1.. SPDX-License-Identifier: GPL-2.0 2 3.. _bootconfig: 4 5================== 6Boot Configuration 7================== 8 9:Author: Masami Hiramatsu <mhiramat@kernel.org> 10 11Overview 12======== 13 14The boot configuration expands the current kernel command line to support 15additional key-value data when booting the kernel in an efficient way. 16This allows administrators 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 26 KEY[.WORD[...]] = VALUE[, VALUE2[...]][;] 27 28Unlike the kernel command line syntax, spaces are OK around the comma and ``=``. 29 30Each key word must contain only alphabets, numbers, dash (``-``) or underscore 31(``_``). And each value only contains printable characters or spaces except 32for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``), 33hash (``#``) and closing brace (``}``). 34 35If you want to use those delimiters in a value, you can use either double- 36quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that 37you can not escape these quotes. 38 39There can be a key which doesn't have value or has an empty value. Those keys 40are used for checking if the key exists or not (like a boolean). 41 42Key-Value Syntax 43---------------- 44 45The boot config file syntax allows user to merge partially same word keys 46by brace. For example:: 47 48 foo.bar.baz = value1 49 foo.bar.qux.quux = value2 50 51These can be written also in:: 52 53 foo.bar { 54 baz = value1 55 qux.quux = value2 56 } 57 58Or more shorter, written as following:: 59 60 foo.bar { baz = value1; qux.quux = value2 } 61 62In both styles, same key words are automatically merged when parsing it 63at boot time. So you can append similar trees or key-values. 64 65Same-key Values 66--------------- 67 68It is prohibited that two or more values or arrays share a same-key. 69For example,:: 70 71 foo = bar, baz 72 foo = qux # !ERROR! we can not re-define same key 73 74If you want to append the value to existing key as an array member, 75you can use ``+=`` operator. For example:: 76 77 foo = bar, baz 78 foo += qux 79 80In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``. 81 82However, a sub-key and a value can not co-exist under a parent key. 83For example, following config is NOT allowed.:: 84 85 foo = value1 86 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist 87 88 89Comments 90-------- 91 92The config syntax accepts shell-script style comments. The comments starting 93with hash ("#") until newline ("\n") will be ignored. 94 95:: 96 97 # comment line 98 foo = value # value is set to foo. 99 bar = 1, # 1st element 100 2, # 2nd element 101 3 # 3rd element 102 103This is parsed as below:: 104 105 foo = value 106 bar = 1, 2, 3 107 108Note that you can not put a comment between value and delimiter(``,`` or 109``;``). This means following config has a syntax error :: 110 111 key = 1 # comment 112 ,2 113 114 115/proc/bootconfig 116================ 117 118/proc/bootconfig is a user-space interface of the boot config. 119Unlike /proc/cmdline, this file shows the key-value style list. 120Each key-value pair is shown in each line with following style:: 121 122 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 123 124 125Boot Kernel With a Boot Config 126============================== 127 128Since the boot configuration file is loaded with initrd, it will be added 129to the end of the initrd (initramfs) image file with size, checksum and 13012-byte magic word as below. 131 132[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n] 133 134The Linux kernel decodes the last part of the initrd image in memory to 135get the boot configuration data. 136Because of this "piggyback" method, there is no need to change or 137update the boot loader and the kernel image itself. 138 139To do this operation, Linux kernel provides "bootconfig" command under 140tools/bootconfig, which allows admin to apply or delete the config file 141to/from initrd image. You can build it by the following command:: 142 143 # make -C tools/bootconfig 144 145To add your boot config file to initrd image, run bootconfig as below 146(Old data is removed automatically if exists):: 147 148 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z 149 150To remove the config from the image, you can use -d option as below:: 151 152 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z 153 154Then add "bootconfig" on the normal kernel command line to tell the 155kernel to look for the bootconfig at the end of the initrd file. 156 157Config File Limitation 158====================== 159 160Currently the maximum config size size is 32KB and the total key-words (not 161key-value entries) must be under 1024 nodes. 162Note: this is not the number of entries but nodes, an entry must consume 163more than 2 nodes (a key-word and a value). So theoretically, it will be 164up to 512 key-value pairs. If keys contains 3 words in average, it can 165contain 256 key-value pairs. In most cases, the number of config items 166will be under 100 entries and smaller than 8KB, so it would be enough. 167If the node number exceeds 1024, parser returns an error even if the file 168size is smaller than 32KB. 169Anyway, since bootconfig command verifies it when appending a boot config 170to initrd image, user can notice it before boot. 171 172 173Bootconfig APIs 174=============== 175 176User can query or loop on key-value pairs, also it is possible to find 177a root (prefix) key node and find key-values under that node. 178 179If you have a key string, you can query the value directly with the key 180using xbc_find_value(). If you want to know what keys exist in the boot 181config, you can use xbc_for_each_key_value() to iterate key-value pairs. 182Note that you need to use xbc_array_for_each_value() for accessing 183each array's value, e.g.:: 184 185 vnode = NULL; 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); 190 } 191 192If you want to focus on keys which have a prefix string, you can use 193xbc_find_node() to find a node by the prefix string, and iterate 194keys under the prefix node with xbc_node_for_each_key_value(). 195 196But the most typical usage is to get the named value under prefix 197or get the named array under prefix as below:: 198 199 root = xbc_find_node("key.prefix"); 200 value = xbc_node_find_value(root, "option", &vnode); 201 ... 202 xbc_node_for_each_array_value(root, "array-option", value, anode) { 203 ... 204 } 205 206This accesses a value of "key.prefix.option" and an array of 207"key.prefix.array-option". 208 209Locking is not needed, since after initialization, the config becomes 210read-only. All data and keys must be copied if you need to modify it. 211 212 213Functions and structures 214======================== 215 216.. kernel-doc:: include/linux/bootconfig.h 217.. kernel-doc:: lib/bootconfig.c 218 219