linux-kernel - Re: [PATCH v6] tty/serial: at91: fix hardware handshake on Atmel platforms

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Date:   Fri, 28 Oct 2016 11:51:51 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>
Cc:     Richard Genoud <richard.genoud@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        linux-kernel@...r.kernel.org, "#4 . 4+" <stable@...r.kernel.org>,
        linux-serial@...r.kernel.org,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6] tty/serial: at91: fix hardware handshake on Atmel
 platforms

On Fri, Oct 28, 2016 at 01:13:31AM +0200, Alexandre Belloni wrote:
> On 27/10/2016 at 20:02:29 +0200, Uwe Kleine-König wrote :
> > Hello Richard,
> > 
> > On Thu, Oct 27, 2016 at 06:04:06PM +0200, Richard Genoud wrote:
> > > diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> > > index fd8aa1f4ba78..168b10cad47b 100644
> > > --- a/drivers/tty/serial/atmel_serial.c
> > > +++ b/drivers/tty/serial/atmel_serial.c
> > > @@ -2132,11 +2132,29 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
> > >  		mode |= ATMEL_US_USMODE_RS485;
> > >  	} else if (termios->c_cflag & CRTSCTS) {
> > >  		/* RS232 with hardware handshake (RTS/CTS) */
> > > -		if (atmel_use_dma_rx(port) && !atmel_use_fifo(port)) {
> > > -			dev_info(port->dev, "not enabling hardware flow control because DMA is used");
> > > -			termios->c_cflag &= ~CRTSCTS;
> > > -		} else {
> > > +		if (atmel_use_fifo(port) &&
> > > +		    !mctrl_gpio_to_gpiod(atmel_port->gpios, UART_GPIO_CTS)) {
> > > +			/*
> > > +			 * with ATMEL_US_USMODE_HWHS set, the controller will
> > > +			 * be able to drive the RTS pin high/low when the RX
> > > +			 * FIFO is above RXFTHRES/below RXFTHRES2.
> > > +			 * It will also disable the transmitter when the CTS
> > > +			 * pin is high.
> > > +			 * This mode is not activated if CTS pin is a GPIO
> > > +			 * because in this case, the transmitter is always
> > > +			 * disabled (there must be an internal pull-up
> > > +			 * responsible for this behaviour).
> > > +			 * If the RTS pin is a GPIO, the controller won't be
> > > +			 * able to drive it according to the FIFO thresholds,
> > > +			 * but it will be handled by the driver.
> > > +			 */
> > >  			mode |= ATMEL_US_USMODE_HWHS;
> > 
> > You use
> > 
> > 	!mctrl_gpio_to_gpiod(atmel_port->gpios, UART_GPIO_CTS)
> > 
> > as indicator that the cts mode of the respective pin is used. Is this
> > reliable? (It's not if there are machines that don't use CTS, neither as
> > gpio nor using the hardware function.) Maybe this needs a dt property to
> > indicate that there is no (hw)handshaking available?
> > 
> 
> We had a call today were we agreed that this should be added in a future
> patch. Let's fix the regression for now.

A machine without CTS (neither gpio nor hw function) used to work fine
before the breaking commit, right? So this case is part of the
regression and needs a fix?

Anyhow, this probably shouldn't stop the commit entering mainline
because there are probably very few such machines (if any).

So:
Acked-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

Best regards
Uwe

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Pengutronix e.K.                           | Uwe Kleine-König            |
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