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

From: Alexandre Belloni <alexandre.belloni@free-electrons.com>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Richard Genoud <richard.genoud@gmail.com>,
	Nicolas Ferre <nicolas.ferre@atmel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Cyrille Pitchen <cyrille.pitchen@atmel.com>,
	linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	"#4 . 4+" <stable@vger.kernel.org>
Subject: Re: [PATCH v6] tty/serial: at91: fix hardware handshake on Atmel platforms
Date: Fri, 28 Oct 2016 01:13:31 +0200	[thread overview]
Message-ID: <20161027231331.gngekbg22wfvnolr@piout.net> (raw)
In-Reply-To: <20161027180229.5faqrvxa2a4pos7i@pengutronix.de>

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.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com