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Diffstat (limited to 'Documentation/admin-guide/bootconfig.rst')
| -rw-r--r-- | Documentation/admin-guide/bootconfig.rst | 127 |
1 files changed, 116 insertions, 11 deletions
diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst index cf2edcd09183..d99994345d41 100644 --- a/Documentation/admin-guide/bootconfig.rst +++ b/Documentation/admin-guide/bootconfig.rst @@ -23,7 +23,7 @@ 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[...]][;] + KEY[.WORD[...]] = VALUE[, VALUE2[...]][;] Unlike the kernel command line syntax, spaces are OK around the comma and ``=``. @@ -71,6 +71,16 @@ 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:: @@ -79,12 +89,35 @@ you can use ``+=`` operator. For example:: 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.:: +Moreover, sub-keys and a value can coexist under a parent key. +For example, following config is allowed.:: foo = value1 - foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist + foo.bar = value2 + foo := value3 # This will update foo's value. + +Note, since there is no syntax to put a raw value directly under a +structured key, you have to define it outside of the brace. For example:: + + foo { + bar = value1 + bar { + baz = value2 + qux = value3 + } + } + +Also, the order of the value node under a key is fixed. If there +are a value and subkeys, the value is always the first child node +of the key. Thus if user specifies subkeys first, e.g.:: + + foo.bar = value1 + foo = value2 + +In the program (and /proc/bootconfig), it will be shown as below:: + foo = value2 + foo.bar = value1 Comments -------- @@ -125,18 +158,33 @@ Each key-value pair is shown in each line with following style:: 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. +There are two options to boot the kernel with bootconfig: attaching the +bootconfig to the initrd image or embedding it in the kernel itself. -[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n] +Attaching a Boot Config to Initrd +--------------------------------- + +Since the boot configuration file is loaded with initrd by default, +it will be added to the end of the initrd (initramfs) image file with +padding, size, checksum and 12-byte magic word as below. + +[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n] + +The size and checksum fields are unsigned 32bit little endian value. + +When the boot configuration is added to the initrd image, the total +file size is aligned to 4 bytes. To fill the gap, null characters +(``\0``) will be added. Thus the ``size`` is the length of the bootconfig +file + padding bytes. 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. +update the boot loader and the kernel image itself as long as the boot +loader passes the correct initrd file size. If by any chance, the boot +loader passes a longer size, the kernel fails to find the bootconfig data. -To do this operation, Linux kernel provides "bootconfig" command under +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:: @@ -154,6 +202,62 @@ To remove the config from the image, you can use -d option as below:: 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. +Embedding a Boot Config into Kernel +----------------------------------- + +If you can not use initrd, you can also embed the bootconfig file in the +kernel by Kconfig options. In this case, you need to recompile the kernel +with the following configs:: + + CONFIG_BOOT_CONFIG_EMBED=y + CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE" + +``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative +path to the bootconfig file from source tree or object tree. +The kernel will embed it as the default bootconfig. + +Just as when attaching the bootconfig to the initrd, you need ``bootconfig`` +option on the kernel command line to enable the embedded bootconfig. + +Note that even if you set this option, you can override the embedded +bootconfig by another bootconfig which attached to the initrd. + +Kernel parameters via Boot Config +================================= + +In addition to the kernel command line, the boot config can be used for +passing the kernel parameters. All the key-value pairs under ``kernel`` +key will be passed to kernel cmdline directly. Moreover, the key-value +pairs under ``init`` will be passed to init process via the cmdline. +The parameters are concatinated with user-given kernel cmdline string +as the following order, so that the command line parameter can override +bootconfig parameters (this depends on how the subsystem handles parameters +but in general, earlier parameter will be overwritten by later one.):: + + [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params] + +Here is an example of the bootconfig file for kernel/init parameters.:: + + kernel { + root = 01234567-89ab-cdef-0123-456789abcd + } + init { + splash + } + +This will be copied into the kernel cmdline string as the following:: + + root="01234567-89ab-cdef-0123-456789abcd" -- splash + +If user gives some other command line like,:: + + ro bootconfig -- quiet + +The final kernel cmdline will be the following:: + + root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet + + Config File Limitation ====================== @@ -165,7 +269,8 @@ 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. +size is smaller than 32KB. (Note that this maximum size is not including +the padding null characters.) Anyway, since bootconfig command verifies it when appending a boot config to initrd image, user can notice it before boot. |