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

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Date:   Wed, 13 Dec 2023 23:14:20 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        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>,
        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

Hi,

On 11.12.23 11:35, Ilpo Järvinen wrote:
> 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.
>

Thats a good point actually. Rolling back is not hard to implement and
although it may not matter right now since currently no driver returns an error
code, this can change very soon.
So I will rework this patch for the next version, thanks!

Regards,
Lino