/Documentation/serial-console.txt

  1                       Linux Serial Console
  2
  3To use a serial port as console you need to compile the support into your
  4kernel - by default it is not compiled in. For PC style serial ports
  5it's the config option next to "Standard/generic (dumb) serial support".
  6You must compile serial support into the kernel and not as a module.
  7
  8It is possible to specify multiple devices for console output. You can
  9define a new kernel command line option to select which device(s) to
 10use for console output.
 11
 12The format of this option is:
 13
 14	console=device,options
 15
 16	device:		tty0 for the foreground virtual console
 17			ttyX for any other virtual console
 18			ttySx for a serial port
 19			lp0 for the first parallel port
 20			ttyUSB0 for the first USB serial device
 21
 22	options:	depend on the driver. For the serial port this
 23			defines the baudrate/parity/bits/flow control of
 24			the port, in the format BBBBPNF, where BBBB is the
 25			speed, P is parity (n/o/e), N is number of bits,
 26			and F is flow control ('r' for RTS). Default is
 27			9600n8. The maximum baudrate is 115200.
 28
 29You can specify multiple console= options on the kernel command line.
 30Output will appear on all of them. The last device will be used when
 31you open /dev/console. So, for example:
 32
 33	console=ttyS1,9600 console=tty0
 34
 35defines that opening /dev/console will get you the current foreground
 36virtual console, and kernel messages will appear on both the VGA
 37console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
 38
 39Note that you can only define one console per device type (serial, video).
 40
 41If no console device is specified, the first device found capable of
 42acting as a system console will be used. At this time, the system
 43first looks for a VGA card and then for a serial port. So if you don't
 44have a VGA card in your system the first serial port will automatically
 45become the console.
 46
 47You will need to create a new device to use /dev/console. The official
 48/dev/console is now character device 5,1.
 49
 50(You can also use a network device as a console.  See
 51Documentation/networking/netconsole.txt for information on that.)
 52
 53Here's an example that will use /dev/ttyS1 (COM2) as the console.
 54Replace the sample values as needed.
 55
 561. Create /dev/console (real console) and /dev/tty0 (master virtual
 57   console):
 58
 59   cd /dev
 60   rm -f console tty0
 61   mknod -m 622 console c 5 1
 62   mknod -m 622 tty0 c 4 0
 63
 642. LILO can also take input from a serial device. This is a very
 65   useful option. To tell LILO to use the serial port:
 66   In lilo.conf (global section): 
 67
 68   serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
 69
 703. Adjust to kernel flags for the new kernel,
 71   again in lilo.conf (kernel section)
 72
 73   append = "console=ttyS1,9600" 
 74
 754. Make sure a getty runs on the serial port so that you can login to
 76   it once the system is done booting. This is done by adding a line
 77   like this to /etc/inittab (exact syntax depends on your getty):
 78
 79   S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
 80
 815. Init and /etc/ioctl.save
 82
 83   Sysvinit remembers its stty settings in a file in /etc, called
 84   `/etc/ioctl.save'. REMOVE THIS FILE before using the serial
 85   console for the first time, because otherwise init will probably
 86   set the baudrate to 38400 (baudrate of the virtual console).
 87
 886. /dev/console and X
 89   Programs that want to do something with the virtual console usually
 90   open /dev/console. If you have created the new /dev/console device,
 91   and your console is NOT the virtual console some programs will fail.
 92   Those are programs that want to access the VT interface, and use
 93   /dev/console instead of /dev/tty0. Some of those programs are:
 94
 95   Xfree86, svgalib, gpm, SVGATextMode
 96
 97   It should be fixed in modern versions of these programs though.
 98
 99   Note that if you boot without a console= option (or with
100   console=/dev/tty0), /dev/console is the same as /dev/tty0. In that
101   case everything will still work.
102
1037. Thanks
104
105   Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
106   for porting the patches from 2.1.4x to 2.1.6x for taking care of
107   the integration of these patches into m68k, ppc and alpha.
108
109Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000