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

From: Maarten Brock <m.brock@vanmierlo.com>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Lukas Wunner <lukas@wunner.de>, Heiko Stuebner <heiko@sntech.de>,
	gregkh@linuxfoundation.org, jslaby@suse.com,
	matwey.kornilov@gmail.com, giulio.benetti@micronovasrl.com,
	linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	christoph.muellner@theobroma-systems.com,
	Heiko Stuebner <heiko.stuebner@theobroma-systems.com>,
	linux-serial-owner@vger.kernel.org
Subject: Re: [PATCH v3 3/5] serial: 8250: Support separate rs485 rx-enable GPIO
Date: Mon, 18 May 2020 19:05:11 +0200	[thread overview]
Message-ID: <f7d408a0ca747086c01999fc0db905da@vanmierlo.com> (raw)
In-Reply-To: <20200518163522.GK1634618@smile.fi.intel.com>

On 2020-05-18 18:35, Andy Shevchenko wrote:
> 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@theobroma-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.

Is there anything wrong with defining GPIOD_OUT_ACTIVE or 
GPIOD_OUT_ASSERTED
for this very purpose? May I suggest to deprecate GPIOD_OUT_HIGH and 
replace it?

Maarten