On Thu, Mar 26, 2020 at 12:14:21AM +0100, Heiko Stuebner wrote:
> From: Heiko Stuebner <heiko.stuebner@xxxxxxxxxxxxxxxxxxxxx>
>
> The RE signal is used to control the duplex mode of transmissions,
> aka receiving data while sending in full duplex mode, while stopping
> receiving data in half-duplex mode.
>
> On a number of boards the !RE signal is tied to ground so reception
> is always enabled except if the UART allows disabling the receiver.
> This can be taken advantage of to implement half-duplex mode - like
> done on 8250_bcm2835aux.
>
> Another solution is to tie !RE to RTS always forcing half-duplex mode.
>
> And finally there is the option to control the RE signal separately,
> like done here by introducing a new rs485-specifc gpio that can be
> set depending on the RX_DURING_TX setting in the common em485 callbacks.
> + if (port->rs485_re_gpio)
Redundant. Same for all other cases.
> + gpiod_set_value(port->rs485_re_gpio, 1);
> + port->rs485_re_gpio = devm_gpiod_get(dev, "rs485-rx-enable",
> + GPIOD_FLAGS_BIT_DIR_OUT);
> + if (IS_ERR(port->rs485_re_gpio)) {
> + ret = PTR_ERR(port->rs485_re_gpio);
> + port->rs485_re_gpio = NULL;
> + if (ret != -ENOENT) {
> + if (ret != -EPROBE_DEFER)
> + dev_err(dev, "Cannot get rs485-rx-enable-gpios\n");
> + return ret;
> + }
> + }
NIH of gpiod_get_optional().
--
With Best Regards,
Andy Shevchenko
