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

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Date:   Mon, 18 May 2020 19:35:22 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Maarten Brock <m.brock@...mierlo.com>
Cc:     Lukas Wunner <lukas@...ner.de>, 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 Mon, May 18, 2020 at 06:13:16PM +0200, Maarten Brock wrote:
> 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.

No, that's exactly the point of the terminology (asserted means active whatever
polarity it is). You need to define active-low in GPIO description.

-- 
With Best Regards,
Andy Shevchenko