Documentation: bootconfig: Add a doc for extended boot config

.. 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

   binderfs

   binfmt-misc

   blockdev/index

   bootconfig

   braille-console

   btmrvl

   cgroup-v1/index

F:	fs/proc/bootconfig.c

F:	include/linux/bootconfig.h

F:	tools/bootconfig/*

F:	Documentation/admin-guide/bootconfig.rst

SUN3/3X

M:	Sam Creasey <sammy@sammy.net>