linux-kernel - Re: [PATCH v3 10/15] serial: stm32-usart: Add STM32 USART Driver

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Message-ID: <55153FD2.7070304@hurleysoftware.com>
Date:	Fri, 27 Mar 2015 07:32:34 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Maxime Coquelin <mcoquelin.stm32@...il.com>
CC:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Andreas Färber <afaerber@...e.de>,
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Subject: Re: [PATCH v3 10/15] serial: stm32-usart: Add STM32 USART Driver

On 03/26/2015 06:03 PM, Maxime Coquelin wrote:
>>> +static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
>>> +                         struct ktermios *old)
>>> +{
>>> +     unsigned int baud;
>>> +     u32 usardiv, mantissa, fraction;
>>> +     tcflag_t cflag;
>>> +     u32 cr1, cr2, cr3;
>>> +     unsigned long flags;
>>> +
>>> +     baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 16);
>>> +     cflag = termios->c_cflag;
>>> +
>>> +     spin_lock_irqsave(&port->lock, flags);
>>> +
>>> +     /* Stop serial port and reset value */
>>> +     writel_relaxed(0, port->membase + USART_CR1);
>>> +
>>> +     cr1 = USART_CR1_TE | USART_CR1_RE | USART_CR1_UE | USART_CR1_RXNEIE;
>>> +
>>> +     if (cflag & CSTOPB)
>>> +             cr2 = USART_CR2_STOP_2B;
>>> +
>>> +     if (cflag & PARENB) {
>>> +             cr1 |= USART_CR1_PCE;
>>> +             if ((cflag & CSIZE) == CS8)
>>> +                     cr1 |= USART_CR1_M;
>>> +     }
>>> +
>>> +     if (cflag & PARODD)
>>> +             cr1 |= USART_CR1_PS;
>>> +
>>> +     if (cflag & CRTSCTS)
>>> +             cr3 = USART_CR3_RTSE | USART_CR3_CTSE;
>>
>> If this means autoflow control, then you need to define
>> throttle()/unthrottle() methods, otherwise the serial core won't
>> be able to throttle the remote when input buffers are about
>> to overflow.
>>
>> And you should only enable the autoCTS and let the serial
>> core enable autoRTS through set_mctrl(TIOCM_RTS).
>>
>> Just let me know if you need more info about how to do this.
> 
> Ok, let's see if I have well understood.
> 
> USART_CR3_RTSE should be set/cleared in set_mctrl(), depending on
> TIOCM_RTS  value.
> The throttle callback should disable the rx interrupt, and the
> unthrottle enable it.
> For CTS, it should be enabled in set_termios() if CRTSCTS, as done here.
> 
> Am I right?

Yeah, basically. You also have to indicate to the serial core that you
require throttle/unthrottle handling in this mode by setting port->status.

Your set_termios() method would look like:

	port->status &= ~(UPSTAT_AUTOCTS | UPSTAT_AUTORTS);
	if (cflag & CRTSCTS) {
		port->status |= UPSTAT_AUTOCTS | UPSTAT_AUTORTS;
		cr3 = USART_CR3_CTSE;
	}

and your set_mctrl() method would look like:

	if ((mctrl & TIOCM_RTS) && (port->status & UPSTAT_AUTORTS))
		stm32_set_bits(port, USART_CR3, USART_CR3_RTSE);
	else
		stm32_clear_bits(port, USART_CR3, USART_CR3_RTSE);

The UPSTAT_AUTOCTS doesn't really do anything right now but please
use it anyway to indicate this driver has that functionality.

Regards,
Peter Hurley

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