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docs: bootconfig: Add how to embed the bootconfig into kernel 

Add a description how to embed the bootconfig file into kernel.

Link: https://lkml.kernel.org/r/164921228987.1090670.16843569536974147213.stgit@devnote2

Cc: Padmanabha Srinivasaiah <treasure4paddy@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Masami Hiramatsu 2022-04-06 11:31:30 +09:00 committed by Steven Rostedt (Google)
parent a2a9d67a26
commit 2f51efc6b7
1 changed files with 28 additions and 3 deletions
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31
Documentation/admin-guide/bootconfig.rst
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@ -158,9 +158,15 @@ Each key-value pair is shown in each line with following style::
Boot Kernel With a Boot Config
==============================
Since the boot configuration file is loaded with initrd, it will be added
to the end of the initrd (initramfs) image file with padding, size,
checksum and 12-byte magic word as below.
There are two options to boot the kernel with bootconfig: attaching the
bootconfig to the initrd image or embedding it in the kernel itself.
Attaching a Boot Config to Initrd
---------------------------------
Since the boot configuration file is loaded with initrd by default,
it will be added to the end of the initrd (initramfs) image file with
padding, size, checksum and 12-byte magic word as below.
[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
@ -196,6 +202,25 @@ 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.
Embedding a Boot Config into Kernel
-----------------------------------
If you can not use initrd, you can also embed the bootconfig file in the
kernel by Kconfig options. In this case, you need to recompile the kernel
with the following configs::
CONFIG_BOOT_CONFIG_EMBED=y
CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE"
``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative
path to the bootconfig file from source tree or object tree.
The kernel will embed it as the default bootconfig.
Just as when attaching the bootconfig to the initrd, you need ``bootconfig``
option on the kernel command line to enable the embedded bootconfig.
Note that even if you set this option, you can override the embedded
bootconfig by another bootconfig which attached to the initrd.
Kernel parameters via Boot Config
=================================