linux-serial.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Lukas Wunner <lukas@wunner.de>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>
Cc: "Matwey V. Kornilov" <matwey@sai.msu.ru>,
	Giulio Benetti <giulio.benetti@micronovasrl.com>,
	Heiko Stuebner <heiko@sntech.de>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Christoph Muellner <christoph.muellner@theobroma-systems.com>,
	linux-serial@vger.kernel.org
Subject: [PATCH v2 4/4] serial: 8250: Support rs485 bus termination GPIO
Date: Tue, 12 May 2020 14:40:04 +0200	[thread overview]
Message-ID: <dafe8ecb9897efdbe530667657377ba96c43b8c7.1589285874.git.lukas@wunner.de> (raw)
In-Reply-To: <cover.1589285873.git.lukas@wunner.de>

Commit e8759ad17d41 ("serial: uapi: Add support for bus termination")
introduced the ability to enable rs485 bus termination from user space.
So far the feature is only used by a single driver, 8250_exar.c, using a
hardcoded GPIO pin specific to Siemens IOT2040 products.

Provide for a more generic solution by allowing specification of an
rs485 bus termination GPIO pin in the device tree:  Amend the serial
core to retrieve the GPIO from the device tree (or ACPI table) and amend
the default ->rs485_config() callback for 8250 drivers to change the
GPIO on request from user space.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
---
Changes v1 -> v2:
* Drop unnecessary devm_gpiod_put(). (Andy)
* Use GPIOD_OUT_LOW macro for brevity. (Andy)
* Document the rationale for disabling termination by default.
* Drop nonsensical read of GPIO after setting its direction to out.

 drivers/tty/serial/8250/8250_port.c |  4 ++++
 drivers/tty/serial/serial_core.c    | 16 ++++++++++++++++
 include/linux/serial_core.h         |  2 ++
 3 files changed, 22 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index f77bf820b7a3..b5b630d02110 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -681,6 +681,10 @@ int serial8250_em485_config(struct uart_port *port, struct serial_rs485 *rs485)
 	memset(rs485->padding, 0, sizeof(rs485->padding));
 	port->rs485 = *rs485;
 
+	if (port->rs485_term_gpio)
+		gpiod_set_value(port->rs485_term_gpio,
+				rs485->flags & SER_RS485_TERMINATE_BUS);
+
 	/*
 	 * Both serial8250_em485_init() and serial8250_em485_destroy()
 	 * are idempotent.
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 43b6682877d5..57840cf90388 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3317,6 +3317,7 @@ int uart_get_rs485_mode(struct uart_port *port)
 	 * to get to a defined state with the following properties:
 	 */
 	rs485conf->flags &= ~(SER_RS485_RX_DURING_TX | SER_RS485_ENABLED |
+			      SER_RS485_TERMINATE_BUS |
 			      SER_RS485_RTS_AFTER_SEND);
 	rs485conf->flags |= SER_RS485_RTS_ON_SEND;
 
@@ -3331,6 +3332,21 @@ int uart_get_rs485_mode(struct uart_port *port)
 		rs485conf->flags |= SER_RS485_RTS_AFTER_SEND;
 	}
 
+	/*
+	 * 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.
+	 */
+	port->rs485_term_gpio = devm_gpiod_get_optional(dev, "rs485-term",
+							GPIOD_OUT_LOW);
+	if (IS_ERR(port->rs485_term_gpio)) {
+		ret = PTR_ERR(port->rs485_term_gpio);
+		port->rs485_term_gpio = NULL;
+		if (ret != -EPROBE_DEFER)
+			dev_err(dev, "Cannot get rs485-term-gpios\n");
+		return ret;
+	}
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(uart_get_rs485_mode);
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index b649a2b894e7..9fd550e7946a 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -10,6 +10,7 @@
 #include <linux/bitops.h>
 #include <linux/compiler.h>
 #include <linux/console.h>
+#include <linux/gpio/consumer.h>
 #include <linux/interrupt.h>
 #include <linux/circ_buf.h>
 #include <linux/spinlock.h>
@@ -251,6 +252,7 @@ struct uart_port {
 	struct attribute_group	*attr_group;		/* port specific attributes */
 	const struct attribute_group **tty_groups;	/* all attributes (serial core use only) */
 	struct serial_rs485     rs485;
+	struct gpio_desc	*rs485_term_gpio;	/* enable RS485 bus termination */
 	struct serial_iso7816   iso7816;
 	void			*private_data;		/* generic platform data pointer */
 };
-- 
2.26.2


  parent reply	other threads:[~2020-05-12 12:48 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-12 12:40 [PATCH v2 0/4] " Lukas Wunner
2020-05-12 12:40 ` [PATCH v2 1/4] serial: 8250: Avoid error message on reprobe Lukas Wunner
2020-05-12 13:45   ` Andy Shevchenko
2020-05-12 12:40 ` [PATCH v2 2/4] serial: Allow uart_get_rs485_mode() to return errno Lukas Wunner
2020-05-12 13:46   ` Andy Shevchenko
2020-05-12 12:40 ` [PATCH v2 3/4] dt-bindings: serial: Add binding for rs485 bus termination GPIO Lukas Wunner
2020-05-12 12:40 ` Lukas Wunner [this message]
2020-05-12 14:09   ` [PATCH v2 4/4] serial: 8250: Support " Andy Shevchenko

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=dafe8ecb9897efdbe530667657377ba96c43b8c7.1589285874.git.lukas@wunner.de \
    --to=lukas@wunner.de \
    --cc=andriy.shevchenko@linux.intel.com \
    --cc=christoph.muellner@theobroma-systems.com \
    --cc=giulio.benetti@micronovasrl.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=heiko@sntech.de \
    --cc=jslaby@suse.com \
    --cc=linux-serial@vger.kernel.org \
    --cc=matwey@sai.msu.ru \
    --subject='Re: [PATCH v2 4/4] serial: 8250: Support rs485 bus termination GPIO' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox