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
65Same-key Values
66---------------
67
68It is prohibited that two or more values or arrays share a same-key.
69For example,::
70
71 foo = bar, baz
72 foo = qux  # !ERROR! we can not re-define same key
73
74If you want to append the value to existing key as an array member,
75you can use ``+=`` operator. For example::
76
77 foo = bar, baz
78 foo += qux
79
80In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
81
82However, a sub-key and a value can not co-exist under a parent key.
83For example, following config is NOT allowed.::
84
85 foo = value1
86 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
87
88
89Comments
90--------
91
92The config syntax accepts shell-script style comments. The comments starting
93with hash ("#") until newline ("\n") will be ignored.
94
95::
96
97 # comment line
98 foo = value # value is set to foo.
99 bar = 1, # 1st element
100       2, # 2nd element
101       3  # 3rd element
102
103This is parsed as below::
104
105 foo = value
106 bar = 1, 2, 3
107
108Note that you can not put a comment between value and delimiter(``,`` or
109``;``). This means following config has a syntax error ::
110
111 key = 1 # comment
112       ,2
113
114
115/proc/bootconfig
116================
117
118/proc/bootconfig is a user-space interface of the boot config.
119Unlike /proc/cmdline, this file shows the key-value style list.
120Each key-value pair is shown in each line with following style::
121
122 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
123
124
125Boot Kernel With a Boot Config
126==============================
127
128Since the boot configuration file is loaded with initrd, it will be added
129to the end of the initrd (initramfs) image file with size, checksum and
13012-byte magic word as below.
131
132[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n]
133
134The Linux kernel decodes the last part of the initrd image in memory to
135get the boot configuration data.
136Because of this "piggyback" method, there is no need to change or
137update the boot loader and the kernel image itself.
138
139To do this operation, Linux kernel provides "bootconfig" command under
140tools/bootconfig, which allows admin to apply or delete the config file
141to/from initrd image. You can build it by the following command::
142
143 # make -C tools/bootconfig
144
145To add your boot config file to initrd image, run bootconfig as below
146(Old data is removed automatically if exists)::
147
148 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
149
150To remove the config from the image, you can use -d option as below::
151
152 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
153
154Then add "bootconfig" on the normal kernel command line to tell the
155kernel to look for the bootconfig at the end of the initrd file.
156
157Config File Limitation
158======================
159
160Currently the maximum config size size is 32KB and the total key-words (not
161key-value entries) must be under 1024 nodes.
162Note: this is not the number of entries but nodes, an entry must consume
163more than 2 nodes (a key-word and a value). So theoretically, it will be
164up to 512 key-value pairs. If keys contains 3 words in average, it can
165contain 256 key-value pairs. In most cases, the number of config items
166will be under 100 entries and smaller than 8KB, so it would be enough.
167If the node number exceeds 1024, parser returns an error even if the file
168size is smaller than 32KB.
169Anyway, since bootconfig command verifies it when appending a boot config
170to initrd image, user can notice it before boot.
171
172
173Bootconfig APIs
174===============
175
176User can query or loop on key-value pairs, also it is possible to find
177a root (prefix) key node and find key-values under that node.
178
179If you have a key string, you can query the value directly with the key
180using xbc_find_value(). If you want to know what keys exist in the boot
181config, you can use xbc_for_each_key_value() to iterate key-value pairs.
182Note that you need to use xbc_array_for_each_value() for accessing
183each array's value, e.g.::
184
185 vnode = NULL;
186 xbc_find_value("key.word", &vnode);
187 if (vnode && xbc_node_is_array(vnode))
188    xbc_array_for_each_value(vnode, value) {
189      printk("%s ", value);
190    }
191
192If you want to focus on keys which have a prefix string, you can use
193xbc_find_node() to find a node by the prefix string, and iterate
194keys under the prefix node with xbc_node_for_each_key_value().
195
196But the most typical usage is to get the named value under prefix
197or get the named array under prefix as below::
198
199 root = xbc_find_node("key.prefix");
200 value = xbc_node_find_value(root, "option", &vnode);
201 ...
202 xbc_node_for_each_array_value(root, "array-option", value, anode) {
203    ...
204 }
205
206This accesses a value of "key.prefix.option" and an array of
207"key.prefix.array-option".
208
209Locking is not needed, since after initialization, the config becomes
210read-only. All data and keys must be copied if you need to modify it.
211
212
213Functions and structures
214========================
215
216.. kernel-doc:: include/linux/bootconfig.h
217.. kernel-doc:: lib/bootconfig.c
218
219