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 26KEY[.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 74Also, a sub-key and a value can not co-exist under a parent key. 75For example, following config is NOT allowed.:: 76 77 foo = value1 78 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist 79 80 81Comments 82-------- 83 84The config syntax accepts shell-script style comments. The comments starting 85with hash ("#") until newline ("\n") will be ignored. 86 87:: 88 89 # comment line 90 foo = value # value is set to foo. 91 bar = 1, # 1st element 92 2, # 2nd element 93 3 # 3rd element 94 95This is parsed as below:: 96 97 foo = value 98 bar = 1, 2, 3 99 100Note that you can not put a comment between value and delimiter(``,`` or 101``;``). This means following config has a syntax error :: 102 103 key = 1 # comment 104 ,2 105 106 107/proc/bootconfig 108================ 109 110/proc/bootconfig is a user-space interface of the boot config. 111Unlike /proc/cmdline, this file shows the key-value style list. 112Each key-value pair is shown in each line with following style:: 113 114 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 115 116 117Boot Kernel With a Boot Config 118============================== 119 120Since the boot configuration file is loaded with initrd, it will be added 121to the end of the initrd (initramfs) image file with size, checksum and 12212-byte magic word as below. 123 124[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n] 125 126The Linux kernel decodes the last part of the initrd image in memory to 127get the boot configuration data. 128Because of this "piggyback" method, there is no need to change or 129update the boot loader and the kernel image itself. 130 131To do this operation, Linux kernel provides "bootconfig" command under 132tools/bootconfig, which allows admin to apply or delete the config file 133to/from initrd image. You can build it by the following command:: 134 135 # make -C tools/bootconfig 136 137To add your boot config file to initrd image, run bootconfig as below 138(Old data is removed automatically if exists):: 139 140 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z 141 142To remove the config from the image, you can use -d option as below:: 143 144 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z 145 146Then add "bootconfig" on the normal kernel command line to tell the 147kernel to look for the bootconfig at the end of the initrd file. 148 149Config File Limitation 150====================== 151 152Currently the maximum config size size is 32KB and the total key-words (not 153key-value entries) must be under 1024 nodes. 154Note: this is not the number of entries but nodes, an entry must consume 155more than 2 nodes (a key-word and a value). So theoretically, it will be 156up to 512 key-value pairs. If keys contains 3 words in average, it can 157contain 256 key-value pairs. In most cases, the number of config items 158will be under 100 entries and smaller than 8KB, so it would be enough. 159If the node number exceeds 1024, parser returns an error even if the file 160size is smaller than 32KB. 161Anyway, since bootconfig command verifies it when appending a boot config 162to initrd image, user can notice it before boot. 163 164 165Bootconfig APIs 166=============== 167 168User can query or loop on key-value pairs, also it is possible to find 169a root (prefix) key node and find key-values under that node. 170 171If you have a key string, you can query the value directly with the key 172using xbc_find_value(). If you want to know what keys exist in the boot 173config, you can use xbc_for_each_key_value() to iterate key-value pairs. 174Note that you need to use xbc_array_for_each_value() for accessing 175each array's value, e.g.:: 176 177 vnode = NULL; 178 xbc_find_value("key.word", &vnode); 179 if (vnode && xbc_node_is_array(vnode)) 180 xbc_array_for_each_value(vnode, value) { 181 printk("%s ", value); 182 } 183 184If you want to focus on keys which have a prefix string, you can use 185xbc_find_node() to find a node by the prefix string, and iterate 186keys under the prefix node with xbc_node_for_each_key_value(). 187 188But the most typical usage is to get the named value under prefix 189or get the named array under prefix as below:: 190 191 root = xbc_find_node("key.prefix"); 192 value = xbc_node_find_value(root, "option", &vnode); 193 ... 194 xbc_node_for_each_array_value(root, "array-option", value, anode) { 195 ... 196 } 197 198This accesses a value of "key.prefix.option" and an array of 199"key.prefix.array-option". 200 201Locking is not needed, since after initialization, the config becomes 202read-only. All data and keys must be copied if you need to modify it. 203 204 205Functions and structures 206======================== 207 208.. kernel-doc:: include/linux/bootconfig.h 209.. kernel-doc:: lib/bootconfig.c 210 211