1.. SPDX-License-Identifier: GPL-2.0 2 3================== 4Boot Configuration 5================== 6 7:Author: Masami Hiramatsu <mhiramat@kernel.org> 8 9Overview 10======== 11 12The boot configuration is expanding current kernel cmdline to support 13additional key-value data when boot the kernel in an efficient way. 14This allows adoministrators to pass a structured-Key config file. 15 16Config File Syntax 17================== 18 19The boot config syntax is a simple structured key-value. Each key consists 20of dot-connected-words, and key and value are connected by "=". The value 21has to be terminated by semi-colon (``;``) or newline (``\n``). 22For array value, array entries are separated by comma (``,``). :: 23 24KEY[.WORD[...]] = VALUE[, VALUE2[...]][;] 25 26Each key word must contain only alphabets, numbers, dash (``-``) or underscore 27(``_``). And each value only contains printable characters or spaces except 28for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``), 29hash (``#``) and closing brace (``}``). 30 31If you want to use those delimiters in a value, you can use either double- 32quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that 33you can not escape these quotes. 34 35There can be a key which doesn't have value or has an empty value. Those keys 36are used for checking the key exists or not (like a boolean). 37 38Key-Value Syntax 39---------------- 40 41The boot config file syntax allows user to merge partially same word keys 42by brace. For example:: 43 44 foo.bar.baz = value1 45 foo.bar.qux.quux = value2 46 47These can be written also in:: 48 49 foo.bar { 50 baz = value1 51 qux.quux = value2 52 } 53 54Or more shorter, written as following:: 55 56 foo.bar { baz = value1; qux.quux = value2 } 57 58In both styles, same key words are automatically merged when parsing it 59at boot time. So you can append similar trees or key-values. 60 61Comments 62-------- 63 64The config syntax accepts shell-script style comments. The comments start 65with hash ("#") until newline ("\n") will be ignored. 66 67:: 68 69 # comment line 70 foo = value # value is set to foo. 71 bar = 1, # 1st element 72 2, # 2nd element 73 3 # 3rd element 74 75This is parsed as below:: 76 77 foo = value 78 bar = 1, 2, 3 79 80Note that you can not put a comment between value and delimiter(``,`` or 81``;``). This means following config has a syntax error :: 82 83 key = 1 # comment 84 ,2 85 86 87/proc/bootconfig 88================ 89 90/proc/bootconfig is a user-space interface of the boot config. 91Unlike /proc/cmdline, this file shows the key-value style list. 92Each key-value pair is shown in each line with following style:: 93 94 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 95 96 97Boot Kernel With a Boot Config 98============================== 99 100Since the boot configuration file is loaded with initrd, it will be added 101to the end of the initrd (initramfs) image file. The Linux kernel decodes 102the last part of the initrd image in memory to get the boot configuration 103data. 104Because of this "piggyback" method, there is no need to change or 105update the boot loader and the kernel image itself. 106 107To do this operation, Linux kernel provides "bootconfig" command under 108tools/bootconfig, which allows admin to apply or delete the config file 109to/from initrd image. You can build it by follwoing command:: 110 111 # make -C tools/bootconfig 112 113To add your boot config file to initrd image, run bootconfig as below 114(Old data is removed automatically if exists):: 115 116 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z 117 118To remove the config from the image, you can use -d option as below:: 119 120 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z 121 122 123C onfig File Limitation 124====================== 125 126Currently the maximum config size size is 32KB and the total key-words (not 127key-value entries) must be under 1024 nodes. 128Note: this is not the number of entries but nodes, an entry must consume 129more than 2 nodes (a key-word and a value). So theoretically, it will be 130up to 512 key-value pairs. If keys contains 3 words in average, it can 131contain 256 key-value pairs. In most cases, the number of config items 132will be under 100 entries and smaller than 8KB, so it would be enough. 133If the node number exceeds 1024, parser returns an error even if the file 134size is smaller than 32KB. 135Anyway, since bootconfig command verifies it when appending a boot config 136to initrd image, user can notice it before boot. 137 138 139Bootconfig APIs 140=============== 141 142User can query or loop on key-value pairs, also it is possible to find 143a root (prefix) key node and find key-values under that node. 144 145If you have a key string, you can query the value directly with the key 146using xbc_find_value(). If you want to know what keys exist in the SKC 147tree, you can use xbc_for_each_key_value() to iterate key-value pairs. 148Note that you need to use xbc_array_for_each_value() for accessing 149each arraies value, e.g.:: 150 151 vnode = NULL; 152 xbc_find_value("key.word", &vnode); 153 if (vnode && xbc_node_is_array(vnode)) 154 xbc_array_for_each_value(vnode, value) { 155 printk("%s ", value); 156 } 157 158If you want to focus on keys which has a prefix string, you can use 159xbc_find_node() to find a node which prefix key words, and iterate 160keys under the prefix node with xbc_node_for_each_key_value(). 161 162But the most typical usage is to get the named value under prefix 163or get the named array under prefix as below:: 164 165 root = xbc_find_node("key.prefix"); 166 value = xbc_node_find_value(root, "option", &vnode); 167 ... 168 xbc_node_for_each_array_value(root, "array-option", value, anode) { 169 ... 170 } 171 172This accesses a value of "key.prefix.option" and an array of 173"key.prefix.array-option". 174 175Locking is not needed, since after initialized, the config becomes readonly. 176All data and keys must be copied if you need to modify it. 177 178 179Functions and structures 180======================== 181 182.. kernel-doc:: include/linux/bootconfig.h 183.. kernel-doc:: lib/bootconfig.c 184 185