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 Moreover, sub-keys and a value can coexist under a parent key.
93 For example, following config is allowed.:: 93 For example, following config is allowed.::
94 94
95 foo = value1 95 foo = value1
96 foo.bar = value2 96 foo.bar = value2
97 foo := value3 # This will update foo's value. 97 foo := value3 # This will update foo's value.
98 98
99 Note, since there is no syntax to put a raw va 99 Note, since there is no syntax to put a raw value directly under a
100 structured key, you have to define it outside 100 structured key, you have to define it outside of the brace. For example::
101 101
102 foo { 102 foo {
103 bar = value1 103 bar = value1
104 bar { 104 bar {
105 baz = value2 105 baz = value2
106 qux = value3 106 qux = value3
107 } 107 }
108 } 108 }
109 109
110 Also, the order of the value node under a key 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 t 111 are a value and subkeys, the value is always the first child node
112 of the key. Thus if user specifies subkeys fir 112 of the key. Thus if user specifies subkeys first, e.g.::
113 113
114 foo.bar = value1 114 foo.bar = value1
115 foo = value2 115 foo = value2
116 116
117 In the program (and /proc/bootconfig), it will 117 In the program (and /proc/bootconfig), it will be shown as below::
118 118
119 foo = value2 119 foo = value2
120 foo.bar = value1 120 foo.bar = value1
121 121
122 Comments 122 Comments
123 -------- 123 --------
124 124
125 The config syntax accepts shell-script style c 125 The config syntax accepts shell-script style comments. The comments starting
126 with hash ("#") until newline ("\n") will be i 126 with hash ("#") until newline ("\n") will be ignored.
127 127
128 :: 128 ::
129 129
130 # comment line 130 # comment line
131 foo = value # value is set to foo. 131 foo = value # value is set to foo.
132 bar = 1, # 1st element 132 bar = 1, # 1st element
133 2, # 2nd element 133 2, # 2nd element
134 3 # 3rd element 134 3 # 3rd element
135 135
136 This is parsed as below:: 136 This is parsed as below::
137 137
138 foo = value 138 foo = value
139 bar = 1, 2, 3 139 bar = 1, 2, 3
140 140
141 Note that you can not put a comment between va 141 Note that you can not put a comment between value and delimiter(``,`` or
142 ``;``). This means following config has a synt 142 ``;``). This means following config has a syntax error ::
143 143
144 key = 1 # comment 144 key = 1 # comment
145 ,2 145 ,2
146 146
147 147
148 /proc/bootconfig 148 /proc/bootconfig
149 ================ 149 ================
150 150
151 /proc/bootconfig is a user-space interface of 151 /proc/bootconfig is a user-space interface of the boot config.
152 Unlike /proc/cmdline, this file shows the key- 152 Unlike /proc/cmdline, this file shows the key-value style list.
153 Each key-value pair is shown in each line with 153 Each key-value pair is shown in each line with following style::
154 154
155 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 155 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
156 156
157 157
158 Boot Kernel With a Boot Config 158 Boot Kernel With a Boot Config
159 ============================== 159 ==============================
160 160
161 There are two options to boot the kernel with !! 161 Since the boot configuration file is loaded with initrd, it will be added
162 bootconfig to the initrd image or embedding it !! 162 to the end of the initrd (initramfs) image file with padding, size,
163 !! 163 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 <<
170 164
171 [initrd][bootconfig][padding][size(le32)][chec 165 [initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
172 166
173 The size and checksum fields are unsigned 32bi 167 The size and checksum fields are unsigned 32bit little endian value.
174 168
175 When the boot configuration is added to the in 169 When the boot configuration is added to the initrd image, the total
176 file size is aligned to 4 bytes. To fill the g 170 file size is aligned to 4 bytes. To fill the gap, null characters
177 (``\0``) will be added. Thus the ``size`` is t 171 (``\0``) will be added. Thus the ``size`` is the length of the bootconfig
178 file + padding bytes. 172 file + padding bytes.
179 173
180 The Linux kernel decodes the last part of the 174 The Linux kernel decodes the last part of the initrd image in memory to
181 get the boot configuration data. 175 get the boot configuration data.
182 Because of this "piggyback" method, there is n 176 Because of this "piggyback" method, there is no need to change or
183 update the boot loader and the kernel image it 177 update the boot loader and the kernel image itself as long as the boot
184 loader passes the correct initrd file size. If 178 loader passes the correct initrd file size. If by any chance, the boot
185 loader passes a longer size, the kernel fails 179 loader passes a longer size, the kernel fails to find the bootconfig data.
186 180
187 To do this operation, Linux kernel provides `` 181 To do this operation, Linux kernel provides ``bootconfig`` command under
188 tools/bootconfig, which allows admin to apply 182 tools/bootconfig, which allows admin to apply or delete the config file
189 to/from initrd image. You can build it by the 183 to/from initrd image. You can build it by the following command::
190 184
191 # make -C tools/bootconfig 185 # make -C tools/bootconfig
192 186
193 To add your boot config file to initrd image, 187 To add your boot config file to initrd image, run bootconfig as below
194 (Old data is removed automatically if exists): 188 (Old data is removed automatically if exists)::
195 189
196 # tools/bootconfig/bootconfig -a your-config 190 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
197 191
198 To remove the config from the image, you can u 192 To remove the config from the image, you can use -d option as below::
199 193
200 # tools/bootconfig/bootconfig -d /boot/initrd 194 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
201 195
202 Then add "bootconfig" on the normal kernel com 196 Then add "bootconfig" on the normal kernel command line to tell the
203 kernel to look for the bootconfig at the end o 197 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. <<
225 198
226 Note that even if you set this option, you can <<
227 bootconfig by another bootconfig which attache <<
228 199
229 Kernel parameters via Boot Config 200 Kernel parameters via Boot Config
230 ================================= 201 =================================
231 202
232 In addition to the kernel command line, the bo 203 In addition to the kernel command line, the boot config can be used for
233 passing the kernel parameters. All the key-val 204 passing the kernel parameters. All the key-value pairs under ``kernel``
234 key will be passed to kernel cmdline directly. 205 key will be passed to kernel cmdline directly. Moreover, the key-value
235 pairs under ``init`` will be passed to init pr 206 pairs under ``init`` will be passed to init process via the cmdline.
236 The parameters are concatenated with user-give !! 207 The parameters are concatinated with user-given kernel cmdline string
237 as the following order, so that the command li 208 as the following order, so that the command line parameter can override
238 bootconfig parameters (this depends on how the 209 bootconfig parameters (this depends on how the subsystem handles parameters
239 but in general, earlier parameter will be over 210 but in general, earlier parameter will be overwritten by later one.)::
240 211
241 [bootconfig params][cmdline params] -- [bootc 212 [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params]
242 213
243 Here is an example of the bootconfig file for 214 Here is an example of the bootconfig file for kernel/init parameters.::
244 215
245 kernel { 216 kernel {
246 root = 01234567-89ab-cdef-0123-456789abcd 217 root = 01234567-89ab-cdef-0123-456789abcd
247 } 218 }
248 init { 219 init {
249 splash 220 splash
250 } 221 }
251 222
252 This will be copied into the kernel cmdline st 223 This will be copied into the kernel cmdline string as the following::
253 224
254 root="01234567-89ab-cdef-0123-456789abcd" -- 225 root="01234567-89ab-cdef-0123-456789abcd" -- splash
255 226
256 If user gives some other command line like,:: 227 If user gives some other command line like,::
257 228
258 ro bootconfig -- quiet 229 ro bootconfig -- quiet
259 230
260 The final kernel cmdline will be the following 231 The final kernel cmdline will be the following::
261 232
262 root="01234567-89ab-cdef-0123-456789abcd" ro 233 root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet
263 234
264 235
265 Config File Limitation 236 Config File Limitation
266 ====================== 237 ======================
267 238
268 Currently the maximum config size size is 32KB 239 Currently the maximum config size size is 32KB and the total key-words (not
269 key-value entries) must be under 1024 nodes. 240 key-value entries) must be under 1024 nodes.
270 Note: this is not the number of entries but no 241 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 242 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 3 243 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 244 contain 256 key-value pairs. In most cases, the number of config items
274 will be under 100 entries and smaller than 8KB 245 will be under 100 entries and smaller than 8KB, so it would be enough.
275 If the node number exceeds 1024, parser return 246 If the node number exceeds 1024, parser returns an error even if the file
276 size is smaller than 32KB. (Note that this max 247 size is smaller than 32KB. (Note that this maximum size is not including
277 the padding null characters.) 248 the padding null characters.)
278 Anyway, since bootconfig command verifies it w 249 Anyway, since bootconfig command verifies it when appending a boot config
279 to initrd image, user can notice it before boo 250 to initrd image, user can notice it before boot.
280 251
281 252
282 Bootconfig APIs 253 Bootconfig APIs
283 =============== 254 ===============
284 255
285 User can query or loop on key-value pairs, als 256 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 257 a root (prefix) key node and find key-values under that node.
287 258
288 If you have a key string, you can query the va 259 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 260 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 261 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 262 Note that you need to use xbc_array_for_each_value() for accessing
292 each array's value, e.g.:: 263 each array's value, e.g.::
293 264
294 vnode = NULL; 265 vnode = NULL;
295 xbc_find_value("key.word", &vnode); 266 xbc_find_value("key.word", &vnode);
296 if (vnode && xbc_node_is_array(vnode)) 267 if (vnode && xbc_node_is_array(vnode))
297 xbc_array_for_each_value(vnode, value) { 268 xbc_array_for_each_value(vnode, value) {
298 printk("%s ", value); 269 printk("%s ", value);
299 } 270 }
300 271
301 If you want to focus on keys which have a pref 272 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 273 xbc_find_node() to find a node by the prefix string, and iterate
303 keys under the prefix node with xbc_node_for_e 274 keys under the prefix node with xbc_node_for_each_key_value().
304 275
305 But the most typical usage is to get the named 276 But the most typical usage is to get the named value under prefix
306 or get the named array under prefix as below:: 277 or get the named array under prefix as below::
307 278
308 root = xbc_find_node("key.prefix"); 279 root = xbc_find_node("key.prefix");
309 value = xbc_node_find_value(root, "option", & 280 value = xbc_node_find_value(root, "option", &vnode);
310 ... 281 ...
311 xbc_node_for_each_array_value(root, "array-op 282 xbc_node_for_each_array_value(root, "array-option", value, anode) {
312 ... 283 ...
313 } 284 }
314 285
315 This accesses a value of "key.prefix.option" a 286 This accesses a value of "key.prefix.option" and an array of
316 "key.prefix.array-option". 287 "key.prefix.array-option".
317 288
318 Locking is not needed, since after initializat 289 Locking is not needed, since after initialization, the config becomes
319 read-only. All data and keys must be copied if 290 read-only. All data and keys must be copied if you need to modify it.
320 291
321 292
322 Functions and structures 293 Functions and structures
323 ======================== 294 ========================
324 295
325 .. kernel-doc:: include/linux/bootconfig.h 296 .. kernel-doc:: include/linux/bootconfig.h
326 .. kernel-doc:: lib/bootconfig.c 297 .. kernel-doc:: lib/bootconfig.c
327 298
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