linux-kernel - Re: [PATCHv2 3/3] tty/serial: at91: fix hardware handshake on SAM9x5 (without GPIOs)

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Date:   Mon, 12 Sep 2016 11:53:17 +0200
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Richard Genoud <richard.genoud@...il.com>
Cc:     Cyrille Pitchen <cyrille.pitchen@...el.com>,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 3/3] tty/serial: at91: fix hardware handshake on SAM9x5
 (without GPIOs)

On 12/09/2016 at 11:47:33 +0200, Richard Genoud wrote :
> Commit 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when
> hardware handshake is enabled") broke the hardware handshake on SAM9x5
> platforms.
> 
> On Atmel platforms, the USART can only handle the handware handshake
> (ATMEL_US_USMODE_HWHS) if FIFOs or PDC are used.
> 
> Thus, ATMEL_US_USMODE_HWHS mode should only be used in this case.
> 
> For SAM9x5, there's no FIFOs nor PDC for the USART, so the mode should
> be ATMEL_US_USMODE_NORMAL and the RTS pin should be controlled by the
> driver.
> 
> NB: -stable is not Cced because it doesn't cleanly apply on 4.1+
> 
> Tested on SAM9G35-CM with and without DMA
> 
> Signed-off-by: Richard Genoud <richard.genoud@...il.com>
> Fixes: 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when hardware handshake is enabled")
> ---
>  drivers/tty/serial/atmel_serial.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index e9b4fbf88c2d..32154e7231ce 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -2130,15 +2130,19 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
>  	} else if ((termios->c_cflag & CRTSCTS) &&
>  		   !mctrl_gpio_use_rtscts(atmel_port->gpios)) {
>  		/*
> -		 * RS232 with hardware handshake (RTS/CTS)
> -		 * handled by the controller.
> +		 * Automatic hardware handshake (RTS/CTS) only work with
> +		 * FIFOs or PDC.
> +		 * Meaning that on SAM9x5 the controller can't handle
> +		 * the hardware handshake (no FIFOs nor PDC on these platforms).
>  		 */
> -		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_pdc_rx(port) || atmel_use_fifo(port))
>  			mode |= ATMEL_US_USMODE_HWHS;
> -		}
> +		else
> +			/*
> +			 * The hardware handshake won't be handle by the
> +			 * controller but by the driver.
> +			 */
> +			mode |= ATMEL_US_USMODE_NORMAL;

You still need the case where HWHS is impossible and there are no gpio
configured. You need to inform userspace that the configuration was not
applied instead of silently ignoring the error.

>  	} else {
>  		/* RS232 without hadware handshake or controlled by GPIOs */
>  		mode |= ATMEL_US_USMODE_NORMAL;

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