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
26  KEY[.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 update the value, you must use the override operator
75``:=`` explicitly. For example::
76
77 foo = bar, baz
78 foo := qux
79
80then, the ``qux`` is assigned to ``foo`` key. This is useful for
81overriding the default value by adding (partial) custom bootconfigs
82without parsing the default bootconfig.
83
84If you want to append the value to existing key as an array member,
85you can use ``+=`` operator. For example::
86
87 foo = bar, baz
88 foo += qux
89
90In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
91
92However, a sub-key and a value can not co-exist under a parent key.
93For example, following config is NOT allowed.::
94
95 foo = value1
96 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
97 foo.bar := value2 # !ERROR! even with the override operator, this is NOT allowed.
98
99
100Comments
101--------
102
103The config syntax accepts shell-script style comments. The comments starting
104with hash ("#") until newline ("\n") will be ignored.
105
106::
107
108 # comment line
109 foo = value # value is set to foo.
110 bar = 1, # 1st element
111       2, # 2nd element
112       3  # 3rd element
113
114This is parsed as below::
115
116 foo = value
117 bar = 1, 2, 3
118
119Note that you can not put a comment between value and delimiter(``,`` or
120``;``). This means following config has a syntax error ::
121
122 key = 1 # comment
123       ,2
124
125
126/proc/bootconfig
127================
128
129/proc/bootconfig is a user-space interface of the boot config.
130Unlike /proc/cmdline, this file shows the key-value style list.
131Each key-value pair is shown in each line with following style::
132
133 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
134
135
136Boot Kernel With a Boot Config
137==============================
138
139Since the boot configuration file is loaded with initrd, it will be added
140to the end of the initrd (initramfs) image file with size, checksum and
14112-byte magic word as below.
142
143[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n]
144
145The Linux kernel decodes the last part of the initrd image in memory to
146get the boot configuration data.
147Because of this "piggyback" method, there is no need to change or
148update the boot loader and the kernel image itself.
149
150To do this operation, Linux kernel provides "bootconfig" command under
151tools/bootconfig, which allows admin to apply or delete the config file
152to/from initrd image. You can build it by the following command::
153
154 # make -C tools/bootconfig
155
156To add your boot config file to initrd image, run bootconfig as below
157(Old data is removed automatically if exists)::
158
159 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
160
161To remove the config from the image, you can use -d option as below::
162
163 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
164
165Then add "bootconfig" on the normal kernel command line to tell the
166kernel to look for the bootconfig at the end of the initrd file.
167
168Config File Limitation
169======================
170
171Currently the maximum config size size is 32KB and the total key-words (not
172key-value entries) must be under 1024 nodes.
173Note: this is not the number of entries but nodes, an entry must consume
174more than 2 nodes (a key-word and a value). So theoretically, it will be
175up to 512 key-value pairs. If keys contains 3 words in average, it can
176contain 256 key-value pairs. In most cases, the number of config items
177will be under 100 entries and smaller than 8KB, so it would be enough.
178If the node number exceeds 1024, parser returns an error even if the file
179size is smaller than 32KB.
180Anyway, since bootconfig command verifies it when appending a boot config
181to initrd image, user can notice it before boot.
182
183
184Bootconfig APIs
185===============
186
187User can query or loop on key-value pairs, also it is possible to find
188a root (prefix) key node and find key-values under that node.
189
190If you have a key string, you can query the value directly with the key
191using xbc_find_value(). If you want to know what keys exist in the boot
192config, you can use xbc_for_each_key_value() to iterate key-value pairs.
193Note that you need to use xbc_array_for_each_value() for accessing
194each array's value, e.g.::
195
196 vnode = NULL;
197 xbc_find_value("key.word", &vnode);
198 if (vnode && xbc_node_is_array(vnode))
199    xbc_array_for_each_value(vnode, value) {
200      printk("%s ", value);
201    }
202
203If you want to focus on keys which have a prefix string, you can use
204xbc_find_node() to find a node by the prefix string, and iterate
205keys under the prefix node with xbc_node_for_each_key_value().
206
207But the most typical usage is to get the named value under prefix
208or get the named array under prefix as below::
209
210 root = xbc_find_node("key.prefix");
211 value = xbc_node_find_value(root, "option", &vnode);
212 ...
213 xbc_node_for_each_array_value(root, "array-option", value, anode) {
214    ...
215 }
216
217This accesses a value of "key.prefix.option" and an array of
218"key.prefix.array-option".
219
220Locking is not needed, since after initialization, the config becomes
221read-only. All data and keys must be copied if you need to modify it.
222
223
224Functions and structures
225========================
226
227.. kernel-doc:: include/linux/bootconfig.h
228.. kernel-doc:: lib/bootconfig.c
229
230