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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 | 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 | |
29 | You can specify multiple console= options on the kernel command line. |
30 | Output will appear on all of them. The last device will be used when |
31 | you open /dev/console. So, for example: |
32 | |
33 | console=ttyS1,9600 console=tty0 |
34 | |
35 | defines that opening /dev/console will get you the current foreground |
36 | virtual console, and kernel messages will appear on both the VGA |
37 | console and the 2nd serial port (ttyS1 or COM2) at 9600 baud. |
38 | |
39 | Note that you can only define one console per device type (serial, video). |
40 | |
41 | If no console device is specified, the first device found capable of |
42 | acting as a system console will be used. At this time, the system |
43 | first looks for a VGA card and then for a serial port. So if you don't |
44 | have a VGA card in your system the first serial port will automatically |
45 | become the console. |
46 | |
47 | You 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 |
51 | Documentation/networking/netconsole.txt for information on that.) |
52 | |
53 | Here's an example that will use /dev/ttyS1 (COM2) as the console. |
54 | Replace the sample values as needed. |
55 | |
56 | 1. 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 | |
64 | 2. 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 | |
70 | 3. Adjust to kernel flags for the new kernel, |
71 | again in lilo.conf (kernel section) |
72 | |
73 | append = "console=ttyS1,9600" |
74 | |
75 | 4. 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 | |
81 | 5. 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 | |
88 | 6. /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 | |
103 | 7. 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 | |
109 | Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000 |
110 |
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