[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 12:23:23PM +0200, Baruch Siach wrote:
> Thanks for your prompt response.
> 
> On Sun, Feb 28, 2016 at 10:56:01AM +0100, Uwe Kleine-K?nig wrote:
> > 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.
> 
> I thought it is a good idea to mention this fact in the commit log anyway. It 
> is not hard to imagine broken userspace being affected by this change.
> 
> > > 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?!
> 
> Sounds like a good idea. But if I take your comment to its logical conclusion, 
> thread B might just disable SER_RS485_ENABLED entirely. Would it make sense to 
> restore RXEN outside the 'if (port->rs485.flags & SER_RS485_ENABLED)' block?  
> Or maybe we should just set RXEN in imx_rs485_config() when 
> SER_RS485_RX_DURING_TX is enabled?

The latter sounds like the right thing to do.

Best regards
Uwe

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