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Diffstat (limited to 'Documentation/admin-guide/serial-console.rst')
| -rw-r--r-- | Documentation/admin-guide/serial-console.rst | 36 |
1 files changed, 33 insertions, 3 deletions
diff --git a/Documentation/admin-guide/serial-console.rst b/Documentation/admin-guide/serial-console.rst index 58b32832e50a..8c8b94e54e26 100644 --- a/Documentation/admin-guide/serial-console.rst +++ b/Documentation/admin-guide/serial-console.rst @@ -33,8 +33,11 @@ The format of this option is:: 9600n8. The maximum baudrate is 115200. You can specify multiple console= options on the kernel command line. -Output will appear on all of them. The last device will be used when -you open ``/dev/console``. So, for example:: + +The behavior is well defined when each device type is mentioned only once. +In this case, the output will appear on all requested consoles. And +the last device will be used when you open ``/dev/console``. +So, for example:: console=ttyS1,9600 console=tty0 @@ -42,7 +45,34 @@ defines that opening ``/dev/console`` will get you the current foreground virtual console, and kernel messages will appear on both the VGA console and the 2nd serial port (ttyS1 or COM2) at 9600 baud. -Note that you can only define one console per device type (serial, video). +The behavior is more complicated when the same device type is defined more +times. In this case, there are the following two rules: + +1. The output will appear only on the first device of each defined type. + +2. ``/dev/console`` will be associated with the first registered device. + Where the registration order depends on how kernel initializes various + subsystems. + + This rule is used also when the last console= parameter is not used + for other reasons. For example, because of a typo or because + the hardware is not available. + +The result might be surprising. For example, the following two command +lines have the same result: + + console=ttyS1,9600 console=tty0 console=tty1 + console=tty0 console=ttyS1,9600 console=tty1 + +The kernel messages are printed only on ``tty0`` and ``ttyS1``. And +``/dev/console`` gets associated with ``tty0``. It is because kernel +tries to register graphical consoles before serial ones. It does it +because of the default behavior when no console device is specified, +see below. + +Note that the last ``console=tty1`` parameter still makes a difference. +The kernel command line is used also by systemd. It would use the last +defined ``tty1`` as the login console. If no console device is specified, the first device found capable of acting as a system console will be used. At this time, the system |