1 .. _serial_console: 1 .. _serial_console:
2 2
3 Linux Serial Console 3 Linux Serial Console
4 ==================== 4 ====================
5 5
6 To use a serial port as console you need to co 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 7 kernel - by default it is not compiled in. For PC style serial ports
8 it's the config option next to menu option: 8 it's the config option next to menu option:
9 9
10 :menuselection:`Character devices --> Serial d 10 :menuselection:`Character devices --> Serial drivers --> 8250/16550 and compatible serial support --> Console on 8250/16550 and compatible serial port`
11 11
12 You must compile serial support into the kerne 12 You must compile serial support into the kernel and not as a module.
13 13
14 It is possible to specify multiple devices for 14 It is possible to specify multiple devices for console output. You can
15 define a new kernel command line option to sel 15 define a new kernel command line option to select which device(s) to
16 use for console output. 16 use for console output.
17 17
18 The format of this option is:: 18 The format of this option is::
19 19
20 console=device,options 20 console=device,options
21 21
22 device: tty0 for the foregroun 22 device: tty0 for the foreground virtual console
23 ttyX for any other vir 23 ttyX for any other virtual console
24 ttySx for a serial por 24 ttySx for a serial port
25 lp0 for the first para 25 lp0 for the first parallel port
26 ttyUSB0 for the first 26 ttyUSB0 for the first USB serial device
27 27
28 options: depend on the driver. 28 options: depend on the driver. For the serial port this
29 defines the baudrate/p 29 defines the baudrate/parity/bits/flow control of
30 the port, in the forma 30 the port, in the format BBBBPNF, where BBBB is the
31 speed, P is parity (n/ 31 speed, P is parity (n/o/e), N is number of bits,
32 and F is flow control 32 and F is flow control ('r' for RTS). Default is
33 9600n8. The maximum ba 33 9600n8. The maximum baudrate is 115200.
34 34
35 You can specify multiple console= options on t 35 You can specify multiple console= options on the kernel command line.
36 !! 36 Output will appear on all of them. The last device will be used when
37 The behavior is well defined when each device !! 37 you open ``/dev/console``. So, for example::
38 In this case, the output will appear on all re <<
39 the last device will be used when you open ``/ <<
40 So, for example:: <<
41 38
42 console=ttyS1,9600 console=tty0 39 console=ttyS1,9600 console=tty0
43 40
44 defines that opening ``/dev/console`` will get 41 defines that opening ``/dev/console`` will get you the current foreground
45 virtual console, and kernel messages will appe 42 virtual console, and kernel messages will appear on both the VGA
46 console and the 2nd serial port (ttyS1 or COM2 43 console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
47 44
48 The behavior is more complicated when the same !! 45 Note that you can only define one console per device type (serial, video).
49 times. In this case, there are the following t <<
50 <<
51 1. The output will appear only on the first de <<
52 <<
53 2. ``/dev/console`` will be associated with th <<
54 Where the registration order depends on how <<
55 subsystems. <<
56 <<
57 This rule is used also when the last consol <<
58 for other reasons. For example, because of <<
59 the hardware is not available. <<
60 <<
61 The result might be surprising. For example, t <<
62 lines have the same result:: <<
63 <<
64 console=ttyS1,9600 console=tty0 consol <<
65 console=tty0 console=ttyS1,9600 consol <<
66 <<
67 The kernel messages are printed only on ``tty0 <<
68 ``/dev/console`` gets associated with ``tty0`` <<
69 tries to register graphical consoles before se <<
70 because of the default behavior when no consol <<
71 see below. <<
72 <<
73 Note that the last ``console=tty1`` parameter <<
74 The kernel command line is used also by system <<
75 defined ``tty1`` as the login console. <<
76 46
77 If no console device is specified, the first d 47 If no console device is specified, the first device found capable of
78 acting as a system console will be used. At th 48 acting as a system console will be used. At this time, the system
79 first looks for a VGA card and then for a seri 49 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 seria 50 have a VGA card in your system the first serial port will automatically
81 become the console. 51 become the console.
82 52
83 You will need to create a new device to use `` 53 You will need to create a new device to use ``/dev/console``. The official
84 ``/dev/console`` is now character device 5,1. 54 ``/dev/console`` is now character device 5,1.
85 55
86 (You can also use a network device as a consol 56 (You can also use a network device as a console. See
87 ``Documentation/networking/netconsole.rst`` fo !! 57 ``Documentation/networking/netconsole.txt`` for information on that.)
88 58
89 Here's an example that will use ``/dev/ttyS1`` 59 Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
90 Replace the sample values as needed. 60 Replace the sample values as needed.
91 61
92 1. Create ``/dev/console`` (real console) and 62 1. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
93 console):: 63 console)::
94 64
95 cd /dev 65 cd /dev
96 rm -f console tty0 66 rm -f console tty0
97 mknod -m 622 console c 5 1 67 mknod -m 622 console c 5 1
98 mknod -m 622 tty0 c 4 0 68 mknod -m 622 tty0 c 4 0
99 69
100 2. LILO can also take input from a serial devi 70 2. LILO can also take input from a serial device. This is a very
101 useful option. To tell LILO to use the seri 71 useful option. To tell LILO to use the serial port:
102 In lilo.conf (global section):: 72 In lilo.conf (global section)::
103 73
104 serial = 1,9600n8 (ttyS1, 9600 bd, no pa 74 serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
105 75
106 3. Adjust to kernel flags for the new kernel, 76 3. Adjust to kernel flags for the new kernel,
107 again in lilo.conf (kernel section):: 77 again in lilo.conf (kernel section)::
108 78
109 append = "console=ttyS1,9600" 79 append = "console=ttyS1,9600"
110 80
111 4. Make sure a getty runs on the serial port s 81 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 82 it once the system is done booting. This is done by adding a line
113 like this to ``/etc/inittab`` (exact syntax 83 like this to ``/etc/inittab`` (exact syntax depends on your getty)::
114 84
115 S1:23:respawn:/sbin/getty -L ttyS1 9600 v 85 S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
116 86
117 5. Init and ``/etc/ioctl.save`` 87 5. Init and ``/etc/ioctl.save``
118 88
119 Sysvinit remembers its stty settings in a f 89 Sysvinit remembers its stty settings in a file in ``/etc``, called
120 ``/etc/ioctl.save``. REMOVE THIS FILE befor 90 ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
121 console for the first time, because otherwi 91 console for the first time, because otherwise init will probably
122 set the baudrate to 38400 (baudrate of the 92 set the baudrate to 38400 (baudrate of the virtual console).
123 93
124 6. ``/dev/console`` and X 94 6. ``/dev/console`` and X
125 Programs that want to do something with the 95 Programs that want to do something with the virtual console usually
126 open ``/dev/console``. If you have created 96 open ``/dev/console``. If you have created the new ``/dev/console`` device,
127 and your console is NOT the virtual console 97 and your console is NOT the virtual console some programs will fail.
128 Those are programs that want to access the 98 Those are programs that want to access the VT interface, and use
129 ``/dev/console instead of /dev/tty0``. Some 99 ``/dev/console instead of /dev/tty0``. Some of those programs are::
130 100
131 Xfree86, svgalib, gpm, SVGATextMode 101 Xfree86, svgalib, gpm, SVGATextMode
132 102
133 It should be fixed in modern versions of th 103 It should be fixed in modern versions of these programs though.
134 104
135 Note that if you boot without a ``console=` 105 Note that if you boot without a ``console=`` option (or with
136 ``console=/dev/tty0``), ``/dev/console`` is 106 ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
137 In that case everything will still work. 107 In that case everything will still work.
138 108
139 7. Thanks 109 7. Thanks
140 110
141 Thanks to Geert Uytterhoeven <geert@linux-m6 111 Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
142 for porting the patches from 2.1.4x to 2.1. 112 for porting the patches from 2.1.4x to 2.1.6x for taking care of
143 the integration of these patches into m68k, 113 the integration of these patches into m68k, ppc and alpha.
144 114
145 Miquel van Smoorenburg <miquels@cistron.nl>, 11 115 Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
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