[2/8] serial: core, 8250: set RS485 termination gpio in serial core

From: Lino Sanfilippo <l.sanfilippo@kunbus.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.

This also makes setting the termination GPIO generic for all uart drivers.

Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>
---
 drivers/tty/serial/8250/8250_port.c |  3 ---
 drivers/tty/serial/serial_core.c    | 12 ++++++++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

On Wed, 22 Jun 2022, Lino Sanfilippo wrote:

> From: Lino Sanfilippo <l.sanfilippo@kunbus.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.
> 
> This also makes setting the termination GPIO generic for all uart drivers.
> 
> Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>
> ---
>  drivers/tty/serial/8250/8250_port.c |  3 ---
>  drivers/tty/serial/serial_core.c    | 12 ++++++++++++
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 3e3d784aa628..5245c179cc51 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -675,9 +675,6 @@ int serial8250_em485_config(struct uart_port *port, struct serial_rs485 *rs485)
>  		rs485->flags &= ~SER_RS485_RTS_AFTER_SEND;
>  	}
>  
> -	gpiod_set_value(port->rs485_term_gpio,
> -			rs485->flags & SER_RS485_TERMINATE_BUS);
> -
>  	/*
>  	 * Both serial8250_em485_init() and serial8250_em485_destroy()
>  	 * are idempotent.
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 015f4e1da647..b387376e6fa2 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1352,12 +1352,23 @@ static void uart_sanitize_serial_rs485(struct uart_port *port, struct serial_rs4
>  	memset(rs485->padding, 0, sizeof(rs485->padding));
>  }
>  
> +static void uart_set_rs485_termination(struct uart_port *port,
> +				       const struct serial_rs485 *rs485)
> +{
> +	if (!port->rs485_term_gpio || !(rs485->flags & SER_RS485_ENABLED))
> +		return;
> +
> +	gpiod_set_value_cansleep(port->rs485_term_gpio,
> +				 !!(rs485->flags & SER_RS485_TERMINATE_BUS));
> +}
> +
>  int uart_rs485_config(struct uart_port *port)
>  {
>  	struct serial_rs485 *rs485 = &port->rs485;
>  	int ret;
>  
>  	uart_sanitize_serial_rs485(port, rs485);
> +	uart_set_rs485_termination(port, rs485);
>  
>  	ret = port->rs485_config(port, rs485);
>  	if (ret)
> @@ -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);

When port->rs485_config(port, &rs485) returns non-zero, the input got 
partially applied?

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@kunbus.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().

Regards,
Lino