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

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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?

> ...
> 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()

>, 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.