linux-kernel - Re: [PATCH 2/2 v5] tty/serial/8250: use mctrl

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Date:   Tue, 11 Jun 2019 16:02:54 +0200
From:   Stefan Roese <sr@...x.de>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yegor Yefremov <yegorslists@...glemail.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Giulio Benetti <giulio.benetti@...ronovasrl.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 2/2 v5] tty/serial/8250: use mctrl_gpio helpers

On 11.06.19 14:44, Andy Shevchenko wrote:
> On Tue, Jun 11, 2019 at 12:56:03PM +0200, Stefan Roese wrote:
>> From: Yegor Yefremov <yegorslists@...glemail.com>
>>
>> This patch permits the usage for GPIOs to control
>> the CTS/RTS/DTR/DSR/DCD/RI signals.
> 
>>   static inline void serial8250_out_MCR(struct uart_8250_port *up, int value)
>>   {
>>   	serial_out(up, UART_MCR, value);
>> +
>> +	if (up->gpios) {
>> +		int mctrl_gpio = 0;
>> +
>> +		if (value & UART_MCR_RTS)
>> +			mctrl_gpio |= TIOCM_RTS;
>> +		if (value & UART_MCR_DTR)
>> +			mctrl_gpio |= TIOCM_DTR;
>> +
>> +		mctrl_gpio_set(up->gpios, mctrl_gpio);
>> +	}
>>   }
>>   
>>   static inline int serial8250_in_MCR(struct uart_8250_port *up)
>>   {
>> -	return serial_in(up, UART_MCR);
>> +	int mctrl;
>> +
>> +	mctrl = serial_in(up, UART_MCR);
>> +
>> +	if (up->gpios) {
>> +		int mctrl_gpio = 0;
>> +
>> +		/* save current MCR values */
>> +		if (mctrl & UART_MCR_RTS)
>> +			mctrl_gpio |= TIOCM_RTS;
>> +		if (mctrl & UART_MCR_DTR)
>> +			mctrl_gpio |= TIOCM_DTR;
>> +
>> +		mctrl_gpio = mctrl_gpio_get_outputs(up->gpios, &mctrl_gpio);
>> +		if (mctrl_gpio & TIOCM_RTS)
>> +			mctrl |= UART_MCR_RTS;
>> +		else
>> +			mctrl &= ~UART_MCR_RTS;
>> +
>> +		if (mctrl_gpio & TIOCM_DTR)
>> +			mctrl |= UART_MCR_DTR;
>> +		else
>> +			mctrl &= ~UART_MCR_DTR;
>> +	}
>> +
>> +	return mctrl;
>>   }
> 
> These are using OR logic with potentially volatile data. Shouldn't we mask
> unused bits in UART_MCR in case of up->gpios != NULL?

Sorry, I don't see, which bits you are referring to? Could you please be
a bit more specific with the variable / macro meant (example)?
  
>> +	if (up->gpios == 0)
> 
> This is type inconsistency with this check as far as I understand.
> I guess you have to do either (up->gpios == NULL), or (!up->gpios).

Ah, right. Thanks for spotting.

Thanks,
Stefan