1.. SPDX-License-Identifier: GPL-2.0
2
3.. _bootconfig:
4
5==================
6Boot Configuration
7==================
8
9:Author: Masami Hiramatsu <[email protected]>
10
11Overview
12========
13
14The boot configuration is expanding current kernel cmdline to support
15additional key-value data when boot the kernel in an efficient way.
16This allows adoministrators 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
28Each key word must contain only alphabets, numbers, dash (``-``) or underscore
29(``_``). And each value only contains printable characters or spaces except
30for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
31hash (``#``) and closing brace (``}``).
32
33If you want to use those delimiters in a value, you can use either double-
34quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
35you can not escape these quotes.
36
37There can be a key which doesn't have value or has an empty value. Those keys
38are used for checking the key exists or not (like a boolean).
39
40Key-Value Syntax
41----------------
42
43The boot config file syntax allows user to merge partially same word keys
44by brace. For example::
45
46 foo.bar.baz = value1
47 foo.bar.qux.quux = value2
48
49These can be written also in::
50
51 foo.bar {
52    baz = value1
53    qux.quux = value2
54 }
55
56Or more shorter, written as following::
57
58 foo.bar { baz = value1; qux.quux = value2 }
59
60In both styles, same key words are automatically merged when parsing it
61at boot time. So you can append similar trees or key-values.
62
63Comments
64--------
65
66The config syntax accepts shell-script style comments. The comments start
67with hash ("#") until newline ("\n") will be ignored.
68
69::
70
71 # comment line
72 foo = value # value is set to foo.
73 bar = 1, # 1st element
74       2, # 2nd element
75       3  # 3rd element
76
77This is parsed as below::
78
79 foo = value
80 bar = 1, 2, 3
81
82Note that you can not put a comment between value and delimiter(``,`` or
83``;``). This means following config has a syntax error ::
84
85 key = 1 # comment
86       ,2
87
88
89/proc/bootconfig
90================
91
92/proc/bootconfig is a user-space interface of the boot config.
93Unlike /proc/cmdline, this file shows the key-value style list.
94Each key-value pair is shown in each line with following style::
95
96 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
97
98
99Boot Kernel With a Boot Config
100==============================
101
102Since the boot configuration file is loaded with initrd, it will be added
103to the end of the initrd (initramfs) image file. The Linux kernel decodes
104the last part of the initrd image in memory to get the boot configuration
105data.
106Because of this "piggyback" method, there is no need to change or
107update the boot loader and the kernel image itself.
108
109To do this operation, Linux kernel provides "bootconfig" command under
110tools/bootconfig, which allows admin to apply or delete the config file
111to/from initrd image. You can build it by follwoing command::
112
113 # make -C tools/bootconfig
114
115To add your boot config file to initrd image, run bootconfig as below
116(Old data is removed automatically if exists)::
117
118 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
119
120To remove the config from the image, you can use -d option as below::
121
122 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
123
124
125C onfig File Limitation
126======================
127
128Currently the maximum config size size is 32KB and the total key-words (not
129key-value entries) must be under 1024 nodes.
130Note: this is not the number of entries but nodes, an entry must consume
131more than 2 nodes (a key-word and a value). So theoretically, it will be
132up to 512 key-value pairs. If keys contains 3 words in average, it can
133contain 256 key-value pairs. In most cases, the number of config items
134will be under 100 entries and smaller than 8KB, so it would be enough.
135If the node number exceeds 1024, parser returns an error even if the file
136size is smaller than 32KB.
137Anyway, since bootconfig command verifies it when appending a boot config
138to initrd image, user can notice it before boot.
139
140
141Bootconfig APIs
142===============
143
144User can query or loop on key-value pairs, also it is possible to find
145a root (prefix) key node and find key-values under that node.
146
147If you have a key string, you can query the value directly with the key
148using xbc_find_value(). If you want to know what keys exist in the SKC
149tree, you can use xbc_for_each_key_value() to iterate key-value pairs.
150Note that you need to use xbc_array_for_each_value() for accessing
151each arraies value, e.g.::
152
153 vnode = NULL;
154 xbc_find_value("key.word", &vnode);
155 if (vnode && xbc_node_is_array(vnode))
156    xbc_array_for_each_value(vnode, value) {
157      printk("%s ", value);
158    }
159
160If you want to focus on keys which has a prefix string, you can use
161xbc_find_node() to find a node which prefix key words, and iterate
162keys under the prefix node with xbc_node_for_each_key_value().
163
164But the most typical usage is to get the named value under prefix
165or get the named array under prefix as below::
166
167 root = xbc_find_node("key.prefix");
168 value = xbc_node_find_value(root, "option", &vnode);
169 ...
170 xbc_node_for_each_array_value(root, "array-option", value, anode) {
171    ...
172 }
173
174This accesses a value of "key.prefix.option" and an array of
175"key.prefix.array-option".
176
177Locking is not needed, since after initialized, the config becomes readonly.
178All data and keys must be copied if you need to modify it.
179
180
181Functions and structures
182========================
183
184.. kernel-doc:: include/linux/bootconfig.h
185.. kernel-doc:: lib/bootconfig.c
186
187