146 lines
5.5 KiB
ReStructuredText
146 lines
5.5 KiB
ReStructuredText
.. _serial_console:
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Linux Serial Console
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====================
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To use a serial port as console you need to compile the support into your
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kernel - by default it is not compiled in. For PC style serial ports
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it's the config option next to menu option:
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:menuselection:`Character devices --> Serial drivers --> 8250/16550 and compatible serial support --> Console on 8250/16550 and compatible serial port`
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You must compile serial support into the kernel and not as a module.
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It is possible to specify multiple devices for console output. You can
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define a new kernel command line option to select which device(s) to
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use for console output.
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The format of this option is::
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console=device,options
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device: tty0 for the foreground virtual console
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ttyX for any other virtual console
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ttySx for a serial port
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lp0 for the first parallel port
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ttyUSB0 for the first USB serial device
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options: depend on the driver. For the serial port this
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defines the baudrate/parity/bits/flow control of
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the port, in the format BBBBPNF, where BBBB is the
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speed, P is parity (n/o/e), N is number of bits,
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and F is flow control ('r' for RTS). Default is
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9600n8. The maximum baudrate is 115200.
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You can specify multiple console= options on the kernel command line.
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The behavior is well defined when each device type is mentioned only once.
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In this case, the output will appear on all requested consoles. And
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the last device will be used when you open ``/dev/console``.
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So, for example::
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console=ttyS1,9600 console=tty0
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defines that opening ``/dev/console`` will get you the current foreground
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virtual console, and kernel messages will appear on both the VGA
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console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
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The behavior is more complicated when the same device type is defined more
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times. In this case, there are the following two rules:
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1. The output will appear only on the first device of each defined type.
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2. ``/dev/console`` will be associated with the first registered device.
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Where the registration order depends on how kernel initializes various
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subsystems.
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This rule is used also when the last console= parameter is not used
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for other reasons. For example, because of a typo or because
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the hardware is not available.
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The result might be surprising. For example, the following two command
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lines have the same result::
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console=ttyS1,9600 console=tty0 console=tty1
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console=tty0 console=ttyS1,9600 console=tty1
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The kernel messages are printed only on ``tty0`` and ``ttyS1``. And
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``/dev/console`` gets associated with ``tty0``. It is because kernel
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tries to register graphical consoles before serial ones. It does it
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because of the default behavior when no console device is specified,
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see below.
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Note that the last ``console=tty1`` parameter still makes a difference.
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The kernel command line is used also by systemd. It would use the last
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defined ``tty1`` as the login console.
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If no console device is specified, the first device found capable of
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acting as a system console will be used. At this time, the system
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first looks for a VGA card and then for a serial port. So if you don't
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have a VGA card in your system the first serial port will automatically
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become the console.
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You will need to create a new device to use ``/dev/console``. The official
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``/dev/console`` is now character device 5,1.
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(You can also use a network device as a console. See
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``Documentation/networking/netconsole.rst`` for information on that.)
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Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
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Replace the sample values as needed.
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1. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
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console)::
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cd /dev
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rm -f console tty0
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mknod -m 622 console c 5 1
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mknod -m 622 tty0 c 4 0
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2. LILO can also take input from a serial device. This is a very
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useful option. To tell LILO to use the serial port:
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In lilo.conf (global section)::
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serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
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3. Adjust to kernel flags for the new kernel,
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again in lilo.conf (kernel section)::
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append = "console=ttyS1,9600"
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4. Make sure a getty runs on the serial port so that you can login to
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it once the system is done booting. This is done by adding a line
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like this to ``/etc/inittab`` (exact syntax depends on your getty)::
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S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
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5. Init and ``/etc/ioctl.save``
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Sysvinit remembers its stty settings in a file in ``/etc``, called
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``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
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console for the first time, because otherwise init will probably
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set the baudrate to 38400 (baudrate of the virtual console).
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6. ``/dev/console`` and X
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Programs that want to do something with the virtual console usually
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open ``/dev/console``. If you have created the new ``/dev/console`` device,
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and your console is NOT the virtual console some programs will fail.
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Those are programs that want to access the VT interface, and use
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``/dev/console instead of /dev/tty0``. Some of those programs are::
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Xfree86, svgalib, gpm, SVGATextMode
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It should be fixed in modern versions of these programs though.
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Note that if you boot without a ``console=`` option (or with
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``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
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In that case everything will still work.
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7. Thanks
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Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
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for porting the patches from 2.1.4x to 2.1.6x for taking care of
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the integration of these patches into m68k, ppc and alpha.
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Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
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