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

yunhui cui
2025-05-12T03:06:30Z

Hi Ilpo,

On Tue, May 6, 2025 at 8:35 PM Ilpo Järvinen
<ilpo.jarvinen@linux.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@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 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