From 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sat, 27 Apr 2024 12:05:51 +0200
Subject: Adding upstream version 5.10.209.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 Documentation/driver-api/serial/serial-rs485.rst | 103 +++++++++++++++++++++++
 1 file changed, 103 insertions(+)
 create mode 100644 Documentation/driver-api/serial/serial-rs485.rst

(limited to 'Documentation/driver-api/serial/serial-rs485.rst')

diff --git a/Documentation/driver-api/serial/serial-rs485.rst b/Documentation/driver-api/serial/serial-rs485.rst
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+===========================
+RS485 Serial Communications
+===========================
+
+1. Introduction
+===============
+
+   EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the
+   electrical characteristics of drivers and receivers for use in balanced
+   digital multipoint systems.
+   This standard is widely used for communications in industrial automation
+   because it can be used effectively over long distances and in electrically
+   noisy environments.
+
+2. Hardware-related Considerations
+==================================
+
+   Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in
+   half-duplex mode capable of automatically controlling line direction by
+   toggling RTS or DTR signals. That can be used to control external
+   half-duplex hardware like an RS485 transceiver or any RS232-connected
+   half-duplex devices like some modems.
+
+   For these microcontrollers, the Linux driver should be made capable of
+   working in both modes, and proper ioctls (see later) should be made
+   available at user-level to allow switching from one mode to the other, and
+   vice versa.
+
+3. Data Structures Already Available in the Kernel
+==================================================
+
+   The Linux kernel provides the serial_rs485 structure (see [1]) to handle
+   RS485 communications. This data structure is used to set and configure RS485
+   parameters in the platform data and in ioctls.
+
+   The device tree can also provide RS485 boot time parameters (see [2]
+   for bindings). The driver is in charge of filling this data structure from
+   the values given by the device tree.
+
+   Any driver for devices capable of working both as RS232 and RS485 should
+   implement the rs485_config callback in the uart_port structure. The
+   serial_core calls rs485_config to do the device specific part in response
+   to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback
+   receives a pointer to struct serial_rs485.
+
+4. Usage from user-level
+========================
+
+   From user-level, RS485 configuration can be get/set using the previous
+   ioctls. For instance, to set RS485 you can use the following code::
+
+	#include <linux/serial.h>
+
+	/* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */
+	#include <sys/ioctl.h>
+
+	/* Open your specific device (e.g., /dev/mydevice): */
+	int fd = open ("/dev/mydevice", O_RDWR);
+	if (fd < 0) {
+		/* Error handling. See errno. */
+	}
+
+	struct serial_rs485 rs485conf;
+
+	/* Enable RS485 mode: */
+	rs485conf.flags |= SER_RS485_ENABLED;
+
+	/* Set logical level for RTS pin equal to 1 when sending: */
+	rs485conf.flags |= SER_RS485_RTS_ON_SEND;
+	/* or, set logical level for RTS pin equal to 0 when sending: */
+	rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
+
+	/* Set logical level for RTS pin equal to 1 after sending: */
+	rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
+	/* or, set logical level for RTS pin equal to 0 after sending: */
+	rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
+
+	/* Set rts delay before send, if needed: */
+	rs485conf.delay_rts_before_send = ...;
+
+	/* Set rts delay after send, if needed: */
+	rs485conf.delay_rts_after_send = ...;
+
+	/* Set this flag if you want to receive data even while sending data */
+	rs485conf.flags |= SER_RS485_RX_DURING_TX;
+
+	if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
+		/* Error handling. See errno. */
+	}
+
+	/* Use read() and write() syscalls here... */
+
+	/* Close the device when finished: */
+	if (close (fd) < 0) {
+		/* Error handling. See errno. */
+	}
+
+5. References
+=============
+
+ [1]	include/uapi/linux/serial.h
+
+ [2]	Documentation/devicetree/bindings/serial/rs485.txt
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