mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-27 19:29:37 +00:00
![Masami Hiramatsu](/assets/img/avatar_default.png)
Add a sentence about bootconfig override operator (":=") to bootconfig.rst. Link: https://lkml.kernel.org/r/159482884682.126704.7198860675721719878.stgit@devnote2 Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
229 lines
7 KiB
ReStructuredText
229 lines
7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
.. _bootconfig:
|
|
|
|
==================
|
|
Boot Configuration
|
|
==================
|
|
|
|
:Author: Masami Hiramatsu <mhiramat@kernel.org>
|
|
|
|
Overview
|
|
========
|
|
|
|
The boot configuration expands the current kernel command line to support
|
|
additional key-value data when booting the kernel in an efficient way.
|
|
This allows administrators to pass a structured-Key config file.
|
|
|
|
Config File Syntax
|
|
==================
|
|
|
|
The boot config syntax is a simple structured key-value. Each key consists
|
|
of dot-connected-words, and key and value are connected by ``=``. The value
|
|
has to be terminated by semi-colon (``;``) or newline (``\n``).
|
|
For array value, array entries are separated by comma (``,``). ::
|
|
|
|
KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
|
|
|
|
Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.
|
|
|
|
Each key word must contain only alphabets, numbers, dash (``-``) or underscore
|
|
(``_``). And each value only contains printable characters or spaces except
|
|
for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
|
|
hash (``#``) and closing brace (``}``).
|
|
|
|
If you want to use those delimiters in a value, you can use either double-
|
|
quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
|
|
you can not escape these quotes.
|
|
|
|
There can be a key which doesn't have value or has an empty value. Those keys
|
|
are used for checking if the key exists or not (like a boolean).
|
|
|
|
Key-Value Syntax
|
|
----------------
|
|
|
|
The boot config file syntax allows user to merge partially same word keys
|
|
by brace. For example::
|
|
|
|
foo.bar.baz = value1
|
|
foo.bar.qux.quux = value2
|
|
|
|
These can be written also in::
|
|
|
|
foo.bar {
|
|
baz = value1
|
|
qux.quux = value2
|
|
}
|
|
|
|
Or more shorter, written as following::
|
|
|
|
foo.bar { baz = value1; qux.quux = value2 }
|
|
|
|
In both styles, same key words are automatically merged when parsing it
|
|
at boot time. So you can append similar trees or key-values.
|
|
|
|
Same-key Values
|
|
---------------
|
|
|
|
It is prohibited that two or more values or arrays share a same-key.
|
|
For example,::
|
|
|
|
foo = bar, baz
|
|
foo = qux # !ERROR! we can not re-define same key
|
|
|
|
If you want to update the value, you must use the override operator
|
|
``:=`` explicitly. For example::
|
|
|
|
foo = bar, baz
|
|
foo := qux
|
|
|
|
then, the ``qux`` is assigned to ``foo`` key. This is useful for
|
|
overriding the default value by adding (partial) custom bootconfigs
|
|
without parsing the default bootconfig.
|
|
|
|
If you want to append the value to existing key as an array member,
|
|
you can use ``+=`` operator. For example::
|
|
|
|
foo = bar, baz
|
|
foo += qux
|
|
|
|
In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
|
|
|
|
However, a sub-key and a value can not co-exist under a parent key.
|
|
For example, following config is NOT allowed.::
|
|
|
|
foo = value1
|
|
foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
|
|
foo.bar := value2 # !ERROR! even with the override operator, this is NOT allowed.
|
|
|
|
|
|
Comments
|
|
--------
|
|
|
|
The config syntax accepts shell-script style comments. The comments starting
|
|
with hash ("#") until newline ("\n") will be ignored.
|
|
|
|
::
|
|
|
|
# comment line
|
|
foo = value # value is set to foo.
|
|
bar = 1, # 1st element
|
|
2, # 2nd element
|
|
3 # 3rd element
|
|
|
|
This is parsed as below::
|
|
|
|
foo = value
|
|
bar = 1, 2, 3
|
|
|
|
Note that you can not put a comment between value and delimiter(``,`` or
|
|
``;``). This means following config has a syntax error ::
|
|
|
|
key = 1 # comment
|
|
,2
|
|
|
|
|
|
/proc/bootconfig
|
|
================
|
|
|
|
/proc/bootconfig is a user-space interface of the boot config.
|
|
Unlike /proc/cmdline, this file shows the key-value style list.
|
|
Each key-value pair is shown in each line with following style::
|
|
|
|
KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
|
|
|
|
|
|
Boot Kernel With a Boot Config
|
|
==============================
|
|
|
|
Since the boot configuration file is loaded with initrd, it will be added
|
|
to the end of the initrd (initramfs) image file with size, checksum and
|
|
12-byte magic word as below.
|
|
|
|
[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n]
|
|
|
|
The Linux kernel decodes the last part of the initrd image in memory to
|
|
get the boot configuration data.
|
|
Because of this "piggyback" method, there is no need to change or
|
|
update the boot loader and the kernel image itself.
|
|
|
|
To do this operation, Linux kernel provides "bootconfig" command under
|
|
tools/bootconfig, which allows admin to apply or delete the config file
|
|
to/from initrd image. You can build it by the following command::
|
|
|
|
# make -C tools/bootconfig
|
|
|
|
To add your boot config file to initrd image, run bootconfig as below
|
|
(Old data is removed automatically if exists)::
|
|
|
|
# tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
|
|
|
|
To remove the config from the image, you can use -d option as below::
|
|
|
|
# tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
|
|
|
|
Then add "bootconfig" on the normal kernel command line to tell the
|
|
kernel to look for the bootconfig at the end of the initrd file.
|
|
|
|
Config File Limitation
|
|
======================
|
|
|
|
Currently the maximum config size size is 32KB and the total key-words (not
|
|
key-value entries) must be under 1024 nodes.
|
|
Note: this is not the number of entries but nodes, an entry must consume
|
|
more than 2 nodes (a key-word and a value). So theoretically, it will be
|
|
up to 512 key-value pairs. If keys contains 3 words in average, it can
|
|
contain 256 key-value pairs. In most cases, the number of config items
|
|
will be under 100 entries and smaller than 8KB, so it would be enough.
|
|
If the node number exceeds 1024, parser returns an error even if the file
|
|
size is smaller than 32KB.
|
|
Anyway, since bootconfig command verifies it when appending a boot config
|
|
to initrd image, user can notice it before boot.
|
|
|
|
|
|
Bootconfig APIs
|
|
===============
|
|
|
|
User can query or loop on key-value pairs, also it is possible to find
|
|
a root (prefix) key node and find key-values under that node.
|
|
|
|
If you have a key string, you can query the value directly with the key
|
|
using xbc_find_value(). If you want to know what keys exist in the boot
|
|
config, you can use xbc_for_each_key_value() to iterate key-value pairs.
|
|
Note that you need to use xbc_array_for_each_value() for accessing
|
|
each array's value, e.g.::
|
|
|
|
vnode = NULL;
|
|
xbc_find_value("key.word", &vnode);
|
|
if (vnode && xbc_node_is_array(vnode))
|
|
xbc_array_for_each_value(vnode, value) {
|
|
printk("%s ", value);
|
|
}
|
|
|
|
If you want to focus on keys which have a prefix string, you can use
|
|
xbc_find_node() to find a node by the prefix string, and iterate
|
|
keys under the prefix node with xbc_node_for_each_key_value().
|
|
|
|
But the most typical usage is to get the named value under prefix
|
|
or get the named array under prefix as below::
|
|
|
|
root = xbc_find_node("key.prefix");
|
|
value = xbc_node_find_value(root, "option", &vnode);
|
|
...
|
|
xbc_node_for_each_array_value(root, "array-option", value, anode) {
|
|
...
|
|
}
|
|
|
|
This accesses a value of "key.prefix.option" and an array of
|
|
"key.prefix.array-option".
|
|
|
|
Locking is not needed, since after initialization, the config becomes
|
|
read-only. All data and keys must be copied if you need to modify it.
|
|
|
|
|
|
Functions and structures
|
|
========================
|
|
|
|
.. kernel-doc:: include/linux/bootconfig.h
|
|
.. kernel-doc:: lib/bootconfig.c
|
|
|