linux-kernel - [PATCH v2 1/9] serial: core: only get RS485 termination GPIO if supported

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Message-Id: <20220703170039.2058202-2-LinoSanfilippo@gmx.de>
Date:   Sun,  3 Jul 2022 19:00:31 +0200
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     gregkh@...uxfoundation.org, jirislaby@...nel.org
Cc:     ilpo.jarvinen@...ux.intel.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org,
        andriy.shevchenko@...ux.intel.com, vz@...ia.com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        lukas@...ner.de, p.rosenberger@...bus.com,
        Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: [PATCH v2 1/9] serial: core: only get RS485 termination GPIO if supported

From: Lino Sanfilippo <l.sanfilippo@...bus.com>

In uart_get_rs485_mode() only try to get a termination GPIO if RS485 bus
termination is supported by the driver. This prevents from allocating
and holding a GPIO descriptor for the drivers lifetimg that will never be
used.

Signed-off-by: Lino Sanfilippo <l.sanfilippo@...bus.com>
---

NOTE: 
This patch follows the design decision that "rs485_supported" is
set by the driver at initialization and cannot be modified
afterwards. However the better approach would be to let the serial
core modify the termination GPIO support setting based on the
existence of a termination GPIO. If "rs485_supported" is not a 
read-only value any more in future the logic implemented in this
patch should be adjusted accordingly.

 drivers/tty/serial/serial_core.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 85ef7ef00b82..3768663dfa4d 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3404,6 +3404,16 @@ int uart_get_rs485_mode(struct uart_port *port)
 	 */
 	port->rs485_term_gpio = devm_gpiod_get_optional(dev, "rs485-term",
 							GPIOD_OUT_LOW);
+
+	if (port->rs485_term_gpio &&
+	    !(port->rs485_supported->flags & SER_RS485_TERMINATE_BUS)) {
+		dev_warn(port->dev,
+			"%s (%d): RS485 termination gpio not supported by driver\n",
+			port->name, port->line);
+		devm_gpiod_put(dev, port->rs485_term_gpio);
+		port->rs485_term_gpio = NULL;
+	}
+
 	if (IS_ERR(port->rs485_term_gpio)) {
 		ret = PTR_ERR(port->rs485_term_gpio);
 		port->rs485_term_gpio = NULL;
-- 
2.25.1