linux-kernel - Re: [PATCH v3 3/5] serial: 8250: Support separate rs485 rx-enable GPIO

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Message-ID: <20200518151241.GG1634618@smile.fi.intel.com>
Date:   Mon, 18 May 2020 18:12:41 +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, giulio.benetti@...ronovasrl.com,
        lukas@...ner.de, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        christoph.muellner@...obroma-systems.com,
        Heiko Stuebner <heiko.stuebner@...obroma-systems.com>
Subject: Re: [PATCH v3 3/5] serial: 8250: Support separate rs485 rx-enable
 GPIO

On Sun, May 17, 2020 at 11:56:08PM +0200, 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-specific gpio that can be
> set depending on the RX_DURING_TX setting in the common em485 callbacks.

...

> +	port->rs485_re_gpio = devm_gpiod_get_optional(dev, "rs485-rx-enable",
> +						      GPIOD_OUT_HIGH);

While reviewing some other patch I realized that people are missing the point
of these GPIO flags when pin is declared to be output.

HIGH here means "asserted" (consider active-high vs. active-low in general). Is
that the intention here?

Lukas, same question to your patch.

-- 
With Best Regards,
Andy Shevchenko