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
  37 The behavior is well defined when each device type is mentioned only once.
  38 In this case, the output will appear on all requested consoles. And
  39 the last device will be used when you open ``/dev/console``.
  40 So, for example::
  41
  42         console=ttyS1,9600 console=tty0
  43
  44 defines that opening ``/dev/console`` will get you the current foreground
  45 virtual console, and kernel messages will appear on both the VGA
  46 console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
  47
  48 The behavior is more complicated when the same device type is defined more
  49 times. In this case, there are the following two rules:
  50
  51 1. The output will appear only on the first device of each defined type.
  52
  53 2. ``/dev/console`` will be associated with the first registered device.
  54    Where the registration order depends on how kernel initializes various
  55    subsystems.
  56
  57    This rule is used also when the last console= parameter is not used
  58    for other reasons. For example, because of a typo or because
  59    the hardware is not available.
  60
  61 The result might be surprising. For example, the following two command
  62 lines have the same result:
  63
  64         console=ttyS1,9600 console=tty0 console=tty1
  65         console=tty0 console=ttyS1,9600 console=tty1
  66
  67 The kernel messages are printed only on ``tty0`` and ``ttyS1``. And
  68 ``/dev/console`` gets associated with ``tty0``. It is because kernel
  69 tries to register graphical consoles before serial ones. It does it
  70 because of the default behavior when no console device is specified,
  71 see below.
  72
  73 Note that the last ``console=tty1`` parameter still makes a difference.
  74 The kernel command line is used also by systemd. It would use the last
  75 defined ``tty1`` as the login console.
  76
  77 If no console device is specified, the first device found capable of
  78 acting as a system console will be used. At this time, the system
  79 first looks for a VGA card and then for a serial port. So if you don't
  80 have a VGA card in your system the first serial port will automatically
  81 become the console.
  82
  83 You will need to create a new device to use ``/dev/console``. The official
  84 ``/dev/console`` is now character device 5,1.
  85
  86 (You can also use a network device as a console.  See
  87 ``Documentation/networking/netconsole.rst`` for information on that.)
  88
  89 Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
  90 Replace the sample values as needed.
  91
  92 1. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
  93    console)::
  94
  95      cd /dev
  96      rm -f console tty0
  97      mknod -m 622 console c 5 1
  98      mknod -m 622 tty0 c 4 0
  99
 100 2. LILO can also take input from a serial device. This is a very
 101    useful option. To tell LILO to use the serial port:
 102    In lilo.conf (global section)::
 103
 104      serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
 105
 106 3. Adjust to kernel flags for the new kernel,
 107    again in lilo.conf (kernel section)::
 108
 109      append = "console=ttyS1,9600"
 110
 111 4. Make sure a getty runs on the serial port so that you can login to
 112    it once the system is done booting. This is done by adding a line
 113    like this to ``/etc/inittab`` (exact syntax depends on your getty)::
 114
 115      S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
 116
 117 5. Init and ``/etc/ioctl.save``
 118
 119    Sysvinit remembers its stty settings in a file in ``/etc``, called
 120    ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
 121    console for the first time, because otherwise init will probably
 122    set the baudrate to 38400 (baudrate of the virtual console).
 123
 124 6. ``/dev/console`` and X
 125    Programs that want to do something with the virtual console usually
 126    open ``/dev/console``. If you have created the new ``/dev/console`` device,
 127    and your console is NOT the virtual console some programs will fail.
 128    Those are programs that want to access the VT interface, and use
 129    ``/dev/console instead of /dev/tty0``. Some of those programs are::
 130
 131      Xfree86, svgalib, gpm, SVGATextMode
 132
 133    It should be fixed in modern versions of these programs though.
 134
 135    Note that if you boot without a ``console=`` option (or with
 136    ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
 137    In that case everything will still work.
 138
 139 7. Thanks
 140
 141    Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
 142    for porting the patches from 2.1.4x to 2.1.6x for taking care of
 143    the integration of these patches into m68k, ppc and alpha.
 144
 145 Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000