serial: add Documentation about RS485 serial communications

Documentation about RS485 serial communications

Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Russell King <linux@arm.linux.org.uk>
Acked-by: Grant Edwards <grant.b.edwards@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 119 insertions, 0 deletions

diff --git a/Documentation/serial/serial-rs485.txt b/Documentation/serial/serial-rs485.txt
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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 (e.g., Atmel AT91) contain a built-in half-duplex mode capable of
+   automatically controlling line direction by toggling RTS. That can used to
+   control external half-duplex hardware like an RS485 transceiver or any
+   RS232-connected half-duplex device 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.
+
+   Any driver for devices capable of working both as RS232 and RS485 should
+   provide at least the following ioctls:
+
+    - TIOCSRS485 (typically associated with number 0x542F). This ioctl is used
+      to enable/disable RS485 mode from user-space
+
+    - TIOCGRS485 (typically associated with number 0x542E). This ioctl is used
+      to get RS485 mode from kernel-space (i.e., driver) to user-space.
+
+   In other words, the serial driver should contain a code similar to the next
+   one:
+
+        static struct uart_ops atmel_pops = {
+                /* ... */
+                .ioctl          = handle_ioctl,
+        };
+
+        static int handle_ioctl(struct uart_port *port,
+                unsigned int cmd,
+                unsigned long arg)
+        {
+                struct serial_rs485 rs485conf;
+
+                switch (cmd) {
+                case TIOCSRS485:
+                        if (copy_from_user(&rs485conf,
+                                (struct serial_rs485 *) arg,
+                                sizeof(rs485conf)))
+                                        return -EFAULT;
+
+                        /* ... */
+                        break;
+
+                case TIOCGRS485:
+                        if (copy_to_user((struct serial_rs485 *) arg,
+                                ...,
+                                sizeof(rs485conf)))
+                                        return -EFAULT;
+                        /* ... */
+                        break;
+
+                /* ... */
+                }
+        }
+
+
+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>
+
+        /* Driver-specific ioctls: */
+        #define TIOCGRS485      0x542E
+        #define TIOCSRS485      0x542F
+
+        /* 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;
+
+        /* Set RS485 mode: */
+        rs485conf.flags |= SER_RS485_ENABLED;
+
+        /* Set rts delay before send, if needed: */
+        rs485conf.flags |= SER_RS485_RTS_BEFORE_SEND;
+        rs485conf.delay_rts_before_send = ...;
+
+        /* Set rts delay after send, if needed: */
+        rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
+        rs485conf.delay_rts_after_send = ...;
+
+        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/linux/serial.h