linux-kernel - Re: [PATCH v5 1/7] serial: Do not hold the port lock when setting rx-during-tx GPIO

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Message-ID: <e65d73ed-9d7f-8037-78c9-48c817ea3492@linux.intel.com>
Date:   Mon, 11 Dec 2023 12:35:36 +0200 (EET)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Lino Sanfilippo <l.sanfilippo@...bus.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        u.kleine-koenig@...gutronix.de, shawnguo@...nel.org,
        s.hauer@...gutronix.de, mcoquelin.stm32@...il.com,
        alexandre.torgue@...s.st.com, cniedermaier@...electronics.com,
        hugo@...ovil.com, LKML <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        LinoSanfilippo@....de, Lukas Wunner <lukas@...ner.de>,
        p.rosenberger@...bus.com, stable@...r.kernel.org
Subject: Re: [PATCH v5 1/7] serial: Do not hold the port lock when setting
 rx-during-tx GPIO

On Sat, 9 Dec 2023, Lino Sanfilippo wrote:

> Both the imx and stm32 driver set the rx-during-tx GPIO in rs485_config().
> Since this function is called with the port lock held, this can be an
> problem in case that setting the GPIO line can sleep (e.g. if a GPIO
> expander is used which is connected via SPI or I2C).
> 
> Avoid this issue by moving the GPIO setting outside of the port lock into
> the serial core and thus making it a generic feature.
> 
> Fixes: c54d48543689 ("serial: stm32: Add support for rs485 RX_DURING_TX output GPIO")
> Fixes: ca530cfa968c ("serial: imx: Add support for RS485 RX_DURING_TX output GPIO")
> Cc: Shawn Guo <shawnguo@...nel.org>
> Cc: Sascha Hauer <s.hauer@...gutronix.de>
> Cc: stable@...r.kernel.org
> Signed-off-by: Lino Sanfilippo <l.sanfilippo@...bus.com>
> ---
>  drivers/tty/serial/imx.c         |  4 ----
>  drivers/tty/serial/serial_core.c | 12 ++++++++++++
>  drivers/tty/serial/stm32-usart.c |  5 +----
>  3 files changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 708b9852a575..9cffeb23112b 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -1943,10 +1943,6 @@ static int imx_uart_rs485_config(struct uart_port *port, struct ktermios *termio
>  	    rs485conf->flags & SER_RS485_RX_DURING_TX)
>  		imx_uart_start_rx(port);
>  
> -	if (port->rs485_rx_during_tx_gpio)
> -		gpiod_set_value_cansleep(port->rs485_rx_during_tx_gpio,
> -					 !!(rs485conf->flags & SER_RS485_RX_DURING_TX));
> -
>  	return 0;
>  }
>  
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index f1348a509552..a0290a5fe8b3 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1402,6 +1402,16 @@ static void uart_set_rs485_termination(struct uart_port *port,
>  				 !!(rs485->flags & SER_RS485_TERMINATE_BUS));
>  }
>  
> +static void uart_set_rs485_rx_during_tx(struct uart_port *port,
> +					const struct serial_rs485 *rs485)
> +{
> +	if (!(rs485->flags & SER_RS485_ENABLED))
> +		return;
> +
> +	gpiod_set_value_cansleep(port->rs485_rx_during_tx_gpio,
> +				 !!(rs485->flags & SER_RS485_RX_DURING_TX));
> +}
> +
>  static int uart_rs485_config(struct uart_port *port)
>  {
>  	struct serial_rs485 *rs485 = &port->rs485;
> @@ -1413,6 +1423,7 @@ static int uart_rs485_config(struct uart_port *port)
>  
>  	uart_sanitize_serial_rs485(port, rs485);
>  	uart_set_rs485_termination(port, rs485);
> +	uart_set_rs485_rx_during_tx(port, rs485);
>  
>  	uart_port_lock_irqsave(port, &flags);
>  	ret = port->rs485_config(port, NULL, rs485);
> @@ -1457,6 +1468,7 @@ static int uart_set_rs485_config(struct tty_struct *tty, struct uart_port *port,
>  		return ret;
>  	uart_sanitize_serial_rs485(port, &rs485);
>  	uart_set_rs485_termination(port, &rs485);
> +	uart_set_rs485_rx_during_tx(port, &rs485);
>
>  	uart_port_lock_irqsave(port, &flags);
>  	ret = port->rs485_config(port, &tty->termios, &rs485);

Also a nice simplification of driver-side code.

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>

Just noting since this is now in core that if ->rs485_config() fails,
I suppose it's just normal to not rollback gpiod_set_value_cansleep() 
(skimming through existing users in tree, it looks it's practically
never touched on the error rollback paths so I guess it's the normal
practice)?

Anyway, since neither of the users currently don't fail in their 
->rs485_config() so it doesn't seem a critical issue.


-- 
 i.