> in your patch and resubmit it so that it can be accepted into the Linux
> kernel tree.
>
> You are receiving this message because of the following common error(s)
> as indicated below:
>
> - You have marked a patch with a "Fixes:" tag for a commit that is in an
> older released kernel, yet you do not have a cc: stable line in the
> signed-off-by area at all, which means that the patch will not be
> applied to any older kernel releases. To properly fix this, please
> follow the documented rules in the
> Documentation/process/stable-kernel-rules.rst file for how to resolve
> this.

Okay, update under v8.

>
> If you wish to discuss this problem further, or you have questions about
> how to resolve this issue, please feel free to respond to this email and
> Greg will reply once he has dug out from the pending patches received
> from other developers.
>
> thanks,
>
> greg k-h's patch email bot

Thanks,
Yunhui