Re: [PATCH] serial: 8250: 8250_omap: Support native RS485

[Lukas Wunner <lukas@wunner.de>]

On Wed, Sep 28, 2022 at 02:38:40PM +0300, Ilpo Järvinen wrote:
> On Tue, 27 Sep 2022, Lukas Wunner wrote:
> > Recent TI Sitara SoCs such as AM64/AM65 have gained the ability to
> > automatically assert RTS when data is transmitted, obviating the need
> > to emulate this functionality in software.
> > 
> > The feature is controlled through new DIR_EN and DIR_POL bits in the
> > Mode Definition Register 3.  For details see page 8783 and 8890 of the
> > AM65 TRM:  https://www.ti.com/lit/ug/spruid7e/spruid7e.pdf
[...]
> > @@ -1335,10 +1387,7 @@ static int omap8250_probe(struct platform_device *pdev)
> >  	up.port.shutdown = omap_8250_shutdown;
> >  	up.port.throttle = omap_8250_throttle;
> >  	up.port.unthrottle = omap_8250_unthrottle;
> > -	up.port.rs485_config = serial8250_em485_config;
> >  	up.port.rs485_supported = serial8250_em485_supported;
> > -	up.rs485_start_tx = serial8250_em485_start_tx;
> > -	up.rs485_stop_tx = serial8250_em485_stop_tx;
> >  	up.port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_8250_CONSOLE);
> >  
> >  	ret = of_alias_get_id(np, "serial");
> > @@ -1377,6 +1426,14 @@ static int omap8250_probe(struct platform_device *pdev)
> >  			 DEFAULT_CLK_SPEED);
> >  	}
> >  
> > +	if (priv->habit & UART_HAS_NATIVE_RS485) {
> > +		up.port.rs485_config = omap8250_rs485_config;
> > +	} else {
> > +		up.port.rs485_config = serial8250_em485_config;
> > +		up.rs485_start_tx = serial8250_em485_start_tx;
> > +		up.rs485_stop_tx = serial8250_em485_stop_tx;
> > +	}
> 
> I guess .rs485_supported shouldn't be equal in both cases?

I contemplated whether it should be different for hardware-assisted
RS485 but came to the conclusion that it shouldn't:

The polarity may be chosen both for hardware- and software-controlled RTS.

Whether RX_DURING_TX is possible depends on how the RS485 transceiver
is wired to the UART:  If RTS asserts !RE on the transceiver when sending,
the UART cannot receive data, regardless whether hardware- or software-
controlled RTS is used.

TERMINATE_BUS works independently from RTS control.

And ADDRB doesn't seem to be supported in either mode AFAICS.

Am I missing something?

Thanks,

Lukas