Messages in this thread | | | Date | Mon, 18 May 2020 19:05:11 +0200 | From | Maarten Brock <> | Subject | Re: [PATCH v3 3/5] serial: 8250: Support separate rs485 rx-enable GPIO |
| |
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
| |