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Documentation / serial-console.txt

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Based on kernel version 4.9. Page generated on 2016-12-21 14:37 EST.

1	                       Linux Serial Console
3	To use a serial port as console you need to compile the support into your
4	kernel - by default it is not compiled in. For PC style serial ports
5	it's the config option next to "Standard/generic (dumb) serial support".
6	You must compile serial support into the kernel and not as a module.
8	It is possible to specify multiple devices for console output. You can
9	define a new kernel command line option to select which device(s) to
10	use for console output.
12	The format of this option is:
14		console=device,options
16		device:		tty0 for the foreground virtual console
17				ttyX for any other virtual console
18				ttySx for a serial port
19				lp0 for the first parallel port
20				ttyUSB0 for the first USB serial device
22		options:	depend on the driver. For the serial port this
23				defines the baudrate/parity/bits/flow control of
24				the port, in the format BBBBPNF, where BBBB is the
25				speed, P is parity (n/o/e), N is number of bits,
26				and F is flow control ('r' for RTS). Default is
27				9600n8. The maximum baudrate is 115200.
29	You can specify multiple console= options on the kernel command line.
30	Output will appear on all of them. The last device will be used when
31	you open /dev/console. So, for example:
33		console=ttyS1,9600 console=tty0
35	defines that opening /dev/console will get you the current foreground
36	virtual console, and kernel messages will appear on both the VGA
37	console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
39	Note that you can only define one console per device type (serial, video).
41	If no console device is specified, the first device found capable of
42	acting as a system console will be used. At this time, the system
43	first looks for a VGA card and then for a serial port. So if you don't
44	have a VGA card in your system the first serial port will automatically
45	become the console.
47	You will need to create a new device to use /dev/console. The official
48	/dev/console is now character device 5,1.
50	(You can also use a network device as a console.  See
51	Documentation/networking/netconsole.txt for information on that.)
53	Here's an example that will use /dev/ttyS1 (COM2) as the console.
54	Replace the sample values as needed.
56	1. Create /dev/console (real console) and /dev/tty0 (master virtual
57	   console):
59	   cd /dev
60	   rm -f console tty0
61	   mknod -m 622 console c 5 1
62	   mknod -m 622 tty0 c 4 0
64	2. LILO can also take input from a serial device. This is a very
65	   useful option. To tell LILO to use the serial port:
66	   In lilo.conf (global section): 
68	   serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
70	3. Adjust to kernel flags for the new kernel,
71	   again in lilo.conf (kernel section)
73	   append = "console=ttyS1,9600" 
75	4. Make sure a getty runs on the serial port so that you can login to
76	   it once the system is done booting. This is done by adding a line
77	   like this to /etc/inittab (exact syntax depends on your getty):
79	   S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
81	5. Init and /etc/ioctl.save
83	   Sysvinit remembers its stty settings in a file in /etc, called
84	   `/etc/ioctl.save'. REMOVE THIS FILE before using the serial
85	   console for the first time, because otherwise init will probably
86	   set the baudrate to 38400 (baudrate of the virtual console).
88	6. /dev/console and X
89	   Programs that want to do something with the virtual console usually
90	   open /dev/console. If you have created the new /dev/console device,
91	   and your console is NOT the virtual console some programs will fail.
92	   Those are programs that want to access the VT interface, and use
93	   /dev/console instead of /dev/tty0. Some of those programs are:
95	   Xfree86, svgalib, gpm, SVGATextMode
97	   It should be fixed in modern versions of these programs though.
99	   Note that if you boot without a console= option (or with
100	   console=/dev/tty0), /dev/console is the same as /dev/tty0. In that
101	   case everything will still work.
103	7. Thanks
105	   Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
106	   for porting the patches from 2.1.4x to 2.1.6x for taking care of
107	   the integration of these patches into m68k, ppc and alpha.
109	Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
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