Documentation/admin-guide/bootconfig.rst

1 .. SPDX-License-Identifier: GPL-2.0
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[...]][;]
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 (``,``),
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
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).
42 Key-Value Syntax
43 ----------------
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.
65 Same-key Values
66 ---------------
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 update the value, you must use the override operator
80 then, the ``qux`` is assigned to ``foo`` key. This is useful for
81 overriding the default value by adding (partial) custom bootconfigs
84 If you want to append the value to existing key as an array member,
90 In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
92 Moreover, sub-keys and a value can coexist under a parent key.
97  foo := value3 # This will update foo's value.
99 Note, since there is no syntax to put a raw value directly under a
100 structured key, you have to define it outside of the brace. For example::
110 Also, the order of the value node under a key is fixed. If there
111 are a value and subkeys, the value is always the first child node
112 of the key. Thus if user specifies subkeys first, e.g.::
123 --------
125 The config syntax accepts shell-script style comments. The comments starting
131  foo = value # value is set to foo.
138  foo = value
141 Note that you can not put a comment between value and delimiter(``,`` or
144  key = 1 # comment
151 /proc/bootconfig is a user-space interface of the boot config.
152 Unlike /proc/cmdline, this file shows the key-value style list.
153 Each key-value pair is shown in each line with following style::
155  KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
165 ---------------------------------
169 padding, size, checksum and 12-byte magic word as below.
173 The size and checksum fields are unsigned 32bit little endian value.
191  # make -C tools/bootconfig
196  # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
198 To remove the config from the image, you can use -d option as below::
200  # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
208 -----------------------------------
233 passing the kernel parameters. All the key-value pairs under ``kernel``
234 key will be passed to kernel cmdline directly. Moreover, the key-value
236 The parameters are concatenated with user-given kernel cmdline string
241  [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params]
246    root = 01234567-89ab-cdef-0123-456789abcd
254  root="01234567-89ab-cdef-0123-456789abcd" -- splash
258  ro bootconfig -- quiet
262  root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet
268 Currently the maximum config size size is 32KB and the total key-words (not
269 key-value entries) must be under 1024 nodes.
271 more than 2 nodes (a key-word and a value). So theoretically, it will be
272 up to 512 key-value pairs. If keys contains 3 words in average, it can
273 contain 256 key-value pairs. In most cases, the number of config items
285 User can query or loop on key-value pairs, also it is possible to find
286 a root (prefix) key node and find key-values under that node.
288 If you have a key string, you can query the value directly with the key
290 config, you can use xbc_for_each_key_value() to iterate key-value pairs.
292 each array's value, e.g.::
295  xbc_find_value("key.word", &vnode);
297     xbc_array_for_each_value(vnode, value) {
298       printk("%s ", value);
305 But the most typical usage is to get the named value under prefix
308  root = xbc_find_node("key.prefix");
309  value = xbc_node_find_value(root, "option", &vnode);
311  xbc_node_for_each_array_value(root, "array-option", value, anode) {
315 This accesses a value of "key.prefix.option" and an array of
316 "key.prefix.array-option".
319 read-only. All data and keys must be copied if you need to modify it.
325 .. kernel-doc:: include/linux/bootconfig.h
326 .. kernel-doc:: lib/bootconfig.c