1.. _serial_console:
2
3Linux Serial Console
4====================
5
6To use a serial port as console you need to compile the support into your
7kernel - by default it is not compiled in. For PC style serial ports
8it'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
12You must compile serial support into the kernel and not as a module.
13
14It is possible to specify multiple devices for console output. You can
15define a new kernel command line option to select which device(s) to
16use for console output.
17
18The 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
35You can specify multiple console= options on the kernel command line.
36
37The behavior is well defined when each device type is mentioned only once.
38In this case, the output will appear on all requested consoles. And
39the last device will be used when you open ``/dev/console``.
40So, for example::
41
42	console=ttyS1,9600 console=tty0
43
44defines that opening ``/dev/console`` will get you the current foreground
45virtual console, and kernel messages will appear on both the VGA
46console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
47
48The behavior is more complicated when the same device type is defined more
49times. In this case, there are the following two rules:
50
511. The output will appear only on the first device of each defined type.
52
532. ``/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
61The result might be surprising. For example, the following two command
62lines have the same result::
63
64	console=ttyS1,9600 console=tty0 console=tty1
65	console=tty0 console=ttyS1,9600 console=tty1
66
67The kernel messages are printed only on ``tty0`` and ``ttyS1``. And
68``/dev/console`` gets associated with ``tty0``. It is because kernel
69tries to register graphical consoles before serial ones. It does it
70because of the default behavior when no console device is specified,
71see below.
72
73Note that the last ``console=tty1`` parameter still makes a difference.
74The kernel command line is used also by systemd. It would use the last
75defined ``tty1`` as the login console.
76
77If no console device is specified, the first device found capable of
78acting as a system console will be used. At this time, the system
79first looks for a VGA card and then for a serial port. So if you don't
80have a VGA card in your system the first serial port will automatically
81become the console.
82
83You 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
89Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
90Replace the sample values as needed.
91
921. 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
1002. 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
1063. Adjust to kernel flags for the new kernel,
107   again in lilo.conf (kernel section)::
108
109     append = "console=ttyS1,9600"
110
1114. 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
1175. 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
1246. ``/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
1397. 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
145Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
146