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 65Note that a sub-key and a value can not co-exist under a parent key. 66For example, following config is NOT allowed.:: 67 68 foo = value1 69 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist 70 71 72Comments 73-------- 74 75The config syntax accepts shell-script style comments. The comments starting 76with hash ("#") until newline ("\n") will be ignored. 77 78:: 79 80 # comment line 81 foo = value # value is set to foo. 82 bar = 1, # 1st element 83 2, # 2nd element 84 3 # 3rd element 85 86This is parsed as below:: 87 88 foo = value 89 bar = 1, 2, 3 90 91Note that you can not put a comment between value and delimiter(``,`` or 92``;``). This means following config has a syntax error :: 93 94 key = 1 # comment 95 ,2 96 97 98/proc/bootconfig 99================ 100 101/proc/bootconfig is a user-space interface of the boot config. 102Unlike /proc/cmdline, this file shows the key-value style list. 103Each key-value pair is shown in each line with following style:: 104 105 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 106 107 108Boot Kernel With a Boot Config 109============================== 110 111Since the boot configuration file is loaded with initrd, it will be added 112to the end of the initrd (initramfs) image file with size, checksum and 11312-byte magic word as below. 114 115[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n] 116 117The Linux kernel decodes the last part of the initrd image in memory to 118get the boot configuration data. 119Because of this "piggyback" method, there is no need to change or 120update the boot loader and the kernel image itself. 121 122To do this operation, Linux kernel provides "bootconfig" command under 123tools/bootconfig, which allows admin to apply or delete the config file 124to/from initrd image. You can build it by the following command:: 125 126 # make -C tools/bootconfig 127 128To add your boot config file to initrd image, run bootconfig as below 129(Old data is removed automatically if exists):: 130 131 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z 132 133To remove the config from the image, you can use -d option as below:: 134 135 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z 136 137Then add "bootconfig" on the normal kernel command line to tell the 138kernel to look for the bootconfig at the end of the initrd file. 139 140Config File Limitation 141====================== 142 143Currently the maximum config size size is 32KB and the total key-words (not 144key-value entries) must be under 1024 nodes. 145Note: this is not the number of entries but nodes, an entry must consume 146more than 2 nodes (a key-word and a value). So theoretically, it will be 147up to 512 key-value pairs. If keys contains 3 words in average, it can 148contain 256 key-value pairs. In most cases, the number of config items 149will be under 100 entries and smaller than 8KB, so it would be enough. 150If the node number exceeds 1024, parser returns an error even if the file 151size is smaller than 32KB. 152Anyway, since bootconfig command verifies it when appending a boot config 153to initrd image, user can notice it before boot. 154 155 156Bootconfig APIs 157=============== 158 159User can query or loop on key-value pairs, also it is possible to find 160a root (prefix) key node and find key-values under that node. 161 162If you have a key string, you can query the value directly with the key 163using xbc_find_value(). If you want to know what keys exist in the boot 164config, you can use xbc_for_each_key_value() to iterate key-value pairs. 165Note that you need to use xbc_array_for_each_value() for accessing 166each array's value, e.g.:: 167 168 vnode = NULL; 169 xbc_find_value("key.word", &vnode); 170 if (vnode && xbc_node_is_array(vnode)) 171 xbc_array_for_each_value(vnode, value) { 172 printk("%s ", value); 173 } 174 175If you want to focus on keys which have a prefix string, you can use 176xbc_find_node() to find a node by the prefix string, and iterate 177keys under the prefix node with xbc_node_for_each_key_value(). 178 179But the most typical usage is to get the named value under prefix 180or get the named array under prefix as below:: 181 182 root = xbc_find_node("key.prefix"); 183 value = xbc_node_find_value(root, "option", &vnode); 184 ... 185 xbc_node_for_each_array_value(root, "array-option", value, anode) { 186 ... 187 } 188 189This accesses a value of "key.prefix.option" and an array of 190"key.prefix.array-option". 191 192Locking is not needed, since after initialization, the config becomes 193read-only. All data and keys must be copied if you need to modify it. 194 195 196Functions and structures 197======================== 198 199.. kernel-doc:: include/linux/bootconfig.h 200.. kernel-doc:: lib/bootconfig.c 201 202