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

From: Lino Sanfilippo <LinoSanfilippo@gmx.de>
To: "Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	vz@mleia.com, linux-arm-kernel@lists.infradead.org,
	devicetree@vger.kernel.org,
	linux-serial <linux-serial@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	lukas@wunner.de, p.rosenberger@kunbus.com,
	Lino Sanfilippo <l.sanfilippo@kunbus.com>
Subject: Re: [PATCH 2/8] serial: core, 8250: set RS485 termination gpio in serial core
Date: Sun, 26 Jun 2022 17:41:35 +0200	[thread overview]
Message-ID: <aa851698-ff26-a319-500a-575a9c669d95@gmx.de> (raw)
In-Reply-To: <83762813-70ec-93c3-4015-5676ce9534fd@linux.intel.com>


Hi,

On 25.06.22 at 12:40, Ilpo Järvinen wrote:
>> +
>>  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?
>
>
The thing is we dont know what the state of the termination GPIO (asserted or deasserted)
was before we set it and port->rs485_config() failed, so we cannot restore it.
We could read the GPIO before we change it but AFAIK this is unsafe since it is an output
pin. Maybe add a boolean variable "rs485_termination_gpio_asserted" to uart_port to track the
current state?


Regards,
Lino