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

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Date:   Sun, 26 Jun 2022 15:36:17 +0200
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        ilpo.jarvinen@...ux.intel.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org,
        andriy.shevchenko@...ux.intel.com, vz@...ia.com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-serial@...r.kernel.org, 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 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().

Regards,
Lino