Re: [PATCH v9 1/4] serial: 8250: fix panic due to PSLVERR

[John Ogness <john.ogness@linutronix.de>]

On 2025-06-10, Yunhui Cui <cuiyunhui@bytedance.com> 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>
> Cc: stable@vger.kernel.org

Reviewed-by: John Ogness <john.ogness@linutronix.de>