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, unless the kernel is configured with the 82CONFIG_NULL_TTY_DEFAULT_CONSOLE option, then it will default to using the 83ttynull device. 84 85You will need to create a new device to use ``/dev/console``. The official 86``/dev/console`` is now character device 5,1. 87 88(You can also use a network device as a console. See 89``Documentation/networking/netconsole.rst`` for information on that.) 90 91Here's an example that will use ``/dev/ttyS1`` (COM2) as the console. 92Replace the sample values as needed. 93 941. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual 95 console):: 96 97 cd /dev 98 rm -f console tty0 99 mknod -m 622 console c 5 1 100 mknod -m 622 tty0 c 4 0 101 1022. LILO can also take input from a serial device. This is a very 103 useful option. To tell LILO to use the serial port: 104 In lilo.conf (global section):: 105 106 serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits) 107 1083. Adjust to kernel flags for the new kernel, 109 again in lilo.conf (kernel section):: 110 111 append = "console=ttyS1,9600" 112 1134. Make sure a getty runs on the serial port so that you can login to 114 it once the system is done booting. This is done by adding a line 115 like this to ``/etc/inittab`` (exact syntax depends on your getty):: 116 117 S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100 118 1195. Init and ``/etc/ioctl.save`` 120 121 Sysvinit remembers its stty settings in a file in ``/etc``, called 122 ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial 123 console for the first time, because otherwise init will probably 124 set the baudrate to 38400 (baudrate of the virtual console). 125 1266. ``/dev/console`` and X 127 Programs that want to do something with the virtual console usually 128 open ``/dev/console``. If you have created the new ``/dev/console`` device, 129 and your console is NOT the virtual console some programs will fail. 130 Those are programs that want to access the VT interface, and use 131 ``/dev/console instead of /dev/tty0``. Some of those programs are:: 132 133 Xfree86, svgalib, gpm, SVGATextMode 134 135 It should be fixed in modern versions of these programs though. 136 137 Note that if you boot without a ``console=`` option (or with 138 ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``. 139 In that case everything will still work. 140 1417. Thanks 142 143 Thanks to Geert Uytterhoeven <geert@linux-m68k.org> 144 for porting the patches from 2.1.4x to 2.1.6x for taking care of 145 the integration of these patches into m68k, ppc and alpha. 146 147Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000 148