Subversion Repositories or1k

                       Linux Serial Console

To use a serial port as console you need to compile the support into your

kernel - by default it is not compiled in. For PC style serial ports

it's the config option next to "Standard/generic (dumb) serial support".

You must compile serial support into the kernel and not as a module.

It is possible to specify multiple devices for console output. You can

define a new kernel command line option to select which device(s) to

use for console output.

The format of this option is:

        console=device,options

        device:         tty0 for the foreground virtual console

                        ttyX for any other virtual console

                        ttySx for a serial port

                        lp0 for the first parallel port

        options:        depend on the driver. For the serial port this

                        defines the baudrate/parity/bits of the port,

                        in the format BBBBPN, where BBBB is the speed,

                        P is parity (n/o/e), and N is bits. Default is

                        9600n8. The maximum baudrate is 115200.

You can specify multiple console= options on the kernel command line.

Output will appear on all of them. The last device will be used when

you open /dev/console. So, for example:

        console=ttyS1,9600 console=tty0

defines that opening /dev/console will get you the current foreground

virtual console, and kernel messages will appear on both the VGA

console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.

Note that you can only define one console per device type (serial, video).

If no console device is specified, the first device found capable of

acting as a system console will be used. At this time, the system

first looks for a VGA card and then for a serial port. So if you don't

have a VGA card in your system the first serial port will automatically

become the console.

You will need to create a new device to use /dev/console. The official

/dev/console is now character device 5,1.

Here's an example that will use /dev/ttyS1 (COM2) as the console.

Replace the sample values as needed.

1. Create /dev/console (real console) and /dev/tty0 (master virtual

   console):

   cd /dev

   rm -f console tty0

   mknod -m 622 console c 5 1

   mknod -m 622 tty0 c 4 0

2. LILO can also take input from a serial device. This is a very

   useful option. To tell LILO to use the serial port:

   In lilo.conf (global section):

   serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)

3. Adjust to kernel flags for the new kernel,

   again in lilo.conf (kernel section)

   append = "console=ttyS1,9600"

4. Make sure a getty runs on the serial port so that you can login to

   it once the system is done booting. This is done by adding a line

   like this to /etc/inittab (exact syntax depends on your getty):

   S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100

5. Init and /etc/ioctl.save

   Sysvinit remembers its stty settings in a file in /etc, called

   `/etc/ioctl.save'. REMOVE THIS FILE before using the serial

   console for the first time, because otherwise init will probably

   set the baudrate to 38400 (baudrate of the virtual console).

6. /dev/console and X

   Programs that want to do something with the virtual console usually

   open /dev/console. If you have created the new /dev/console device,

   and your console is NOT the virtual console some programs will fail.

   Those are programs that want to access the VT interface, and use

   /dev/console instead of /dev/tty0. Some of those programs are:

   Xfree86, svgalib, gpm, SVGATextMode

   It should be fixed in modern versions of these programs though.

   Note that if you boot without a console= option (or with

   console=/dev/tty0), /dev/console is the same as /dev/tty0. In that

   case everything will still work.

7. Thanks

   Thanks to Geert Uytterhoeven 

   for porting the patches from 2.1.4x to 2.1.6x for taking care of

   the integration of these patches into m68k, ppc and alpha.

Miquel van Smoorenburg , 11-Jun-2000