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