linux/Documentation/admin-guide/bootconfig.rst

.. SPDX-License-Identifier: GPL-2.0

==================
Boot Configuration
==================

:Author: Masami Hiramatsu <mhiramat@kernel.org>

Overview
========

The boot configuration is expanding current kernel cmdline to support
additional key-value data when boot the kernel in an efficient way.
This allows adoministrators to pass a structured-Key config file.

Config File Syntax
==================

The boot config syntax is a simple structured key-value. Each key consists
of dot-connected-words, and key and value are connected by "=". The value
has to be terminated by semi-colon (``;``) or newline (``\n``).
For array value, array entries are separated by comma (``,``). ::

KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]

Each key word must contain only alphabets, numbers, dash (``-``) or underscore
(``_``). And each value only contains printable characters or spaces except
for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
hash (``#``) and closing brace (``}``).

If you want to use those delimiters in a value, you can use either double-
quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
you can not escape these quotes.

There can be a key which doesn't have value or has an empty value. Those keys
are used for checking the key exists or not (like a boolean).

Key-Value Syntax
----------------

The boot config file syntax allows user to merge partially same word keys
by brace. For example::

 foo.bar.baz = value1
 foo.bar.qux.quux = value2

These can be written also in::

 foo.bar {
    baz = value1
    qux.quux = value2
 }

Or more shorter, written as following::

 foo.bar { baz = value1; qux.quux = value2 }

In both styles, same key words are automatically merged when parsing it
at boot time. So you can append similar trees or key-values.

Comments
--------

The config syntax accepts shell-script style comments. The comments start
with hash ("#") until newline ("\n") will be ignored.

::

 # comment line
 foo = value # value is set to foo.
 bar = 1, # 1st element
       2, # 2nd element
       3  # 3rd element

This is parsed as below::

 foo = value
 bar = 1, 2, 3

Note that you can not put a comment between value and delimiter(``,`` or
``;``). This means following config has a syntax error ::

 key = 1 # comment
       ,2


/proc/bootconfig
================

/proc/bootconfig is a user-space interface of the boot config.
Unlike /proc/cmdline, this file shows the key-value style list.
Each key-value pair is shown in each line with following style::

 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]


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. The Linux kernel decodes
the last part of the initrd image in memory to get the boot configuration
data.
Because of this "piggyback" method, there is no need to change or
update the boot loader and the kernel image itself.

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 follwoing command::

 # make -C tools/bootconfig

To add your boot config file to initrd image, run bootconfig as below
(Old data is removed automatically if exists)::

 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z

To remove the config from the image, you can use -d option as below::

 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z


C onfig File Limitation
======================

Currently the maximum config size size is 32KB and the total key-words (not
key-value entries) must be under 1024 nodes.
Note: this is not the number of entries but nodes, an entry must consume
more than 2 nodes (a key-word and a value). So theoretically, it will be
up to 512 key-value pairs. If keys contains 3 words in average, it can
contain 256 key-value pairs. In most cases, the number of config items
will be under 100 entries and smaller than 8KB, so it would be enough.
If the node number exceeds 1024, parser returns an error even if the file
size is smaller than 32KB.
Anyway, since bootconfig command verifies it when appending a boot config
to initrd image, user can notice it before boot.


Bootconfig APIs
===============

User can query or loop on key-value pairs, also it is possible to find
a root (prefix) key node and find key-values under that node.

If you have a key string, you can query the value directly with the key
using xbc_find_value(). If you want to know what keys exist in the SKC
tree, you can use xbc_for_each_key_value() to iterate key-value pairs.
Note that you need to use xbc_array_for_each_value() for accessing
each arraies value, e.g.::

 vnode = NULL;
 xbc_find_value("key.word", &vnode);
 if (vnode && xbc_node_is_array(vnode))
    xbc_array_for_each_value(vnode, value) {
      printk("%s ", value);
    }

If you want to focus on keys which has a prefix string, you can use
xbc_find_node() to find a node which prefix key words, and iterate
keys under the prefix node with xbc_node_for_each_key_value().

But the most typical usage is to get the named value under prefix
or get the named array under prefix as below::

 root = xbc_find_node("key.prefix");
 value = xbc_node_find_value(root, "option", &vnode);
 ...
 xbc_node_for_each_array_value(root, "array-option", value, anode) {
    ...
 }

This accesses a value of "key.prefix.option" and an array of
"key.prefix.array-option".

Locking is not needed, since after initialized, the config becomes readonly.
All data and keys must be copied if you need to modify it.


Functions and structures
========================

.. kernel-doc:: include/linux/bootconfig.h
.. kernel-doc:: lib/bootconfig.c