FreeBSD/Linux Kernel Cross Reference
sys/Documentation/serial-console.txt

                            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
                            ttyUSB0 for the first USB serial device
    
            options:        depend on the driver. For the serial port this
                            defines the baudrate/parity/bits/flow control of
                            the port, in the format BBBBPNF, where BBBB is the
                            speed, P is parity (n/o/e), N is number of bits,
                            and F is flow control ('r' for RTS). 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.
    
    (You can also use a network device as a console.  See
    Documentation/networking/netconsole.txt for information on that.)
    
    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 <geert@linux-m68k.org>
      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>, 11-Jun-2000

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