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 It is prohibited that two or more values or arrays share a same-key.
72 foo = qux # !ERROR! we can not re-define same key
74 If you want to append the value to existing key as an array member,
75 you can use ``+=`` operator. For example::
80 In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
82 However, a sub-key and a value can not co-exist under a parent key.
83 For example, following config is NOT allowed.::
86 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
92 The config syntax accepts shell-script style comments. The comments starting
93 with hash ("#") until newline ("\n") will be ignored.
98 foo = value # value is set to foo.
99 bar = 1, # 1st element
103 This is parsed as below::
108 Note that you can not put a comment between value and delimiter(``,`` or
109 ``;``). This means following config has a syntax error ::
118 /proc/bootconfig is a user-space interface of the boot config.
119 Unlike /proc/cmdline, this file shows the key-value style list.
120 Each key-value pair is shown in each line with following style::
122 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
125 Boot Kernel With a Boot Config
126 ==============================
128 Since the boot configuration file is loaded with initrd, it will be added
129 to the end of the initrd (initramfs) image file with size, checksum and
130 12-byte magic word as below.
132 [initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n]
134 The Linux kernel decodes the last part of the initrd image in memory to
135 get the boot configuration data.
136 Because of this "piggyback" method, there is no need to change or
137 update the boot loader and the kernel image itself.
139 To do this operation, Linux kernel provides "bootconfig" command under
140 tools/bootconfig, which allows admin to apply or delete the config file
141 to/from initrd image. You can build it by the following command::
143 # make -C tools/bootconfig
145 To add your boot config file to initrd image, run bootconfig as below
146 (Old data is removed automatically if exists)::
148 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
150 To remove the config from the image, you can use -d option as below::
152 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
154 Then add "bootconfig" on the normal kernel command line to tell the
155 kernel to look for the bootconfig at the end of the initrd file.
157 Config File Limitation
158 ======================
160 Currently the maximum config size size is 32KB and the total key-words (not
161 key-value entries) must be under 1024 nodes.
162 Note: this is not the number of entries but nodes, an entry must consume
163 more than 2 nodes (a key-word and a value). So theoretically, it will be
164 up to 512 key-value pairs. If keys contains 3 words in average, it can
165 contain 256 key-value pairs. In most cases, the number of config items
166 will be under 100 entries and smaller than 8KB, so it would be enough.
167 If the node number exceeds 1024, parser returns an error even if the file
168 size is smaller than 32KB.
169 Anyway, since bootconfig command verifies it when appending a boot config
170 to initrd image, user can notice it before boot.
176 User can query or loop on key-value pairs, also it is possible to find
177 a root (prefix) key node and find key-values under that node.
179 If you have a key string, you can query the value directly with the key
180 using xbc_find_value(). If you want to know what keys exist in the boot
181 config, you can use xbc_for_each_key_value() to iterate key-value pairs.
182 Note that you need to use xbc_array_for_each_value() for accessing
183 each array's value, e.g.::
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);
192 If you want to focus on keys which have a prefix string, you can use
193 xbc_find_node() to find a node by the prefix string, and iterate
194 keys under the prefix node with xbc_node_for_each_key_value().
196 But the most typical usage is to get the named value under prefix
197 or get the named array under prefix as below::
199 root = xbc_find_node("key.prefix");
200 value = xbc_node_find_value(root, "option", &vnode);
202 xbc_node_for_each_array_value(root, "array-option", value, anode) {
206 This accesses a value of "key.prefix.option" and an array of
207 "key.prefix.array-option".
209 Locking is not needed, since after initialization, the config becomes
210 read-only. All data and keys must be copied if you need to modify it.
213 Functions and structures
214 ========================
216 .. kernel-doc:: include/linux/bootconfig.h
217 .. kernel-doc:: lib/bootconfig.c