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

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Date:   Tue, 28 Jun 2022 11:31:50 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
cc:     Lukas Wunner <lukas@...ner.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        vz@...ia.com, linux-arm-kernel@...ts.infradead.org,
        devicetree@...r.kernel.org,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, p.rosenberger@...bus.com,
        Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH 2/8] serial: core, 8250: set RS485 termination gpio in
 serial core

On Sun, 26 Jun 2022, Lino Sanfilippo wrote:

> On 25.06.22 at 21:58, Lukas Wunner wrote:
> > On Wed, Jun 22, 2022 at 05:46:53PM +0200, Lino Sanfilippo wrote:
> >> From: Lino Sanfilippo <l.sanfilippo@...bus.com>
> >>
> >> 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.
> 
> As a first step its sufficient to take the lock in each drivers rs485_config()
> function and remove it from uart_set_rs485_config(). Then after time sort out
> the drivers that dont require the lock and remove it from their function.
> 
> However the point of this patch was also to generalize the handling of the
> termination GPIO, so I would still see this placed in uart_set_rs485_config().

Additional thing to consider is that core currently handles also the 
port->rs485 assignment under spinlock when ->rs485_config() was 
successful. TIOCSRS485 ioctl calls are synchronized by other primitives 
wrt. each other (port mutex and now also termios_rwsem) but drivers 
probably would like to see consistent rs485 which can only be realized by 
holding port->lock.


-- 
 i.