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

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Date:   Fri, 28 Oct 2016 01:13:31 +0200
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Richard Genoud <richard.genoud@...il.com>,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        "#4 . 4+" <stable@...r.kernel.org>
Subject: Re: [PATCH v6] tty/serial: at91: fix hardware handshake on Atmel
 platforms

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