1.. SPDX-License-Identifier: GPL-2.0
2
3.. _bootconfig:
4
5==================
6Boot Configuration
7==================
8
9:Author: Masami Hiramatsu <mhiramat@kernel.org>
10
11Overview
12========
13
14The boot configuration expands the current kernel command line to support
15additional key-value data when booting the kernel in an efficient way.
16This allows administrators to pass a structured-Key config file.
17
18Config File Syntax
19==================
20
21The boot config syntax is a simple structured key-value. Each key consists
22of dot-connected-words, and key and value are connected by ``=``. The value
23has to be terminated by semi-colon (``;``) or newline (``\n``).
24For array value, array entries are separated by comma (``,``). ::
25
26KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
27
28Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.
29
30Each key word must contain only alphabets, numbers, dash (``-``) or underscore
31(``_``). And each value only contains printable characters or spaces except
32for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
33hash (``#``) and closing brace (``}``).
34
35If you want to use those delimiters in a value, you can use either double-
36quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
37you can not escape these quotes.
38
39There can be a key which doesn't have value or has an empty value. Those keys
40are used for checking if the key exists or not (like a boolean).
41
42Key-Value Syntax
43----------------
44
45The boot config file syntax allows user to merge partially same word keys
46by brace. For example::
47
48 foo.bar.baz = value1
49 foo.bar.qux.quux = value2
50
51These can be written also in::
52
53 foo.bar {
54    baz = value1
55    qux.quux = value2
56 }
57
58Or more shorter, written as following::
59
60 foo.bar { baz = value1; qux.quux = value2 }
61
62In both styles, same key words are automatically merged when parsing it
63at boot time. So you can append similar trees or key-values.
64
65Comments
66--------
67
68The config syntax accepts shell-script style comments. The comments starting
69with hash ("#") until newline ("\n") will be ignored.
70
71::
72
73 # comment line
74 foo = value # value is set to foo.
75 bar = 1, # 1st element
76       2, # 2nd element
77       3  # 3rd element
78
79This is parsed as below::
80
81 foo = value
82 bar = 1, 2, 3
83
84Note that you can not put a comment between value and delimiter(``,`` or
85``;``). This means following config has a syntax error ::
86
87 key = 1 # comment
88       ,2
89
90
91/proc/bootconfig
92================
93
94/proc/bootconfig is a user-space interface of the boot config.
95Unlike /proc/cmdline, this file shows the key-value style list.
96Each key-value pair is shown in each line with following style::
97
98 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
99
100
101Boot Kernel With a Boot Config
102==============================
103
104Since the boot configuration file is loaded with initrd, it will be added
105to the end of the initrd (initramfs) image file. The Linux kernel decodes
106the last part of the initrd image in memory to get the boot configuration
107data.
108Because of this "piggyback" method, there is no need to change or
109update the boot loader and the kernel image itself.
110
111To do this operation, Linux kernel provides "bootconfig" command under
112tools/bootconfig, which allows admin to apply or delete the config file
113to/from initrd image. You can build it by the following command::
114
115 # make -C tools/bootconfig
116
117To add your boot config file to initrd image, run bootconfig as below
118(Old data is removed automatically if exists)::
119
120 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
121
122To remove the config from the image, you can use -d option as below::
123
124 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
125
126Then add "bootconfig" on the normal kernel command line to tell the
127kernel to look for the bootconfig at the end of the initrd file.
128
129Config File Limitation
130======================
131
132Currently the maximum config size size is 32KB and the total key-words (not
133key-value entries) must be under 1024 nodes.
134Note: this is not the number of entries but nodes, an entry must consume
135more than 2 nodes (a key-word and a value). So theoretically, it will be
136up to 512 key-value pairs. If keys contains 3 words in average, it can
137contain 256 key-value pairs. In most cases, the number of config items
138will be under 100 entries and smaller than 8KB, so it would be enough.
139If the node number exceeds 1024, parser returns an error even if the file
140size is smaller than 32KB.
141Anyway, since bootconfig command verifies it when appending a boot config
142to initrd image, user can notice it before boot.
143
144
145Bootconfig APIs
146===============
147
148User can query or loop on key-value pairs, also it is possible to find
149a root (prefix) key node and find key-values under that node.
150
151If you have a key string, you can query the value directly with the key
152using xbc_find_value(). If you want to know what keys exist in the boot
153config, you can use xbc_for_each_key_value() to iterate key-value pairs.
154Note that you need to use xbc_array_for_each_value() for accessing
155each array's value, e.g.::
156
157 vnode = NULL;
158 xbc_find_value("key.word", &vnode);
159 if (vnode && xbc_node_is_array(vnode))
160    xbc_array_for_each_value(vnode, value) {
161      printk("%s ", value);
162    }
163
164If you want to focus on keys which have a prefix string, you can use
165xbc_find_node() to find a node by the prefix string, and iterate
166keys under the prefix node with xbc_node_for_each_key_value().
167
168But the most typical usage is to get the named value under prefix
169or get the named array under prefix as below::
170
171 root = xbc_find_node("key.prefix");
172 value = xbc_node_find_value(root, "option", &vnode);
173 ...
174 xbc_node_for_each_array_value(root, "array-option", value, anode) {
175    ...
176 }
177
178This accesses a value of "key.prefix.option" and an array of
179"key.prefix.array-option".
180
181Locking is not needed, since after initialization, the config becomes
182read-only. All data and keys must be copied if you need to modify it.
183
184
185Functions and structures
186========================
187
188.. kernel-doc:: include/linux/bootconfig.h
189.. kernel-doc:: lib/bootconfig.c
190
191