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Mauro Carvalho Chehab8e7fbec2016-09-21 14:44:59 -03001.. _serial_console:
2
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -03003Linux Serial Console
4====================
Linus Torvalds1da177e2005-04-16 15:20:36 -07005
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
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -03008it'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
Linus Torvalds1da177e2005-04-16 15:20:36 -070012You 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
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -030018The format of this option is::
Linus Torvalds1da177e2005-04-16 15:20:36 -070019
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
Randy Dunlapf1a1c2d2006-03-25 03:08:17 -080026 ttyUSB0 for the first USB serial device
Linus Torvalds1da177e2005-04-16 15:20:36 -070027
28 options: depend on the driver. For the serial port this
Randy Dunlapf1a1c2d2006-03-25 03:08:17 -080029 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
Linus Torvalds1da177e2005-04-16 15:20:36 -070033 9600n8. The maximum baudrate is 115200.
34
35You can specify multiple console= options on the kernel command line.
Petr Mladek571079f2023-03-08 12:24:33 +010036
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::
Linus Torvalds1da177e2005-04-16 15:20:36 -070041
42 console=ttyS1,9600 console=tty0
43
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -030044defines that opening ``/dev/console`` will get you the current foreground
Linus Torvalds1da177e2005-04-16 15:20:36 -070045virtual console, and kernel messages will appear on both the VGA
46console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
47
Petr Mladek571079f2023-03-08 12:24:33 +010048The 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
Andrei Emeltchenkoac6804f2023-08-25 12:16:26 +030062lines have the same result::
Petr Mladek571079f2023-03-08 12:24:33 +010063
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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070076
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
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -030083You will need to create a new device to use ``/dev/console``. The official
84``/dev/console`` is now character device 5,1.
Linus Torvalds1da177e2005-04-16 15:20:36 -070085
Randy Dunlapf1a1c2d2006-03-25 03:08:17 -080086(You can also use a network device as a console. See
Mauro Carvalho Chehabd9d6ef22020-04-30 18:04:02 +020087``Documentation/networking/netconsole.rst`` for information on that.)
Randy Dunlapf1a1c2d2006-03-25 03:08:17 -080088
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -030089Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
Linus Torvalds1da177e2005-04-16 15:20:36 -070090Replace the sample values as needed.
91
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300921. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
93 console)::
Linus Torvalds1da177e2005-04-16 15:20:36 -070094
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -030095 cd /dev
96 rm -f console tty0
97 mknod -m 622 console c 5 1
98 mknod -m 622 tty0 c 4 0
Linus Torvalds1da177e2005-04-16 15:20:36 -070099
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:
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300102 In lilo.conf (global section)::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700103
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300104 serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700105
1063. Adjust to kernel flags for the new kernel,
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300107 again in lilo.conf (kernel section)::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700108
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300109 append = "console=ttyS1,9600"
Linus Torvalds1da177e2005-04-16 15:20:36 -0700110
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
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300113 like this to ``/etc/inittab`` (exact syntax depends on your getty)::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700114
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300115 S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
Linus Torvalds1da177e2005-04-16 15:20:36 -0700116
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -03001175. Init and ``/etc/ioctl.save``
Linus Torvalds1da177e2005-04-16 15:20:36 -0700118
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300119 Sysvinit remembers its stty settings in a file in ``/etc``, called
120 ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
Linus Torvalds1da177e2005-04-16 15:20:36 -0700121 console for the first time, because otherwise init will probably
122 set the baudrate to 38400 (baudrate of the virtual console).
123
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -03001246. ``/dev/console`` and X
Linus Torvalds1da177e2005-04-16 15:20:36 -0700125 Programs that want to do something with the virtual console usually
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300126 open ``/dev/console``. If you have created the new ``/dev/console`` device,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700127 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
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300129 ``/dev/console instead of /dev/tty0``. Some of those programs are::
Linus Torvalds1da177e2005-04-16 15:20:36 -0700130
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300131 Xfree86, svgalib, gpm, SVGATextMode
Linus Torvalds1da177e2005-04-16 15:20:36 -0700132
133 It should be fixed in modern versions of these programs though.
134
Mauro Carvalho Chehabc2ffd5d2016-09-21 14:08:10 -0300135 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700138
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