Linux Serial Console

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

        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. 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).

5. /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

   I have binary patched the above mentioned programs to use "tty0"
   instead of "console".  This will be reported to the maintainers of
   said programs.

   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.

6. Thanks

   Thanks to Geert Uytterhoeven <Geert.Uytterhoeven@cs.kuleuven.ac.be>
   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 <miquels@cistron.nl>, 21-Mar-1998