1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 .. _bootconfig: 3 .. _bootconfig:
4 4
5 ================== 5 ==================
6 Boot Configuration 6 Boot Configuration
7 ================== 7 ==================
8 8
9 :Author: Masami Hiramatsu <mhiramat@kernel.org> 9 :Author: Masami Hiramatsu <mhiramat@kernel.org>
10 10
11 Overview 11 Overview
12 ======== 12 ========
13 13
14 The boot configuration expands the current ker 14 The boot configuration expands the current kernel command line to support
15 additional key-value data when booting the ker 15 additional key-value data when booting the kernel in an efficient way.
16 This allows administrators to pass a structure 16 This allows administrators to pass a structured-Key config file.
17 17
18 Config File Syntax 18 Config File Syntax
19 ================== 19 ==================
20 20
21 The boot config syntax is a simple structured 21 The boot config syntax is a simple structured key-value. Each key consists
22 of dot-connected-words, and key and value are 22 of dot-connected-words, and key and value are connected by ``=``. The value
23 has to be terminated by semi-colon (``;``) or 23 has to be terminated by semi-colon (``;``) or newline (``\n``).
24 For array value, array entries are separated b 24 For array value, array entries are separated by comma (``,``). ::
25 25
26 KEY[.WORD[...]] = VALUE[, VALUE2[...]][;] 26 KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
27 27
28 Unlike the kernel command line syntax, spaces 28 Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.
29 29
30 Each key word must contain only alphabets, num 30 Each key word must contain only alphabets, numbers, dash (``-``) or underscore
31 (``_``). And each value only contains printabl 31 (``_``). And each value only contains printable characters or spaces except
32 for delimiters such as semi-colon (``;``), new 32 for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
33 hash (``#``) and closing brace (``}``). 33 hash (``#``) and closing brace (``}``).
34 34
35 If you want to use those delimiters in a value 35 If you want to use those delimiters in a value, you can use either double-
36 quotes (``"VALUE"``) or single-quotes (``'VALU 36 quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
37 you can not escape these quotes. 37 you can not escape these quotes.
38 38
39 There can be a key which doesn't have value or 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 40 are used for checking if the key exists or not (like a boolean).
41 41
42 Key-Value Syntax 42 Key-Value Syntax
43 ---------------- 43 ----------------
44 44
45 The boot config file syntax allows user to mer 45 The boot config file syntax allows user to merge partially same word keys
46 by brace. For example:: 46 by brace. For example::
47 47
48 foo.bar.baz = value1 48 foo.bar.baz = value1
49 foo.bar.qux.quux = value2 49 foo.bar.qux.quux = value2
50 50
51 These can be written also in:: 51 These can be written also in::
52 52
53 foo.bar { 53 foo.bar {
54 baz = value1 54 baz = value1
55 qux.quux = value2 55 qux.quux = value2
56 } 56 }
57 57
58 Or more shorter, written as following:: 58 Or more shorter, written as following::
59 59
60 foo.bar { baz = value1; qux.quux = value2 } 60 foo.bar { baz = value1; qux.quux = value2 }
61 61
62 In both styles, same key words are automatical 62 In both styles, same key words are automatically merged when parsing it
63 at boot time. So you can append similar trees 63 at boot time. So you can append similar trees or key-values.
64 64
65 Same-key Values 65 Same-key Values
66 --------------- 66 ---------------
67 67
68 It is prohibited that two or more values or ar 68 It is prohibited that two or more values or arrays share a same-key.
69 For example,:: 69 For example,::
70 70
71 foo = bar, baz 71 foo = bar, baz
72 foo = qux # !ERROR! we can not re-define sam 72 foo = qux # !ERROR! we can not re-define same key
73 73
74 If you want to update the value, you must use 74 If you want to update the value, you must use the override operator
75 ``:=`` explicitly. For example:: 75 ``:=`` explicitly. For example::
76 76
77 foo = bar, baz 77 foo = bar, baz
78 foo := qux 78 foo := qux
79 79
80 then, the ``qux`` is assigned to ``foo`` key. 80 then, the ``qux`` is assigned to ``foo`` key. This is useful for
81 overriding the default value by adding (partia 81 overriding the default value by adding (partial) custom bootconfigs
82 without parsing the default bootconfig. 82 without parsing the default bootconfig.
83 83
84 If you want to append the value to existing ke 84 If you want to append the value to existing key as an array member,
85 you can use ``+=`` operator. For example:: 85 you can use ``+=`` operator. For example::
86 86
87 foo = bar, baz 87 foo = bar, baz
88 foo += qux 88 foo += qux
89 89
90 In this case, the key ``foo`` has ``bar``, ``b 90 In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
91 91
92 Moreover, sub-keys and a value can coexist und !! 92 However, a sub-key and a value can not co-exist under a parent key.
93 For example, following config is allowed.:: !! 93 For example, following config is NOT allowed.::
94 94
95 foo = value1 95 foo = value1
96 foo.bar = value2 !! 96 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
97 foo := value3 # This will update foo's value. !! 97 foo.bar := value2 # !ERROR! even with the override operator, this is NOT allowed.
98 98
99 Note, since there is no syntax to put a raw va <<
100 structured key, you have to define it outside <<
101 <<
102 foo { <<
103 bar = value1 <<
104 bar { <<
105 baz = value2 <<
106 qux = value3 <<
107 } <<
108 } <<
109 <<
110 Also, the order of the value node under a key <<
111 are a value and subkeys, the value is always t <<
112 of the key. Thus if user specifies subkeys fir <<
113 <<
114 foo.bar = value1 <<
115 foo = value2 <<
116 <<
117 In the program (and /proc/bootconfig), it will <<
118 <<
119 foo = value2 <<
120 foo.bar = value1 <<
121 99
122 Comments 100 Comments
123 -------- 101 --------
124 102
125 The config syntax accepts shell-script style c 103 The config syntax accepts shell-script style comments. The comments starting
126 with hash ("#") until newline ("\n") will be i 104 with hash ("#") until newline ("\n") will be ignored.
127 105
128 :: 106 ::
129 107
130 # comment line 108 # comment line
131 foo = value # value is set to foo. 109 foo = value # value is set to foo.
132 bar = 1, # 1st element 110 bar = 1, # 1st element
133 2, # 2nd element 111 2, # 2nd element
134 3 # 3rd element 112 3 # 3rd element
135 113
136 This is parsed as below:: 114 This is parsed as below::
137 115
138 foo = value 116 foo = value
139 bar = 1, 2, 3 117 bar = 1, 2, 3
140 118
141 Note that you can not put a comment between va 119 Note that you can not put a comment between value and delimiter(``,`` or
142 ``;``). This means following config has a synt 120 ``;``). This means following config has a syntax error ::
143 121
144 key = 1 # comment 122 key = 1 # comment
145 ,2 123 ,2
146 124
147 125
148 /proc/bootconfig 126 /proc/bootconfig
149 ================ 127 ================
150 128
151 /proc/bootconfig is a user-space interface of 129 /proc/bootconfig is a user-space interface of the boot config.
152 Unlike /proc/cmdline, this file shows the key- 130 Unlike /proc/cmdline, this file shows the key-value style list.
153 Each key-value pair is shown in each line with 131 Each key-value pair is shown in each line with following style::
154 132
155 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 133 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
156 134
157 135
158 Boot Kernel With a Boot Config 136 Boot Kernel With a Boot Config
159 ============================== 137 ==============================
160 138
161 There are two options to boot the kernel with !! 139 Since the boot configuration file is loaded with initrd, it will be added
162 bootconfig to the initrd image or embedding it !! 140 to the end of the initrd (initramfs) image file with padding, size,
163 !! 141 checksum and 12-byte magic word as below.
164 Attaching a Boot Config to Initrd <<
165 --------------------------------- <<
166 <<
167 Since the boot configuration file is loaded wi <<
168 it will be added to the end of the initrd (ini <<
169 padding, size, checksum and 12-byte magic word <<
170 142
171 [initrd][bootconfig][padding][size(le32)][chec 143 [initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
172 144
173 The size and checksum fields are unsigned 32bi 145 The size and checksum fields are unsigned 32bit little endian value.
174 146
175 When the boot configuration is added to the in 147 When the boot configuration is added to the initrd image, the total
176 file size is aligned to 4 bytes. To fill the g 148 file size is aligned to 4 bytes. To fill the gap, null characters
177 (``\0``) will be added. Thus the ``size`` is t 149 (``\0``) will be added. Thus the ``size`` is the length of the bootconfig
178 file + padding bytes. 150 file + padding bytes.
179 151
180 The Linux kernel decodes the last part of the 152 The Linux kernel decodes the last part of the initrd image in memory to
181 get the boot configuration data. 153 get the boot configuration data.
182 Because of this "piggyback" method, there is n 154 Because of this "piggyback" method, there is no need to change or
183 update the boot loader and the kernel image it 155 update the boot loader and the kernel image itself as long as the boot
184 loader passes the correct initrd file size. If 156 loader passes the correct initrd file size. If by any chance, the boot
185 loader passes a longer size, the kernel fails 157 loader passes a longer size, the kernel fails to find the bootconfig data.
186 158
187 To do this operation, Linux kernel provides `` !! 159 To do this operation, Linux kernel provides "bootconfig" command under
188 tools/bootconfig, which allows admin to apply 160 tools/bootconfig, which allows admin to apply or delete the config file
189 to/from initrd image. You can build it by the 161 to/from initrd image. You can build it by the following command::
190 162
191 # make -C tools/bootconfig 163 # make -C tools/bootconfig
192 164
193 To add your boot config file to initrd image, 165 To add your boot config file to initrd image, run bootconfig as below
194 (Old data is removed automatically if exists): 166 (Old data is removed automatically if exists)::
195 167
196 # tools/bootconfig/bootconfig -a your-config 168 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
197 169
198 To remove the config from the image, you can u 170 To remove the config from the image, you can use -d option as below::
199 171
200 # tools/bootconfig/bootconfig -d /boot/initrd 172 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
201 173
202 Then add "bootconfig" on the normal kernel com 174 Then add "bootconfig" on the normal kernel command line to tell the
203 kernel to look for the bootconfig at the end o 175 kernel to look for the bootconfig at the end of the initrd file.
204 Alternatively, build your kernel with the ``CO <<
205 Kconfig option selected. <<
206 <<
207 Embedding a Boot Config into Kernel <<
208 ----------------------------------- <<
209 <<
210 If you can not use initrd, you can also embed <<
211 kernel by Kconfig options. In this case, you n <<
212 with the following configs:: <<
213 <<
214 CONFIG_BOOT_CONFIG_EMBED=y <<
215 CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTC <<
216 <<
217 ``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an <<
218 path to the bootconfig file from source tree o <<
219 The kernel will embed it as the default bootco <<
220 <<
221 Just as when attaching the bootconfig to the i <<
222 option on the kernel command line to enable th <<
223 alternatively, build your kernel with the ``CO <<
224 Kconfig option selected. <<
225 <<
226 Note that even if you set this option, you can <<
227 bootconfig by another bootconfig which attache <<
228 <<
229 Kernel parameters via Boot Config <<
230 ================================= <<
231 <<
232 In addition to the kernel command line, the bo <<
233 passing the kernel parameters. All the key-val <<
234 key will be passed to kernel cmdline directly. <<
235 pairs under ``init`` will be passed to init pr <<
236 The parameters are concatenated with user-give <<
237 as the following order, so that the command li <<
238 bootconfig parameters (this depends on how the <<
239 but in general, earlier parameter will be over <<
240 <<
241 [bootconfig params][cmdline params] -- [bootc <<
242 <<
243 Here is an example of the bootconfig file for <<
244 <<
245 kernel { <<
246 root = 01234567-89ab-cdef-0123-456789abcd <<
247 } <<
248 init { <<
249 splash <<
250 } <<
251 <<
252 This will be copied into the kernel cmdline st <<
253 <<
254 root="01234567-89ab-cdef-0123-456789abcd" -- <<
255 <<
256 If user gives some other command line like,:: <<
257 <<
258 ro bootconfig -- quiet <<
259 <<
260 The final kernel cmdline will be the following <<
261 <<
262 root="01234567-89ab-cdef-0123-456789abcd" ro <<
263 <<
264 176
265 Config File Limitation 177 Config File Limitation
266 ====================== 178 ======================
267 179
268 Currently the maximum config size size is 32KB 180 Currently the maximum config size size is 32KB and the total key-words (not
269 key-value entries) must be under 1024 nodes. 181 key-value entries) must be under 1024 nodes.
270 Note: this is not the number of entries but no 182 Note: this is not the number of entries but nodes, an entry must consume
271 more than 2 nodes (a key-word and a value). So 183 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 3 184 up to 512 key-value pairs. If keys contains 3 words in average, it can
273 contain 256 key-value pairs. In most cases, th 185 contain 256 key-value pairs. In most cases, the number of config items
274 will be under 100 entries and smaller than 8KB 186 will be under 100 entries and smaller than 8KB, so it would be enough.
275 If the node number exceeds 1024, parser return 187 If the node number exceeds 1024, parser returns an error even if the file
276 size is smaller than 32KB. (Note that this max 188 size is smaller than 32KB. (Note that this maximum size is not including
277 the padding null characters.) 189 the padding null characters.)
278 Anyway, since bootconfig command verifies it w 190 Anyway, since bootconfig command verifies it when appending a boot config
279 to initrd image, user can notice it before boo 191 to initrd image, user can notice it before boot.
280 192
281 193
282 Bootconfig APIs 194 Bootconfig APIs
283 =============== 195 ===============
284 196
285 User can query or loop on key-value pairs, als 197 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 u 198 a root (prefix) key node and find key-values under that node.
287 199
288 If you have a key string, you can query the va 200 If you have a key string, you can query the value directly with the key
289 using xbc_find_value(). If you want to know wh 201 using xbc_find_value(). If you want to know what keys exist in the boot
290 config, you can use xbc_for_each_key_value() t 202 config, you can use xbc_for_each_key_value() to iterate key-value pairs.
291 Note that you need to use xbc_array_for_each_v 203 Note that you need to use xbc_array_for_each_value() for accessing
292 each array's value, e.g.:: 204 each array's value, e.g.::
293 205
294 vnode = NULL; 206 vnode = NULL;
295 xbc_find_value("key.word", &vnode); 207 xbc_find_value("key.word", &vnode);
296 if (vnode && xbc_node_is_array(vnode)) 208 if (vnode && xbc_node_is_array(vnode))
297 xbc_array_for_each_value(vnode, value) { 209 xbc_array_for_each_value(vnode, value) {
298 printk("%s ", value); 210 printk("%s ", value);
299 } 211 }
300 212
301 If you want to focus on keys which have a pref 213 If you want to focus on keys which have a prefix string, you can use
302 xbc_find_node() to find a node by the prefix s 214 xbc_find_node() to find a node by the prefix string, and iterate
303 keys under the prefix node with xbc_node_for_e 215 keys under the prefix node with xbc_node_for_each_key_value().
304 216
305 But the most typical usage is to get the named 217 But the most typical usage is to get the named value under prefix
306 or get the named array under prefix as below:: 218 or get the named array under prefix as below::
307 219
308 root = xbc_find_node("key.prefix"); 220 root = xbc_find_node("key.prefix");
309 value = xbc_node_find_value(root, "option", & 221 value = xbc_node_find_value(root, "option", &vnode);
310 ... 222 ...
311 xbc_node_for_each_array_value(root, "array-op 223 xbc_node_for_each_array_value(root, "array-option", value, anode) {
312 ... 224 ...
313 } 225 }
314 226
315 This accesses a value of "key.prefix.option" a 227 This accesses a value of "key.prefix.option" and an array of
316 "key.prefix.array-option". 228 "key.prefix.array-option".
317 229
318 Locking is not needed, since after initializat 230 Locking is not needed, since after initialization, the config becomes
319 read-only. All data and keys must be copied if 231 read-only. All data and keys must be copied if you need to modify it.
320 232
321 233
322 Functions and structures 234 Functions and structures
323 ======================== 235 ========================
324 236
325 .. kernel-doc:: include/linux/bootconfig.h 237 .. kernel-doc:: include/linux/bootconfig.h
326 .. kernel-doc:: lib/bootconfig.c 238 .. kernel-doc:: lib/bootconfig.c
327 239
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