On Thu, Sep 21, 2023 at 05:24:04PM +0206, John Ogness wrote:
> On 2023-09-21, Lukas Wunner <lukas@xxxxxxxxx> 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.
uart_rs485_config() is only called on probe and resume.
I don't quite see how 2 CPUs can probe or resume the same UART
simultaneously?
Granted, it *is* possible to access or modify port->rs485 from
user space through uart_{get,set}_rs485_config() in parallel to
uart_resume_port(), but as Ilpo has correctly stated, that's
serialized using port->mutex.
To provide some background information:
The majority of products *hardwire* the UART port to the RS485
transceiver and thus the UART should be switched to RS485 mode
on probe (through a "linux,rs485-enabled-at-boot-time" devicetree
property). User space generally has no business fiddling with
RS485 properties in those cases, save for enabling/disabling
termination etc.
A minority of products has serial ports which can be switched
between RS232 and RS485 mode. For those products, user space
may indeed have the need to enable/disable RS485 at runtime.
The Siemens IOT2040 is a case in point:
https://assets.new.siemens.com/siemens/assets/api/uuid:7b2c5580-7436-4081-9ae6-7c4eba29ddd1/version:1562856379/simaticiot2040.pdf
Thanks,
Lukas
