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

[Maarten Brock <m.brock@vanmierlo.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