Documentation/admin-guide/serial-console.rst

   1 .. _serial_console:
   2
   3 Linux Serial Console
   4 ====================
   5
   6 To use a serial port as console you need to compile the support into your
   7 kernel - by default it is not compiled in. For PC style serial ports
   8 it's the config option next to menu option:
   9
  10 :menuselection:`Character devices --> Serial drivers --> 8250/16550 and compatible serial support --> Console on 8250/16550 and compatible serial port`
  11
  12 You must compile serial support into the kernel and not as a module.
  13
  14 It is possible to specify multiple devices for console output. You can
  15 define a new kernel command line option to select which device(s) to
  16 use for console output.
  17
  18 The format of this option is::
  19
  20         console=device,options
  21
  22         device:         tty0 for the foreground virtual console
  23                         ttyX for any other virtual console
  24                         ttySx for a serial port
  25                         lp0 for the first parallel port
  26                         ttyUSB0 for the first USB serial device
  27
  28         options:        depend on the driver. For the serial port this
  29                         defines the baudrate/parity/bits/flow control of
  30                         the port, in the format BBBBPNF, where BBBB is the
  31                         speed, P is parity (n/o/e), N is number of bits,
  32                         and F is flow control ('r' for RTS). Default is
  33                         9600n8. The maximum baudrate is 115200.
  34
  35 You can specify multiple console= options on the kernel command line.
  36 Output will appear on all of them. The last device will be used when
  37 you open ``/dev/console``. So, for example::
  38
  39         console=ttyS1,9600 console=tty0
  40
  41 defines that opening ``/dev/console`` will get you the current foreground
  42 virtual console, and kernel messages will appear on both the VGA
  43 console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
  44
  45 Note that you can only define one console per device type (serial, video).
  46
  47 If no console device is specified, the first device found capable of
  48 acting as a system console will be used. At this time, the system
  49 first looks for a VGA card and then for a serial port. So if you don't
  50 have a VGA card in your system the first serial port will automatically
  51 become the console.
  52
  53 You will need to create a new device to use ``/dev/console``. The official
  54 ``/dev/console`` is now character device 5,1.
  55
  56 (You can also use a network device as a console.  See
  57 ``Documentation/networking/netconsole.txt`` for information on that.)
  58
  59 Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
  60 Replace the sample values as needed.
  61
  62 1. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
  63    console)::
  64
  65      cd /dev
  66      rm -f console tty0
  67      mknod -m 622 console c 5 1
  68      mknod -m 622 tty0 c 4 0
  69
  70 2. LILO can also take input from a serial device. This is a very
  71    useful option. To tell LILO to use the serial port:
  72    In lilo.conf (global section)::
  73
  74      serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
  75
  76 3. Adjust to kernel flags for the new kernel,
  77    again in lilo.conf (kernel section)::
  78
  79      append = "console=ttyS1,9600"
  80
  81 4. Make sure a getty runs on the serial port so that you can login to
  82    it once the system is done booting. This is done by adding a line
  83    like this to ``/etc/inittab`` (exact syntax depends on your getty)::
  84
  85      S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
  86
  87 5. Init and ``/etc/ioctl.save``
  88
  89    Sysvinit remembers its stty settings in a file in ``/etc``, called
  90    ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
  91    console for the first time, because otherwise init will probably
  92    set the baudrate to 38400 (baudrate of the virtual console).
  93
  94 6. ``/dev/console`` and X
  95    Programs that want to do something with the virtual console usually
  96    open ``/dev/console``. If you have created the new ``/dev/console`` device,
  97    and your console is NOT the virtual console some programs will fail.
  98    Those are programs that want to access the VT interface, and use
  99    ``/dev/console instead of /dev/tty0``. Some of those programs are::
 100
 101      Xfree86, svgalib, gpm, SVGATextMode
 102
 103    It should be fixed in modern versions of these programs though.
 104
 105    Note that if you boot without a ``console=`` option (or with
 106    ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
 107    In that case everything will still work.
 108
 109 7. Thanks
 110
 111    Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
 112    for porting the patches from 2.1.4x to 2.1.6x for taking care of
 113    the integration of these patches into m68k, ppc and alpha.
 114
 115 Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000