linux-kernel - Re: [PATCH v2] serial: amba-pl011: add RS485 support

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Date:   Fri, 25 Jun 2021 08:01:36 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     linux@...linux.org.uk, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] serial: amba-pl011: add RS485 support

On 25. 06. 21, 2:15, Lino Sanfilippo wrote:
>>> +static int pl011_rs485_config(struct uart_port *port,
>>> +			      struct serial_rs485 *rs485)
>>> +{
>>> +	struct uart_amba_port *uap =
>>> +		container_of(port, struct uart_amba_port, port);
>>> +
>>> +	/* pick sane settings if the user hasn't */
>>> +	if (!!(rs485->flags & SER_RS485_RTS_ON_SEND) ==
>>
>> Why the !! in an if statement?
>>
>>> +	    !!(rs485->flags & SER_RS485_RTS_AFTER_SEND)) {
>>
>> Same here, why?
>>
> 
> This was copied from serial8250_em485_config(). But I think we can simply use
> 
> 	if (rs485->flags & SER_RS485_RTS_AFTER_SEND)
> 		rs485->flags &= ~SER_RS485_RTS_ON_SEND;
> 	else
> 		rs485->flags |= SER_RS485_RTS_ON_SEND;
> 
> instead. I will adjust the code accordingly.

This is different. You want to set ON_SEND when none is set. And unset 
AFTER_SEND when both are set. In your code, when both are set, you leave 
AFTER_SEND.

regards,
-- 
js
suse labs