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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
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.
36Output will appear on all of them. The last device will be used when
37you open ``/dev/console``. So, for example::
38
39 console=ttyS1,9600 console=tty0
40
41defines that opening ``/dev/console`` will get you the current foreground
42virtual console, and kernel messages will appear on both the VGA
43console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
44
45Note that you can only define one console per device type (serial, video).
46
47If no console device is specified, the first device found capable of
48acting as a system console will be used. At this time, the system
49first looks for a VGA card and then for a serial port. So if you don't
50have a VGA card in your system the first serial port will automatically
51become the console.
52
53You will need to create a new device to use ``/dev/console``. The official
54``/dev/console`` is now character device 5,1.
55
56(You can also use a network device as a console. See
57``Documentation/networking/netconsole.txt`` for information on that.)
58
59Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
60Replace the sample values as needed.
61
621. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
63 console)::
64
65 cd /dev
66 rm -f console tty0
67 mknod -m 622 console c 5 1
68 mknod -m 622 tty0 c 4 0
69
702. LILO can also take input from a serial device. This is a very
71 useful option. To tell LILO to use the serial port:
72 In lilo.conf (global section)::
73
74 serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
75
763. Adjust to kernel flags for the new kernel,
77 again in lilo.conf (kernel section)::
78
79 append = "console=ttyS1,9600"
80
814. Make sure a getty runs on the serial port so that you can login to
82 it once the system is done booting. This is done by adding a line
83 like this to ``/etc/inittab`` (exact syntax depends on your getty)::
84
85 S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
86
875. Init and ``/etc/ioctl.save``
88
89 Sysvinit remembers its stty settings in a file in ``/etc``, called
90 ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
91 console for the first time, because otherwise init will probably
92 set the baudrate to 38400 (baudrate of the virtual console).
93
946. ``/dev/console`` and X
95 Programs that want to do something with the virtual console usually
96 open ``/dev/console``. If you have created the new ``/dev/console`` device,
97 and your console is NOT the virtual console some programs will fail.
98 Those are programs that want to access the VT interface, and use
99 ``/dev/console instead of /dev/tty0``. Some of those programs are::
100
101 Xfree86, svgalib, gpm, SVGATextMode
102
103 It should be fixed in modern versions of these programs though.
104
105 Note that if you boot without a ``console=`` option (or with
106 ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
107 In that case everything will still work.
108
1097. Thanks
110
111 Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
112 for porting the patches from 2.1.4x to 2.1.6x for taking care of
113 the integration of these patches into m68k, ppc and alpha.
114
115Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000