[PATCH] serial: imx: support RS-485 Rx disable on Tx

[u.kleine-koenig@pengutronix.de (Uwe Kleine-König)]

Hello Baruch,

On Sun, Feb 28, 2016 at 11:25:51AM +0200, Baruch Siach wrote:
> Some RS-232 to RS-485 transceivers require Rx to be disabled on Tx to
> avoid echo of Tx data into the Rx buffer. Specifically, the XR3160E
> RS-232/RS-485/RS-422 transceiver behaves this way.
> 
> This commit disables Rx on active Tx when SER_RS485_ENABLED is active and
> SER_RS485_RX_DURING_TX is disabled.
> 
> Note that this is a change in behavior of the driver. Until now

But this change is a good one (assuming it does what it advertises :-).
Userspace got informed before that SER_RS485_RX_DURING_TX is enabled, so
this is not an incompatible change.

Best regards
Uwe

> SER_RS485_RX_DURING_TX was enabled unconditionally even when disabled in
> the TIOCSRS485 ioctl serial_rs485 flags field.
> 
> Cc: Uwe Kleine-K?nig <u.kleine-koenig@pengutronix.de>
> Signed-off-by: Baruch Siach <baruch@tkos.co.il>
> ---
>  drivers/tty/serial/imx.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 9362f54c816c..333d34ff358c 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -361,6 +361,8 @@ static void imx_stop_tx(struct uart_port *port)
>  			imx_port_rts_inactive(sport, &temp);
>  		else
>  			imx_port_rts_active(sport, &temp);
> +		if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
> +			temp |= UCR2_RXEN;
>  		writel(temp, port->membase + UCR2);
>  
>  		temp = readl(port->membase + UCR4);
> @@ -568,6 +570,8 @@ static void imx_start_tx(struct uart_port *port)
>  			imx_port_rts_inactive(sport, &temp);
>  		else
>  			imx_port_rts_active(sport, &temp);
> +		if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
> +			temp &= ~UCR2_RXEN;
>  		writel(temp, port->membase + UCR2);

Can this happen:

 - SER_RS485_RX_DURING_TX is off
 - thread A starts sending (and so disables RX)
 - thread B sets SER_RS485_RX_DURING_TX
 - thread A finishes sending, and doesn't restore RXEN.

?

Even if this cannot happen it might be more robust to restore RXEN
unconditionally in imx_stop_tx?!

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |