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

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Message-ID: <33547f6a596df2ca2ee8e647111e5fa1@vanmierlo.com>
Date:   Mon, 18 May 2020 18:13:16 +0200
From:   Maarten Brock <m.brock@...mierlo.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Heiko Stuebner <heiko@...ech.de>, gregkh@...uxfoundation.org,
        jslaby@...e.com, matwey.kornilov@...il.com,
        giulio.benetti@...ronovasrl.com, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        christoph.muellner@...obroma-systems.com,
        Heiko Stuebner <heiko.stuebner@...obroma-systems.com>,
        linux-serial-owner@...r.kernel.org
Subject: Re: [PATCH v3 3/5] serial: 8250: Support separate rs485 rx-enable
 GPIO

On 2020-05-18 17:22, Lukas Wunner wrote:
> On Mon, May 18, 2020 at 06:12:41PM +0300, Andy Shevchenko wrote:
>> 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.
> 
> Yes.  "High", i.e. asserted, means "termination enabled" in the case of
> my patch and "receiver enabled" in the case of Heiko's patch.

But "High" on a gpio would disable the receiver when connected to !RE.

Maarten