Re: [PATCH 2/8] serial: core, 8250: set RS485 termination gpio in serial core

[Lukas Wunner]

On Wed, Jun 22, 2022 at 05:46:53PM +0200, Lino Sanfilippo wrote:
> From: Lino Sanfilippo <l.sanfilippo@xxxxxxxxxx>
> 
> In serial8250_em485_config() the termination GPIO is set with the uart_port
> spinlock held. This is an issue if setting the GPIO line can sleep (e.g.
> since the concerning GPIO expander is connected via SPI or I2C).
> 
> Fix this by setting the termination line outside of the uart_port spinlock
> in the serial core.
[...]
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1400,6 +1411,7 @@ static int uart_set_rs485_config(struct uart_port *port,
>  	if (ret)
>  		return ret;
>  	uart_sanitize_serial_rs485(port, &rs485);
> +	uart_set_rs485_termination(port, &rs485);
>  
>  	spin_lock_irqsave(&port->lock, flags);
>  	ret = port->rs485_config(port, &rs485);

That's one way to solve the issue.  Another would be to push
acquisition of the port spinlock down into drivers.

I think in most drivers we don't need to take the port spinlock at all
or only for a few specific register accesses.  So taking the lock here
in the midlayer is likely unwarranted.  However, changing that requires
going through every single driver's ->rs485_config() callback and
checking whether it needs the lock or not.  That's painful, but
unavoidable in the long run.  This patch just kicks the can down the road...

Thanks,

Lukas