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docs: initramfs: update compression and mtime descriptions

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Update the document to reflect that initramfs didn't replace initrd
following kernel 2.5.x.
The initramfs buffer format now supports many compression types in
addition to gzip, so include them in the grammar section.
c_mtime use is dependent on CONFIG_INITRAMFS_PRESERVE_MTIME.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20250402033949.852-2-ddiss@suse.de
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
David Disseldorp
2025-04-02 14:39:50 +11:00
committed by Christian Brauner
parent 0af2f6be1b
commit 418556fa57
1 changed files with 21 additions and 13 deletions
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34
Documentation/driver-api/early-userspace/buffer-format.rst
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@@ -4,20 +4,18 @@ initramfs buffer format
Al Viro, H. Peter Anvin
Last revision: 2002-01-13
Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
getting {replaced/complemented} with the new "initial ramfs"
(initramfs) protocol. The initramfs contents is passed using the same
memory buffer protocol used by the initrd protocol, but the contents
With kernel 2.5.x, the old "initial ramdisk" protocol was complemented
with an "initial ramfs" protocol. The initramfs content is passed
using the same memory buffer protocol used by initrd, but the content
is different. The initramfs buffer contains an archive which is
expanded into a ramfs filesystem; this document details the format of
the initramfs buffer format.
expanded into a ramfs filesystem; this document details the initramfs
buffer format.
The initramfs buffer format is based around the "newc" or "crc" CPIO
formats, and can be created with the cpio(1) utility. The cpio
archive can be compressed using gzip(1). One valid version of an
initramfs buffer is thus a single .cpio.gz file.
archive can be compressed using gzip(1), or any other algorithm provided
via CONFIG_DECOMPRESS_*. One valid version of an initramfs buffer is
thus a single .cpio.gz file.
The full format of the initramfs buffer is defined by the following
grammar, where::
@@ -25,12 +23,20 @@ grammar, where::
* is used to indicate "0 or more occurrences of"
(|) indicates alternatives
+ indicates concatenation
GZIP() indicates the gzip(1) of the operand
GZIP() indicates gzip compression of the operand
BZIP2() indicates bzip2 compression of the operand
LZMA() indicates lzma compression of the operand
XZ() indicates xz compression of the operand
LZO() indicates lzo compression of the operand
LZ4() indicates lz4 compression of the operand
ZSTD() indicates zstd compression of the operand
ALGN(n) means padding with null bytes to an n-byte boundary
initramfs := ("\0" | cpio_archive | cpio_gzip_archive)*
initramfs := ("\0" | cpio_archive | cpio_compressed_archive)*
cpio_gzip_archive := GZIP(cpio_archive)
cpio_compressed_archive := (GZIP(cpio_archive) | BZIP2(cpio_archive)
| LZMA(cpio_archive) | XZ(cpio_archive) | LZO(cpio_archive)
| LZ4(cpio_archive) | ZSTD(cpio_archive))
cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
@@ -75,6 +81,8 @@ c_chksum 8 bytes Checksum of data field if c_magic is 070702;
The c_mode field matches the contents of st_mode returned by stat(2)
on Linux, and encodes the file type and file permissions.
c_mtime is ignored unless CONFIG_INITRAMFS_PRESERVE_MTIME=y is set.
The c_filesize should be zero for any file which is not a regular file
or symlink.