linux-kernel - Re: [PATCH v2 6/7] serial: 8250: Support separate rs485 rx-enable GPIO

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Message-ID: <20200330145645.GX1922688@smile.fi.intel.com>
Date:   Mon, 30 Mar 2020 17:56:45 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Heiko Stuebner <heiko@...ech.de>
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com,
        matwey.kornilov@...il.com, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, lukas@...ner.de,
        christoph.muellner@...obroma-systems.com,
        giulio.benetti@...ronovasrl.com,
        Heiko Stuebner <heiko.stuebner@...obroma-systems.com>
Subject: Re: [PATCH v2 6/7] serial: 8250: Support separate rs485 rx-enable
 GPIO

On Thu, Mar 26, 2020 at 12:14:21AM +0100, Heiko Stuebner wrote:
> From: Heiko Stuebner <heiko.stuebner@...obroma-systems.com>
> 
> 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