Re: [PATCH v2] serial: 8250: Clear CON_PRINTBUFFER on port re-registration

[Fushuai Wang <fushuai.wang@linux.dev>]

>> From: Fushuai Wang <wangfushuai@baidu.com>
>> 
>> When two PnP devices map to the same physical port, the serial8250 driver
>> removes and re-registers the console structure for the same port.
>> 
>> During re-registration, the console structure still has CON_PRINTBUFFER set
>> from the initial registration, which causes console_init_seq() to set
>> console->seq to syslog_seq. This results in re-printing the entire
>> system log buffer, which may lead to RCU stall on slow serial consoles.
>> 
>> Clear CON_PRINTBUFFER when re-registering a port to prevent duplicate
>> log printing.
>> 
>> Fixes: 835d844d1a28 ("8250_pnp: do pnp probe before legacy probe")
>> Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
>> ---
>> V1->V2: Add Fixes tag
>> previous discussion: https://lore.kernel.org/all/20260416092917.27301-1-fushuai.wang@linux.dev/T/#u
>> 
>> Please ignore previous email if you received it before. There is something wrong with my email client.
>> 
>>  drivers/tty/serial/8250/8250_core.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
>> index a428e88938eb..01b14392d9f7 100644
>> --- a/drivers/tty/serial/8250/8250_core.c
>> +++ b/drivers/tty/serial/8250/8250_core.c
>> @@ -694,6 +694,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
>>  {
>>  	struct uart_8250_port *uart;
>>  	int ret;
>> +	bool was_removed = false;
>>  
>>  	if (up->port.uartclk == 0)
>>  		return -EINVAL;
>> @@ -716,8 +717,10 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
>>  	if (uart->port.type == PORT_8250_CIR)
>>  		return -ENODEV;
>>  
>> -	if (uart->port.dev)
>> +	if (uart->port.dev) {
>>  		uart_remove_one_port(&serial8250_reg, &uart->port);
>> +		was_removed = true;
>> +	}
>>  
>>  	uart->port.ctrl_id	= up->port.ctrl_id;
>>  	uart->port.port_id	= up->port.port_id;
>> @@ -819,6 +822,10 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
>>  					&uart->capabilities);
>>  
>>  		serial8250_apply_quirks(uart);
>> +
>> +		if (was_removed && uart_console(&uart->port))
>> +			uart->port.cons->flags &= ~CON_PRINTBUFFER;
> 
> Why not set the flag up above when you remove the port?  Why down here?
> 
> thanks,
> 
> greg k-h

Hi, Greg

I just felt it's cleaner to clear it only when re-registration happens.
Do you think there is any problem with doing it right after the removal?

-- 
Regards,
WANG