Re: [PATCH] serial: Deassert RS485 Transmit Enable on probe in driver-specific way

Re: [PATCH] serial: Deassert RS485 Transmit Enable on probe in driver-specific way

[Matthias Schiffer]

On Fri, 2022-09-16 at 09:04 +0200, Lukas Wunner wrote:
> When a UART port is newly registered, uart_configure_port() seeks to
> deassert RS485 Transmit Enable by setting the RTS bit in port->mctrl.
> However a number of UART drivers interpret a set RTS bit as *assertion*
> instead of deassertion:  Affected drivers include those using
> serial8250_em485_config() (except 8250_bcm2835aux.c) and some using
> mctrl_gpio (e.g. imx.c).
> 
> Since the interpretation of the RTS bit is driver-specific, it is not
> suitable as a means to centrally deassert Transmit Enable in the serial
> core.  Instead, the serial core must call on drivers to deassert it in
> their driver-specific way.  One way to achieve that is to call
> ->rs485_config().  It implicitly deasserts Transmit Enable.
> 
> So amend uart_configure_port() and uart_resume_port() to invoke
> uart_rs485_config().  That allows removing calls to uart_rs485_config()
> from drivers' ->probe() hooks and declaring the function static.
> 
> Skip any invocation of ->set_mctrl() if RS485 is enabled.  RS485 has no
> hardware flow control, so the modem control lines are irrelevant and
> need not be touched.  When leaving RS485 mode, reset the modem control
> lines to the state stored in port->mctrl.  That way, UARTs which are
> muxed between RS485 and RS232 transceivers drive the lines correctly
> when switched to RS232.  (serial8250_do_startup() historically raises
> the OUT1 modem signal because otherwise interrupts are not signaled on
> ancient PC UARTs, but I believe that no longer applies to modern,
> RS485-capable UARTs and is thus safe to be skipped.)
> 
> imx.c modifies port->mctrl whenever Transmit Enable is asserted and
> deasserted.  Stop it from doing that so port->mctrl reflects the RS232
> line state.
> 
> 8250_omap.c deasserts Transmit Enable on ->runtime_resume() by calling
> ->set_mctrl().  Because that is now a no-op in RS485 mode, amend the
> function to call serial8250_em485_stop_tx().
> 
> fsl_lpuart.c retrieves and applies the RS485 device tree properties
> after registering the UART port.  Because applying now happens on
> registration in uart_configure_port(), move retrieval of the properties
> ahead of uart_add_one_port().
> 
> Fixes: d3b3404df318 ("serial: Fix incorrect rs485 polarity on uart open")
> Reported-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> Link: https://lore.kernel.org/all/20220329085050.311408-1-matthias.schiffer@ew.tq-group.com/
> Reported-by: Roosen Henri <Henri.Roosen@ginzinger.com>
> Link: https://lore.kernel.org/all/8f538a8903795f22f9acc94a9a31b03c9c4ccacb.camel@ginzinger.com/
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Cc: stable@vger.kernel.org # v4.14+


I had to add one more fix to make RS485 work at all with 8250_omap on
Linux 6.0-rc5+ (submitted a few minutes ago, "serial: 8250: omap: Use
serial8250_em485_supported"), but the RTS polarity is correct now, so:

Tested-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>

Thanks!


> ---
>  Based on v6.0-rc3 + this dependency:
>  https://lore.kernel.org/linux-serial/72fb646c1b0b11c989850c55f52f9ff343d1b2fa.1662884345.git.lukas@wunner.de/
> 
>  drivers/tty/serial/8250/8250_omap.c |  3 +++
>  drivers/tty/serial/8250/8250_pci.c  |  9 +--------
>  drivers/tty/serial/8250/8250_port.c | 10 ++++++----
>  drivers/tty/serial/fsl_lpuart.c     | 10 ++++------
>  drivers/tty/serial/imx.c            |  8 ++------
>  drivers/tty/serial/serial_core.c    | 36 ++++++++++++++++++++----------------
>  include/linux/serial_core.h         |  1 -
>  7 files changed, 36 insertions(+), 41 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
> index b43894e..2e67754 100644
> --- a/drivers/tty/serial/8250/8250_omap.c
> +++ b/drivers/tty/serial/8250/8250_omap.c
> @@ -342,6 +342,9 @@ static void omap8250_restore_regs(struct uart_8250_port *up)
>  	omap8250_update_mdr1(up, priv);
>  
>  	up->port.ops->set_mctrl(&up->port, up->port.mctrl);
> +
> +	if (up->port.rs485.flags & SER_RS485_ENABLED)
> +		serial8250_em485_stop_tx(up);
>  }
>  
>  /*
> diff --git a/drivers/tty/serial/8250/8250_pci.c b/drivers/tty/serial/8250/8250_pci.c
> index 6f66dc2..b260c55 100644
> --- a/drivers/tty/serial/8250/8250_pci.c
> +++ b/drivers/tty/serial/8250/8250_pci.c
> @@ -1627,7 +1627,6 @@ static int pci_fintek_init(struct pci_dev *dev)
>  	resource_size_t bar_data[3];
>  	u8 config_base;
>  	struct serial_private *priv = pci_get_drvdata(dev);
> -	struct uart_8250_port *port;
>  
>  	if (!(pci_resource_flags(dev, 5) & IORESOURCE_IO) ||
>  			!(pci_resource_flags(dev, 4) & IORESOURCE_IO) ||
> @@ -1674,13 +1673,7 @@ static int pci_fintek_init(struct pci_dev *dev)
>  
>  		pci_write_config_byte(dev, config_base + 0x06, dev->irq);
>  
> -		if (priv) {
> -			/* re-apply RS232/485 mode when
> -			 * pciserial_resume_ports()
> -			 */
> -			port = serial8250_get_port(priv->line[i]);
> -			uart_rs485_config(&port->port);
> -		} else {
> +		if (!priv) {
>  			/* First init without port data
>  			 * force init to RS232 Mode
>  			 */
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 907c5ff..a6f03b1 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -600,7 +600,7 @@ void serial8250_rpm_put(struct uart_8250_port *p)
>  static int serial8250_em485_init(struct uart_8250_port *p)
>  {
>  	if (p->em485)
> -		return 0;
> +		goto deassert_rts;
>  
>  	p->em485 = kmalloc(sizeof(struct uart_8250_em485), GFP_ATOMIC);
>  	if (!p->em485)
> @@ -616,6 +616,7 @@ static int serial8250_em485_init(struct uart_8250_port *p)
>  	p->em485->active_timer = NULL;
>  	p->em485->tx_stopped = true;
>  
> +deassert_rts:
>  	p->rs485_stop_tx(p);
>  
>  	return 0;
> @@ -2047,6 +2048,10 @@ void serial8250_do_set_mctrl(struct uart_port *port, unsigned int mctrl)
>  
>  static void serial8250_set_mctrl(struct uart_port *port, unsigned int mctrl)
>  {
> +
> +	if (port->rs485.flags & SER_RS485_ENABLED)
> +		return;
> +
>  	if (port->set_mctrl)
>  		port->set_mctrl(port, mctrl);
>  	else
> @@ -3189,9 +3194,6 @@ static void serial8250_config_port(struct uart_port *port, int flags)
>  	if (flags & UART_CONFIG_TYPE)
>  		autoconfig(up);
>  
> -	if (port->rs485.flags & SER_RS485_ENABLED)
> -		uart_rs485_config(port);
> -
>  	/* if access method is AU, it is a 16550 with a quirk */
>  	if (port->type == PORT_16550A && port->iotype == UPIO_AU)
>  		up->bugs |= UART_BUG_NOMSR;
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index e9d5b48..dfc73fe 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -2724,15 +2724,13 @@ static int lpuart_probe(struct platform_device *pdev)
>  	if (ret)
>  		goto failed_reset;
>  
> -	ret = uart_add_one_port(&lpuart_reg, &sport->port);
> -	if (ret)
> -		goto failed_attach_port;
> -
>  	ret = uart_get_rs485_mode(&sport->port);
>  	if (ret)
>  		goto failed_get_rs485;
>  
> -	uart_rs485_config(&sport->port);
> +	ret = uart_add_one_port(&lpuart_reg, &sport->port);
> +	if (ret)
> +		goto failed_attach_port;
>  
>  	ret = devm_request_irq(&pdev->dev, sport->port.irq, handler, 0,
>  				DRIVER_NAME, sport);
> @@ -2742,9 +2740,9 @@ static int lpuart_probe(struct platform_device *pdev)
>  	return 0;
>  
>  failed_irq_request:
> -failed_get_rs485:
>  	uart_remove_one_port(&lpuart_reg, &sport->port);
>  failed_attach_port:
> +failed_get_rs485:
>  failed_reset:
>  	lpuart_disable_clks(sport);
>  	return ret;
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 5875ee6..05b432d 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -380,8 +380,7 @@ static void imx_uart_rts_active(struct imx_port *sport, u32 *ucr2)
>  {
>  	*ucr2 &= ~(UCR2_CTSC | UCR2_CTS);
>  
> -	sport->port.mctrl |= TIOCM_RTS;
> -	mctrl_gpio_set(sport->gpios, sport->port.mctrl);
> +	mctrl_gpio_set(sport->gpios, sport->port.mctrl | TIOCM_RTS);
>  }
>  
>  /* called with port.lock taken and irqs caller dependent */
> @@ -390,8 +389,7 @@ static void imx_uart_rts_inactive(struct imx_port *sport, u32 *ucr2)
>  	*ucr2 &= ~UCR2_CTSC;
>  	*ucr2 |= UCR2_CTS;
>  
> -	sport->port.mctrl &= ~TIOCM_RTS;
> -	mctrl_gpio_set(sport->gpios, sport->port.mctrl);
> +	mctrl_gpio_set(sport->gpios, sport->port.mctrl & ~TIOCM_RTS);
>  }
>  
>  static void start_hrtimer_ms(struct hrtimer *hrt, unsigned long msec)
> @@ -2347,8 +2345,6 @@ static int imx_uart_probe(struct platform_device *pdev)
>  		dev_err(&pdev->dev,
>  			"low-active RTS not possible when receiver is off, enabling receiver\n");
>  
> -	uart_rs485_config(&sport->port);
> -
>  	/* Disable interrupts before requesting them */
>  	ucr1 = imx_uart_readl(sport, UCR1);
>  	ucr1 &= ~(UCR1_ADEN | UCR1_TRDYEN | UCR1_IDEN | UCR1_RRDYEN | UCR1_RTSDEN);
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index c711318..179ee19 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -158,15 +158,10 @@ static void uart_start(struct tty_struct *tty)
>  	unsigned long flags;
>  	unsigned int old;
>  
> -	if (port->rs485.flags & SER_RS485_ENABLED) {
> -		set &= ~TIOCM_RTS;
> -		clear &= ~TIOCM_RTS;
> -	}
> -
>  	spin_lock_irqsave(&port->lock, flags);
>  	old = port->mctrl;
>  	port->mctrl = (old & ~clear) | set;
> -	if (old != port->mctrl)
> +	if (old != port->mctrl && !(port->rs485.flags & SER_RS485_ENABLED))
>  		port->ops->set_mctrl(port, port->mctrl);
>  	spin_unlock_irqrestore(&port->lock, flags);
>  }
> @@ -1391,7 +1386,7 @@ static void uart_set_rs485_termination(struct uart_port *port,
>  				 !!(rs485->flags & SER_RS485_TERMINATE_BUS));
>  }
>  
> -int uart_rs485_config(struct uart_port *port)
> +static int uart_rs485_config(struct uart_port *port)
>  {
>  	struct serial_rs485 *rs485 = &port->rs485;
>  	int ret;
> @@ -1405,7 +1400,6 @@ int uart_rs485_config(struct uart_port *port)
>  
>  	return ret;
>  }
> -EXPORT_SYMBOL_GPL(uart_rs485_config);
>  
>  static int uart_get_rs485_config(struct uart_port *port,
>  			 struct serial_rs485 __user *rs485)
> @@ -1444,8 +1438,13 @@ static int uart_set_rs485_config(struct tty_struct *tty, struct uart_port *port,
>  
>  	spin_lock_irqsave(&port->lock, flags);
>  	ret = port->rs485_config(port, &tty->termios, &rs485);
> -	if (!ret)
> +	if (!ret) {
>  		port->rs485 = rs485;
> +
> +		/* Reset RTS and other mctrl lines when disabling RS485 */
> +		if (!(rs485.flags & SER_RS485_ENABLED))
> +			port->ops->set_mctrl(port, port->mctrl);
> +	}
>  	spin_unlock_irqrestore(&port->lock, flags);
>  	if (ret)
>  		return ret;
> @@ -2352,7 +2351,8 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *uport)
>  
>  		spin_lock_irq(&uport->lock);
>  		ops->stop_tx(uport);
> -		ops->set_mctrl(uport, 0);
> +		if (!(uport->rs485.flags & SER_RS485_ENABLED))
> +			ops->set_mctrl(uport, 0);
>  		/* save mctrl so it can be restored on resume */
>  		mctrl = uport->mctrl;
>  		uport->mctrl = 0;
> @@ -2440,7 +2440,8 @@ int uart_resume_port(struct uart_driver *drv, struct uart_port *uport)
>  
>  		uart_change_pm(state, UART_PM_STATE_ON);
>  		spin_lock_irq(&uport->lock);
> -		ops->set_mctrl(uport, 0);
> +		if (!(uport->rs485.flags & SER_RS485_ENABLED))
> +			ops->set_mctrl(uport, 0);
>  		spin_unlock_irq(&uport->lock);
>  		if (console_suspend_enabled || !uart_console(uport)) {
>  			/* Protected by port mutex for now */
> @@ -2451,7 +2452,10 @@ int uart_resume_port(struct uart_driver *drv, struct uart_port *uport)
>  				if (tty)
>  					uart_change_speed(tty, state, NULL);
>  				spin_lock_irq(&uport->lock);
> -				ops->set_mctrl(uport, uport->mctrl);
> +				if (!(uport->rs485.flags & SER_RS485_ENABLED))
> +					ops->set_mctrl(uport, uport->mctrl);
> +				else
> +					uart_rs485_config(uport);
>  				ops->start_tx(uport);
>  				spin_unlock_irq(&uport->lock);
>  				tty_port_set_initialized(port, 1);
> @@ -2558,10 +2562,10 @@ int uart_resume_port(struct uart_driver *drv, struct uart_port *uport)
>  		 */
>  		spin_lock_irqsave(&port->lock, flags);
>  		port->mctrl &= TIOCM_DTR;
> -		if (port->rs485.flags & SER_RS485_ENABLED &&
> -		    !(port->rs485.flags & SER_RS485_RTS_AFTER_SEND))
> -			port->mctrl |= TIOCM_RTS;
> -		port->ops->set_mctrl(port, port->mctrl);
> +		if (!(port->rs485.flags & SER_RS485_ENABLED))
> +			port->ops->set_mctrl(port, port->mctrl);
> +		else
> +			uart_rs485_config(port);
>  		spin_unlock_irqrestore(&port->lock, flags);
>  
>  		/*
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 02a4299..0354369 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -933,5 +933,4 @@ static inline int uart_handle_break(struct uart_port *port)
>  					 !((cflag) & CLOCAL))
>  
>  int uart_get_rs485_mode(struct uart_port *port);
> -int uart_rs485_config(struct uart_port *port);
>  #endif /* LINUX_SERIAL_CORE_H */

Re: [PATCH] serial: Deassert RS485 Transmit Enable on probe in driver-specific way

[Ilpo Järvinen]

On Fri, 16 Sep 2022, Lukas Wunner wrote:

> When a UART port is newly registered, uart_configure_port() seeks to
> deassert RS485 Transmit Enable by setting the RTS bit in port->mctrl.
> However a number of UART drivers interpret a set RTS bit as *assertion*
> instead of deassertion:  Affected drivers include those using
> serial8250_em485_config() (except 8250_bcm2835aux.c) and some using
> mctrl_gpio (e.g. imx.c).
> 
> Since the interpretation of the RTS bit is driver-specific, it is not
> suitable as a means to centrally deassert Transmit Enable in the serial
> core.  Instead, the serial core must call on drivers to deassert it in
> their driver-specific way.  One way to achieve that is to call
> ->rs485_config().  It implicitly deasserts Transmit Enable.
> 
> So amend uart_configure_port() and uart_resume_port() to invoke
> uart_rs485_config().  That allows removing calls to uart_rs485_config()
> from drivers' ->probe() hooks and declaring the function static.
> 
> Skip any invocation of ->set_mctrl() if RS485 is enabled.  RS485 has no
> hardware flow control, so the modem control lines are irrelevant and
> need not be touched.  When leaving RS485 mode, reset the modem control
> lines to the state stored in port->mctrl.  That way, UARTs which are
> muxed between RS485 and RS232 transceivers drive the lines correctly
> when switched to RS232.  (serial8250_do_startup() historically raises
> the OUT1 modem signal because otherwise interrupts are not signaled on
> ancient PC UARTs, but I believe that no longer applies to modern,
> RS485-capable UARTs and is thus safe to be skipped.)
> 
> imx.c modifies port->mctrl whenever Transmit Enable is asserted and
> deasserted.  Stop it from doing that so port->mctrl reflects the RS232
> line state.
> 
> 8250_omap.c deasserts Transmit Enable on ->runtime_resume() by calling
> ->set_mctrl().  Because that is now a no-op in RS485 mode, amend the
> function to call serial8250_em485_stop_tx().
> 
> fsl_lpuart.c retrieves and applies the RS485 device tree properties
> after registering the UART port.  Because applying now happens on
> registration in uart_configure_port(), move retrieval of the properties
> ahead of uart_add_one_port().
> 
> Fixes: d3b3404df318 ("serial: Fix incorrect rs485 polarity on uart open")
> Reported-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> Link: https://lore.kernel.org/all/20220329085050.311408-1-matthias.schiffer@ew.tq-group.com/
> Reported-by: Roosen Henri <Henri.Roosen@ginzinger.com>
> Link: https://lore.kernel.org/all/8f538a8903795f22f9acc94a9a31b03c9c4ccacb.camel@ginzinger.com/
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Cc: stable@vger.kernel.org # v4.14+
> ---
>  Based on v6.0-rc3 + this dependency:
>  https://lore.kernel.org/linux-serial/72fb646c1b0b11c989850c55f52f9ff343d1b2fa.1662884345.git.lukas@wunner.de/

> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 907c5ff..a6f03b1 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -600,7 +600,7 @@ void serial8250_rpm_put(struct uart_8250_port *p)
>  static int serial8250_em485_init(struct uart_8250_port *p)
>  {
>  	if (p->em485)
> -		return 0;
> +		goto deassert_rts;
>  
>  	p->em485 = kmalloc(sizeof(struct uart_8250_em485), GFP_ATOMIC);
>  	if (!p->em485)
> @@ -616,6 +616,7 @@ static int serial8250_em485_init(struct uart_8250_port *p)
>  	p->em485->active_timer = NULL;
>  	p->em485->tx_stopped = true;
>  
> +deassert_rts:
>  	p->rs485_stop_tx(p);

	if (p->em485->tx_stopped)
		p->rs485_stop_tx(p);

?

Because if p->em485->tx_stopped is false and serial8250_em485_init() is 
called again (the comment above it says it is safe to do so), it would 
stop tx at wrong point.

-- 
 i.

Re: [PATCH] serial: Deassert RS485 Transmit Enable on probe in driver-specific way

[Lukas Wunner]

On Fri, Sep 16, 2022 at 03:27:58PM +0300, Ilpo Järvinen wrote:
> On Fri, 16 Sep 2022, Lukas Wunner wrote:
> > When a UART port is newly registered, uart_configure_port() seeks to
> > deassert RS485 Transmit Enable by setting the RTS bit in port->mctrl.
> > However a number of UART drivers interpret a set RTS bit as *assertion*
> > instead of deassertion:  Affected drivers include those using
> > serial8250_em485_config() (except 8250_bcm2835aux.c) and some using
> > mctrl_gpio (e.g. imx.c).
> > 
> > Since the interpretation of the RTS bit is driver-specific, it is not
> > suitable as a means to centrally deassert Transmit Enable in the serial
> > core.  Instead, the serial core must call on drivers to deassert it in
> > their driver-specific way.  One way to achieve that is to call
> > ->rs485_config().  It implicitly deasserts Transmit Enable.
> > 
> > So amend uart_configure_port() and uart_resume_port() to invoke
> > uart_rs485_config().  That allows removing calls to uart_rs485_config()
> > from drivers' ->probe() hooks and declaring the function static.
> > 
> > Skip any invocation of ->set_mctrl() if RS485 is enabled.  RS485 has no
> > hardware flow control, so the modem control lines are irrelevant and
> > need not be touched.  When leaving RS485 mode, reset the modem control
> > lines to the state stored in port->mctrl.  That way, UARTs which are
> > muxed between RS485 and RS232 transceivers drive the lines correctly
> > when switched to RS232.  (serial8250_do_startup() historically raises
> > the OUT1 modem signal because otherwise interrupts are not signaled on
> > ancient PC UARTs, but I believe that no longer applies to modern,
> > RS485-capable UARTs and is thus safe to be skipped.)
> > 
> > imx.c modifies port->mctrl whenever Transmit Enable is asserted and
> > deasserted.  Stop it from doing that so port->mctrl reflects the RS232
> > line state.
> > 
> > 8250_omap.c deasserts Transmit Enable on ->runtime_resume() by calling
> > ->set_mctrl().  Because that is now a no-op in RS485 mode, amend the
> > function to call serial8250_em485_stop_tx().
> > 
> > fsl_lpuart.c retrieves and applies the RS485 device tree properties
> > after registering the UART port.  Because applying now happens on
> > registration in uart_configure_port(), move retrieval of the properties
> > ahead of uart_add_one_port().
> > 
> > Fixes: d3b3404df318 ("serial: Fix incorrect rs485 polarity on uart open")
> > Reported-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> > Link: https://lore.kernel.org/all/20220329085050.311408-1-matthias.schiffer@ew.tq-group.com/
> > Reported-by: Roosen Henri <Henri.Roosen@ginzinger.com>
> > Link: https://lore.kernel.org/all/8f538a8903795f22f9acc94a9a31b03c9c4ccacb.camel@ginzinger.com/
> > Signed-off-by: Lukas Wunner <lukas@wunner.de>
> > Cc: stable@vger.kernel.org # v4.14+
> > ---
> >  Based on v6.0-rc3 + this dependency:
> >  https://lore.kernel.org/linux-serial/72fb646c1b0b11c989850c55f52f9ff343d1b2fa.1662884345.git.lukas@wunner.de/
> 
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index 907c5ff..a6f03b1 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -600,7 +600,7 @@ void serial8250_rpm_put(struct uart_8250_port *p)
> >  static int serial8250_em485_init(struct uart_8250_port *p)
> >  {
> >  	if (p->em485)
> > -		return 0;
> > +		goto deassert_rts;
> >  
> >  	p->em485 = kmalloc(sizeof(struct uart_8250_em485), GFP_ATOMIC);
> >  	if (!p->em485)
> > @@ -616,6 +616,7 @@ static int serial8250_em485_init(struct uart_8250_port *p)
> >  	p->em485->active_timer = NULL;
> >  	p->em485->tx_stopped = true;
> >  
> > +deassert_rts:
> >  	p->rs485_stop_tx(p);
> 
> 	if (p->em485->tx_stopped)
> 		p->rs485_stop_tx(p);
> 
> ?
> 
> Because if p->em485->tx_stopped is false and serial8250_em485_init() is 
> called again (the comment above it says it is safe to do so), it would 
> stop tx at wrong point.

Right, sounds reasonable.

The ->rs485_config is serialized with e.g. serial8250_tx_chars() through
the port spinlock, but I think it may be called while the TX FIFO still
holds characters and no other function is holding the lock.  In that case,
indeed we don't want to deassert RTS.

I'll wait for other comments and respin probably mid next week.

Thanks,

Lukas