linux-kernel - [PATCH v4 1/2] serial: core: Add support for enabling RS485 mode via GPIO at boot time

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Message-Id: <3Za.ZZs}.ndXI8CMee4.1bN6eQ@seznam.cz>
Date:   Tue, 21 Nov 2023 09:28:10 +0100 (CET)
From:   "Tomas Paukrt" <tomaspaukrt@...il.cz>
To:     "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Jiri Slaby" <jirislaby@...nel.org>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Krzysztof Kozlowski" <krzysztof.kozlowski+dt@...aro.org>,
        "Conor Dooley" <conor+dt@...nel.org>
Cc:     <linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>,
        <devicetree@...r.kernel.org>
Subject: [PATCH v4 1/2] serial: core: Add support for enabling RS485 mode via GPIO at boot time

Add an option to enable the RS485 mode at boot time based on
the state of a GPIO pin (DIP switch or configuration jumper).
The GPIO is defined by the device tree property "linux,rs485-mode-gpio".

Signed-off-by: Tomas Paukrt <tomaspaukrt@...il.cz>
---
 drivers/tty/serial/serial_core.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index f1348a5..f1bf0b9 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3603,6 +3603,18 @@ int uart_get_rs485_mode(struct uart_port *port)
 	}
 
 	/*
+	 * Enable the RS485 mode based on the state of a GPIO pin.
+	 */
+	desc = devm_gpiod_get_optional(dev, "linux,rs485-mode", GPIOD_IN);
+	if (IS_ERR(desc))
+		return dev_err_probe(dev, PTR_ERR(desc), "Cannot get linux,rs485-mode-gpio\n");
+	if (desc) {
+		if (gpiod_get_value(desc))
+			rs485conf->flags |= SER_RS485_ENABLED;
+		devm_gpiod_put(dev, desc);
+	}
+
+	/*
 	 * Disabling termination by default is the safe choice:  Else if many
 	 * bus participants enable it, no communication is possible at all.
 	 * Works fine for short cables and users may enable for longer cables.
-- 
2.7.4