105 lines
3.8 KiB
Plaintext
105 lines
3.8 KiB
Plaintext
Linux Serial Console
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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 "Standard/generic (dumb) serial support".
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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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options: depend on the driver. For the serial port this
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defines the baudrate/parity/bits of the port,
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in the format BBBBPN, where BBBB is the speed,
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P is parity (n/o/e), and N is bits. 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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Output will appear on all of them. The last device will be used when
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you open /dev/console. 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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Note that you can only define one console per device type (serial, video).
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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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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. In that
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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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