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docs: bootconfig: Add how to use bootconfig for kernel parameters

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Add a section to describe how to use the bootconfig for
specifying kernel and init parameters. This is an important
section because it is the reason why this document is under
the admin-guide.

Link: https://lkml.kernel.org/r/163077086399.222577.5881779375643977991.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Masami Hiramatsu 2021-09-05 00:54:24 +09:00 committed by Steven Rostedt (VMware)
parent b66fbbe8d4
commit 26c9c72fd0
1 changed files with 38 additions and 1 deletions
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39
Documentation/admin-guide/bootconfig.rst
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@ -178,7 +178,7 @@ update the boot loader and the kernel image itself as long as the boot
loader passes the correct initrd file size. If by any chance, the boot
loader passes a longer size, the kernel fails to find the bootconfig data.
To do this operation, Linux kernel provides "bootconfig" command under
To do this operation, Linux kernel provides ``bootconfig`` command under
tools/bootconfig, which allows admin to apply or delete the config file
to/from initrd image. You can build it by the following command::
@ -196,6 +196,43 @@ To remove the config from the image, you can use -d option as below::
Then add "bootconfig" on the normal kernel command line to tell the
kernel to look for the bootconfig at the end of the initrd file.
Kernel parameters via Boot Config
=================================
In addition to the kernel command line, the boot config can be used for
passing the kernel parameters. All the key-value pairs under ``kernel``
key will be passed to kernel cmdline directly. Moreover, the key-value
pairs under ``init`` will be passed to init process via the cmdline.
The parameters are concatinated with user-given kernel cmdline string
as the following order, so that the command line parameter can override
bootconfig parameters (this depends on how the subsystem handles parameters
but in general, earlier parameter will be overwritten by later one.)::
[bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params]
Here is an example of the bootconfig file for kernel/init parameters.::
kernel {
root = 01234567-89ab-cdef-0123-456789abcd
}
init {
splash
}
This will be copied into the kernel cmdline string as the following::
root="01234567-89ab-cdef-0123-456789abcd" -- splash
If user gives some other command line like,::
ro bootconfig -- quiet
The final kernel cmdline will be the following::
root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet
Config File Limitation
======================