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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 | ||