Linux Serial Console

  1                        Linux Serial Console
  2 
  3 It is possible to specify multiple devices for console output. You
can
  4 define a new kernel command line option to select which device(s) to
  5 use for console output.
  6 
  7 The format of this option is:
  8 
  9         console=device,options
 10 
 11         device:         tty0 for the foreground virtual console
 12                         ttyX for any other virtual console
 13                         ttySx for a serial port
 14 
 15         options:        depend on the driver. For the serial port
this
 16                         defines the baudrate/parity/bits of the
port,
 17                         in the format BBBBPN, where BBBB is the
speed,
 18                         P is parity (n/o/e), and N is bits. Default
is
 19                         9600n8. The maximum baudrate is 115200.
 20 
 21 You can specify multiple console= options on the kernel command
line.
 22 Output will appear on all of them. The last device will be used when
 23 you open /dev/console. So, for example:
 24 
 25         console=ttyS1,9600 console=tty0
 26 
 27 defines that opening /dev/console will get you the current
foreground
 28 virtual console, and kernel messages will appear on both the VGA
 29 console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
 30 
 31 Note that you can only define one console per device type (serial,
video).
 32 
 33 If no console device is specified, the first device found capable of
 34 acting as a system console will be used. At this time, the system
 35 first looks for a VGA card and then for a serial port. So if you
don't
 36 have a VGA card in your system the first serial port will
automatically
 37 become the console.
 38 
 39 You will need to create a new device to use /dev/console. The
official
 40 /dev/console is now character device 5,1.
 41 
 42 Here's an example that will use /dev/ttyS1 (COM2) as the console.
 43 Replace the sample values as needed.
 44 
 45 1. Create /dev/console (real console) and /dev/tty0 (master virtual
 46    console):
 47 
 48    cd /dev
 49    rm -f console tty0
 50    mknod -m 622 console c 5 1
 51    mknod -m 622 tty0 c 4 0
 52 
 53 2. LILO can also take input from a serial device. This is a very
 54    useful option. To tell LILO to use the serial port:
 55    In lilo.conf (global section): 
 56 
 57    serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
 58 
 59 3. Adjust to kernel flags for the new kernel,
 60    again in lilo.conf (kernel section)
 61 
 62    append = "console=ttyS1,9600" 
 63 
 64 4. Init and /etc/ioctl.save
 65 
 66    Sysvinit remembers its stty settings in a file in /etc, called
 67    `/etc/ioctl.save'. REMOVE THIS FILE before using the serial
 68    console for the first time, because otherwise init will probably
 69    set the baudrate to 38400 (baudrate of the virtual console).
 70 
 71 5. /dev/console and X
 72    Programs that want to do something with the virtual console
usually
 73    open /dev/console. If you have created the new /dev/console
device,
 74    and your console is NOT the virtual console some programs will
fail.
 75    Those are programs that want to access the VT interface, and use
 76    /dev/console instead of /dev/tty0. Some of those programs are:
 77 
 78    Xfree86, svgalib, gpm, SVGATextMode
 79 
 80    I have binary patched the above mentioned programs to use "tty0"
 81    instead of "console".  This will be reported to the maintainers
of
 82    said programs.
 83 
 84    Note that if you boot without a console= option (or with
 85    console=/dev/tty0), /dev/console is the same as /dev/tty0. In
that
 86    case everything will still work.
 87 
 88 6. Thanks
 89 
 90    Thanks to Geert Uytterhoeven  for porting
 91    the patches from 2.1.4x to 2.1.6x and for taking care of the
 92    integration of these patches into m68k, ppc and alpha.
 93 
 94 Miquel van Smoorenburg , 21-Mar-1998


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