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

Maarten Brock
2020-05-18T16:14:01Z

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.

Maarten