| 1 | .. _serial_console:
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| 2 |
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| 3 | Linux Serial Console
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| 4 | ====================
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| 5 |
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| 6 | To use a serial port as console you need to compile the support into your
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| 7 | kernel - by default it is not compiled in. For PC style serial ports
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| 8 | it's the config option next to menu option:
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| 9 |
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| 10 | :menuselection:`Character devices --> Serial drivers --> 8250/16550 and compatible serial support --> Console on 8250/16550 and compatible serial port`
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| 11 |
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| 12 | You must compile serial support into the kernel and not as a module.
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| 13 |
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| 14 | It is possible to specify multiple devices for console output. You can
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| 15 | define a new kernel command line option to select which device(s) to
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| 16 | use for console output.
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| 17 |
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| 18 | The format of this option is::
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| 19 |
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| 20 | console=device,options
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| 21 |
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| 22 | device: tty0 for the foreground virtual console
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| 23 | ttyX for any other virtual console
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| 24 | ttySx for a serial port
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| 25 | lp0 for the first parallel port
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| 26 | ttyUSB0 for the first USB serial device
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| 27 |
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| 28 | options: depend on the driver. For the serial port this
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| 29 | defines the baudrate/parity/bits/flow control of
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| 30 | the port, in the format BBBBPNF, where BBBB is the
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| 31 | speed, P is parity (n/o/e), N is number of bits,
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| 32 | and F is flow control ('r' for RTS). Default is
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| 33 | 9600n8. The maximum baudrate is 115200.
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| 34 |
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| 35 | You can specify multiple console= options on the kernel command line.
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| 36 |
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| 37 | The behavior is well defined when each device type is mentioned only once.
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| 38 | In this case, the output will appear on all requested consoles. And
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| 39 | the last device will be used when you open ``/dev/console``.
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| 40 | So, for example::
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| 41 |
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| 42 | console=ttyS1,9600 console=tty0
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| 43 |
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| 44 | defines that opening ``/dev/console`` will get you the current foreground
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| 45 | virtual console, and kernel messages will appear on both the VGA
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| 46 | console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
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| 47 |
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| 48 | The behavior is more complicated when the same device type is defined more
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| 49 | times. In this case, there are the following two rules:
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| 50 |
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| 51 | 1. The output will appear only on the first device of each defined type.
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| 52 |
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| 53 | 2. ``/dev/console`` will be associated with the first registered device.
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| 54 | Where the registration order depends on how kernel initializes various
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| 55 | subsystems.
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| 56 |
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| 57 | This rule is used also when the last console= parameter is not used
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| 58 | for other reasons. For example, because of a typo or because
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| 59 | the hardware is not available.
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| 60 |
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| 61 | The result might be surprising. For example, the following two command
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| 62 | lines have the same result::
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| 63 |
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| 64 | console=ttyS1,9600 console=tty0 console=tty1
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| 65 | console=tty0 console=ttyS1,9600 console=tty1
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| 66 |
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| 67 | The kernel messages are printed only on ``tty0`` and ``ttyS1``. And
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| 68 | ``/dev/console`` gets associated with ``tty0``. It is because kernel
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| 69 | tries to register graphical consoles before serial ones. It does it
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| 70 | because of the default behavior when no console device is specified,
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| 71 | see below.
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| 72 |
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| 73 | Note that the last ``console=tty1`` parameter still makes a difference.
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| 74 | The kernel command line is used also by systemd. It would use the last
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| 75 | defined ``tty1`` as the login console.
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| 76 |
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| 77 | If no console device is specified, the first device found capable of
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| 78 | acting as a system console will be used. At this time, the system
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| 79 | first looks for a VGA card and then for a serial port. So if you don't
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| 80 | have a VGA card in your system the first serial port will automatically
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| 81 | become the console, unless the kernel is configured with the
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| 82 | CONFIG_NULL_TTY_DEFAULT_CONSOLE option, then it will default to using the
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| 83 | ttynull device.
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| 84 |
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| 85 | You will need to create a new device to use ``/dev/console``. The official
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| 86 | ``/dev/console`` is now character device 5,1.
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| 87 |
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| 88 | (You can also use a network device as a console. See
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| 89 | ``Documentation/networking/netconsole.rst`` for information on that.)
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| 90 |
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| 91 | Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
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| 92 | Replace the sample values as needed.
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| 93 |
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| 94 | 1. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
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| 95 | console)::
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| 96 |
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| 97 | cd /dev
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| 98 | rm -f console tty0
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| 99 | mknod -m 622 console c 5 1
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| 100 | mknod -m 622 tty0 c 4 0
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| 101 |
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| 102 | 2. LILO can also take input from a serial device. This is a very
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| 103 | useful option. To tell LILO to use the serial port:
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| 104 | In lilo.conf (global section)::
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| 105 |
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| 106 | serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
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| 107 |
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| 108 | 3. Adjust to kernel flags for the new kernel,
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| 109 | again in lilo.conf (kernel section)::
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| 110 |
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| 111 | append = "console=ttyS1,9600"
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| 112 |
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| 113 | 4. Make sure a getty runs on the serial port so that you can login to
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| 114 | it once the system is done booting. This is done by adding a line
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| 115 | like this to ``/etc/inittab`` (exact syntax depends on your getty)::
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| 116 |
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| 117 | S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
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| 118 |
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| 119 | 5. Init and ``/etc/ioctl.save``
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| 120 |
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| 121 | Sysvinit remembers its stty settings in a file in ``/etc``, called
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| 122 | ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
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| 123 | console for the first time, because otherwise init will probably
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| 124 | set the baudrate to 38400 (baudrate of the virtual console).
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| 125 |
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| 126 | 6. ``/dev/console`` and X
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| 127 | Programs that want to do something with the virtual console usually
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| 128 | open ``/dev/console``. If you have created the new ``/dev/console`` device,
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| 129 | and your console is NOT the virtual console some programs will fail.
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| 130 | Those are programs that want to access the VT interface, and use
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| 131 | ``/dev/console instead of /dev/tty0``. Some of those programs are::
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| 132 |
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| 133 | Xfree86, svgalib, gpm, SVGATextMode
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| 134 |
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| 135 | It should be fixed in modern versions of these programs though.
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| 136 |
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| 137 | Note that if you boot without a ``console=`` option (or with
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| 138 | ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
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| 139 | In that case everything will still work.
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| 140 |
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| 141 | 7. Thanks
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| 142 |
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| 143 | Thanks to Geert Uytterhoeven <[email protected]>
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| 144 | for porting the patches from 2.1.4x to 2.1.6x for taking care of
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| 145 | the integration of these patches into m68k, ppc and alpha.
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| 146 |
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| 147 | Miquel van Smoorenburg <[email protected]>, 11-Jun-2000
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