Re: [PATCH tty-linus] serial: Reduce spinlocked portion of uart_rs485_config()

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

On 2023-09-21, Lukas Wunner <lukas@wunner.de> wrote:
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 7bdc21d5e13b..ca26a8aef2cb 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1404,12 +1404,18 @@ static void uart_set_rs485_termination(struct uart_port *port,
>  static int uart_rs485_config(struct uart_port *port)
>  {
>  	struct serial_rs485 *rs485 = &port->rs485;
> +	unsigned long flags;
>  	int ret;
>  
> +	if (!(rs485->flags & SER_RS485_ENABLED))
> +		return 0;
> +
>  	uart_sanitize_serial_rs485(port, rs485);
>  	uart_set_rs485_termination(port, rs485);
>  
> +	spin_lock_irqsave(&port->lock, flags);
>  	ret = port->rs485_config(port, NULL, rs485);
> +	spin_unlock_irqrestore(&port->lock, flags);
>  	if (ret)
>  		memset(rs485, 0, sizeof(*rs485));

Is there some kind of guarantee that 2 CPUs cannot go into
uart_rs485_config() simultaneously? Otherwise it seems dangerous to be
using and clearing @port->rs485 outside of the spin_lock.

John Ogness