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

Greg KH
2025-06-06T10:40:36Z

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,

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