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 65Comments 66-------- 67 68The config syntax accepts shell-script style comments. The comments starting 69with hash ("#") until newline ("\n") will be ignored. 70 71:: 72 73 # comment line 74 foo = value # value is set to foo. 75 bar = 1, # 1st element 76 2, # 2nd element 77 3 # 3rd element 78 79This is parsed as below:: 80 81 foo = value 82 bar = 1, 2, 3 83 84Note that you can not put a comment between value and delimiter(``,`` or 85``;``). This means following config has a syntax error :: 86 87 key = 1 # comment 88 ,2 89 90 91/proc/bootconfig 92================ 93 94/proc/bootconfig is a user-space interface of the boot config. 95Unlike /proc/cmdline, this file shows the key-value style list. 96Each key-value pair is shown in each line with following style:: 97 98 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 99 100 101Boot Kernel With a Boot Config 102============================== 103 104Since the boot configuration file is loaded with initrd, it will be added 105to the end of the initrd (initramfs) image file with size, checksum and 10612-byte magic word as below. 107 108[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n] 109 110The Linux kernel decodes the last part of the initrd image in memory to 111get the boot configuration data. 112Because of this "piggyback" method, there is no need to change or 113update the boot loader and the kernel image itself. 114 115To do this operation, Linux kernel provides "bootconfig" command under 116tools/bootconfig, which allows admin to apply or delete the config file 117to/from initrd image. You can build it by the following command:: 118 119 # make -C tools/bootconfig 120 121To add your boot config file to initrd image, run bootconfig as below 122(Old data is removed automatically if exists):: 123 124 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z 125 126To remove the config from the image, you can use -d option as below:: 127 128 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z 129 130Then add "bootconfig" on the normal kernel command line to tell the 131kernel to look for the bootconfig at the end of the initrd file. 132 133Config File Limitation 134====================== 135 136Currently the maximum config size size is 32KB and the total key-words (not 137key-value entries) must be under 1024 nodes. 138Note: this is not the number of entries but nodes, an entry must consume 139more than 2 nodes (a key-word and a value). So theoretically, it will be 140up to 512 key-value pairs. If keys contains 3 words in average, it can 141contain 256 key-value pairs. In most cases, the number of config items 142will be under 100 entries and smaller than 8KB, so it would be enough. 143If the node number exceeds 1024, parser returns an error even if the file 144size is smaller than 32KB. 145Anyway, since bootconfig command verifies it when appending a boot config 146to initrd image, user can notice it before boot. 147 148 149Bootconfig APIs 150=============== 151 152User can query or loop on key-value pairs, also it is possible to find 153a root (prefix) key node and find key-values under that node. 154 155If you have a key string, you can query the value directly with the key 156using xbc_find_value(). If you want to know what keys exist in the boot 157config, you can use xbc_for_each_key_value() to iterate key-value pairs. 158Note that you need to use xbc_array_for_each_value() for accessing 159each array's value, e.g.:: 160 161 vnode = NULL; 162 xbc_find_value("key.word", &vnode); 163 if (vnode && xbc_node_is_array(vnode)) 164 xbc_array_for_each_value(vnode, value) { 165 printk("%s ", value); 166 } 167 168If you want to focus on keys which have a prefix string, you can use 169xbc_find_node() to find a node by the prefix string, and iterate 170keys under the prefix node with xbc_node_for_each_key_value(). 171 172But the most typical usage is to get the named value under prefix 173or get the named array under prefix as below:: 174 175 root = xbc_find_node("key.prefix"); 176 value = xbc_node_find_value(root, "option", &vnode); 177 ... 178 xbc_node_for_each_array_value(root, "array-option", value, anode) { 179 ... 180 } 181 182This accesses a value of "key.prefix.option" and an array of 183"key.prefix.array-option". 184 185Locking is not needed, since after initialization, the config becomes 186read-only. All data and keys must be copied if you need to modify it. 187 188 189Functions and structures 190======================== 191 192.. kernel-doc:: include/linux/bootconfig.h 193.. kernel-doc:: lib/bootconfig.c 194 195