serial-console.rst 4.3 KB

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