Subject: [PATCH] atmel_serial: Atmel RS485 support v2

From: [email-masked] (Claudio Scordino)
Date: Fri, 28 May 2010 17:05:26 +0200

Wolfram Sang ha scritto:
> Hi Claudio,
> 
> kudos for getting a RS485-implementation into the kernel! Very useful.

Many thanks!

I hope other people will find it useful too.

> 
>>>     the latest version of the patch has been already merged in the
>>> latest Linus' git tree through Russell King's patchsystem 
> 
> Nitpick: Wasn't there some documentation promised after the final patch ;)

Here is the first attempt.

Best regards,

		Claudio

Documentation about RS485

Signed-off-by: Claudio Scordino <[email-masked]>
---
 Documentation/serial/00-INDEX         |    2 +
 Documentation/serial/serial-rs485.txt |  118 +++++++++++++++++++++++++++++++++
 2 files changed, 120 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/serial/serial-rs485.txt

diff --git a/Documentation/serial/00-INDEX b/Documentation/serial/00-INDEX
index 07dcdb0..e09468a 100644
--- a/Documentation/serial/00-INDEX
+++ b/Documentation/serial/00-INDEX
@@ -14,6 +14,8 @@ riscom8.txt
 	- notes on using the RISCom/8 multi-port serial driver.
 rocket.txt
 	- info on the Comtrol RocketPort multiport serial driver.
+serial-rs485.txt
+	- info about RS485 structures and support in the kernel.
 specialix.txt
 	- info on hardware/driver for specialix IO8+ multiport serial card.
 stallion.txt
diff --git a/Documentation/serial/serial-rs485.txt b/Documentation/serial/serial-rs485.txt
new file mode 100644
index 0000000..dac98c3
--- /dev/null
+++ b/Documentation/serial/serial-rs485.txt
@@ -0,0 +1,118 @@
+                        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.
+   Even though the data is transmitted over a 2-wire twisted pair bus, all
+   EIA-485 transceivers interpret the voltage levels of the differential
+   signals with respect to a third common voltage. Without this common
+   reference, a set of transceivers may interpret the differential signals
+   incorrectly.
+   See [1] for more information.
+
+
+2. HARDWARE-RELATED CONSIDERATIONS
+
+   Some CPUs (e.g., Atmel AT91) contain a transceiver capable of working both
+   as RS232 and RS485. For these microcontrollers, the Linux driver should be
+   able 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
+   viceversa.
+   
+   On some other CPUs (e.g., Freescale imx25) the RS485 transceiver is not
+   integrated inside the microcontroller itself. Therefore, manufacturers who
+   use these microcontrollers to produce embedded boards need to connect an
+   external transceiver to some pin of the CPU.
+   On these architectures, therefore, no assumptions can be done at the
+   CPU-level about the presence of a RS485 transceiver, because the connection
+   (if any) is done outside the microcontroller. Moreover, even in case of
+   RS485 transceiver, the manufacturer is free to choose the CPU pin used for
+   the connection.
+
+
+3. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL
+
+   The Linux kernel provides the serial_rs485 structure (inside
+   include/linux/serial.h) 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 interfaces 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 specific device: */
+	int fd = open ("/dev/mydevice", O_RDWR); 
+	struct serial_rs485 rs485conf;
+
+	/* Set RS485 mode: */
+	rs485conf.flags |= SER_RS485_ENABLED;
+	
+	/* Set delay: */
+	rs485conf.delay_rts_before_send = 0x00000004;
+	ioctl (fd, TIOCSRS485, &rs485conf);
+	
+	close (fd);
+
+5. REFERENCES
+
+ [1]	http://en.wikipedia.org/wiki/Rs485
+
+
-- 
1.6.0.4