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