1 .. _serial_console:
2
3 Linux Serial Console
4 ====================
5
6 To use a serial port as console you need to compile the support into your
7 kernel - by default it is not compiled in. For PC style serial ports
8 it's the config option next to menu option:
9
10 :menuselection:`Character devices --> Serial drivers --> 8250/16550 and compatible serial support --> Console on 8250/16550 and compatible serial port`
11
12 You must compile serial support into the kernel and not as a module.
13
14 It is possible to specify multiple devices for console output. You can
15 define a new kernel command line option to select which device(s) to
16 use for console output.
17
18 The format of this option is::
19
20 console=device,options
21
22 device: tty0 for the foreground virtual console
23 ttyX for any other virtual console
24 ttySx for a serial port
25 lp0 for the first parallel port
26 ttyUSB0 for the first USB serial device
27
28 options: depend on the driver. For the serial port this
29 defines the baudrate/parity/bits/flow control of
30 the port, in the format BBBBPNF, where BBBB is the
31 speed, P is parity (n/o/e), N is number of bits,
32 and F is flow control ('r' for RTS). Default is
33 9600n8. The maximum baudrate is 115200.
34
35 You can specify multiple console= options on the kernel command line.
36
37 The behavior is well defined when each device type is mentioned only once.
38 In this case, the output will appear on all requested consoles. And
39 the last device will be used when you open ``/dev/console``.
40 So, for example::
41
42 console=ttyS1,9600 console=tty0
43
44 defines that opening ``/dev/console`` will get you the current foreground
45 virtual console, and kernel messages will appear on both the VGA
46 console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
47
48 The behavior is more complicated when the same device type is defined more
49 times. In this case, there are the following two rules:
50
51 1. The output will appear only on the first device of each defined type.
52
53 2. ``/dev/console`` will be associated with the first registered device.
54 Where the registration order depends on how kernel initializes various
55 subsystems.
56
57 This rule is used also when the last console= parameter is not used
58 for other reasons. For example, because of a typo or because
59 the hardware is not available.
60
61 The result might be surprising. For example, the following two command
62 lines have the same result::
63
64 console=ttyS1,9600 console=tty0 console=tty1
65 console=tty0 console=ttyS1,9600 console=tty1
66
67 The kernel messages are printed only on ``tty0`` and ``ttyS1``. And
68 ``/dev/console`` gets associated with ``tty0``. It is because kernel
69 tries to register graphical consoles before serial ones. It does it
70 because of the default behavior when no console device is specified,
71 see below.
72
73 Note that the last ``console=tty1`` parameter still makes a difference.
74 The kernel command line is used also by systemd. It would use the last
75 defined ``tty1`` as the login console.
76
77 If no console device is specified, the first device found capable of
78 acting as a system console will be used. At this time, the system
79 first looks for a VGA card and then for a serial port. So if you don't
80 have a VGA card in your system the first serial port will automatically
81 become the console.
82
83 You will need to create a new device to use ``/dev/console``. The official
84 ``/dev/console`` is now character device 5,1.
85
86 (You can also use a network device as a console. See
87 ``Documentation/networking/netconsole.rst`` for information on that.)
88
89 Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
90 Replace the sample values as needed.
91
92 1. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
93 console)::
94
95 cd /dev
96 rm -f console tty0
97 mknod -m 622 console c 5 1
98 mknod -m 622 tty0 c 4 0
99
100 2. LILO can also take input from a serial device. This is a very
101 useful option. To tell LILO to use the serial port:
102 In lilo.conf (global section)::
103
104 serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
105
106 3. Adjust to kernel flags for the new kernel,
107 again in lilo.conf (kernel section)::
108
109 append = "console=ttyS1,9600"
110
111 4. Make sure a getty runs on the serial port so that you can login to
112 it once the system is done booting. This is done by adding a line
113 like this to ``/etc/inittab`` (exact syntax depends on your getty)::
114
115 S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
116
117 5. Init and ``/etc/ioctl.save``
118
119 Sysvinit remembers its stty settings in a file in ``/etc``, called
120 ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
121 console for the first time, because otherwise init will probably
122 set the baudrate to 38400 (baudrate of the virtual console).
123
124 6. ``/dev/console`` and X
125 Programs that want to do something with the virtual console usually
126 open ``/dev/console``. If you have created the new ``/dev/console`` device,
127 and your console is NOT the virtual console some programs will fail.
128 Those are programs that want to access the VT interface, and use
129 ``/dev/console instead of /dev/tty0``. Some of those programs are::
130
131 Xfree86, svgalib, gpm, SVGATextMode
132
133 It should be fixed in modern versions of these programs though.
134
135 Note that if you boot without a ``console=`` option (or with
136 ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
137 In that case everything will still work.
138
139 7. Thanks
140
141 Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
142 for porting the patches from 2.1.4x to 2.1.6x for taking care of
143 the integration of these patches into m68k, ppc and alpha.
144
145 Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
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