docs: early-userspace: move to driver-api guide

Those documents describe a kAPI. So, add to the driver-api
book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

4 files changed, 290 insertions, 0 deletions

diff --git a/Documentation/driver-api/early-userspace/buffer-format.rst b/Documentation/driver-api/early-userspace/buffer-format.rst
new file mode 100644
index 000000000000..7f74e301fdf3
--- /dev/null
+++ b/Documentation/driver-api/early-userspace/buffer-format.rst
@@ -0,0 +1,119 @@
+=======================
+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
+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.
+
+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.
+
+The full format of the initramfs buffer is defined by the following
+grammar, where::
+
+	*	is used to indicate "0 or more occurrences of"
+	(|)	indicates alternatives
+	+	indicates concatenation
+	GZIP()	indicates the gzip(1) of the operand
+	ALGN(n)	means padding with null bytes to an n-byte boundary
+
+	initramfs  := ("\0" | cpio_archive | cpio_gzip_archive)*
+
+	cpio_gzip_archive := GZIP(cpio_archive)
+
+	cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
+
+	cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data
+
+	cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4)
+
+
+In human terms, the initramfs buffer contains a collection of
+compressed and/or uncompressed cpio archives (in the "newc" or "crc"
+formats); arbitrary amounts zero bytes (for padding) can be added
+between members.
+
+The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is
+not ignored; see "handling of hard links" below.
+
+The structure of the cpio_header is as follows (all fields contain
+hexadecimal ASCII numbers fully padded with '0' on the left to the
+full width of the field, for example, the integer 4780 is represented
+by the ASCII string "000012ac"):
+
+============= ================== ==============================================
+Field name    Field size	 Meaning
+============= ================== ==============================================
+c_magic	      6 bytes		 The string "070701" or "070702"
+c_ino	      8 bytes		 File inode number
+c_mode	      8 bytes		 File mode and permissions
+c_uid	      8 bytes		 File uid
+c_gid	      8 bytes		 File gid
+c_nlink	      8 bytes		 Number of links
+c_mtime	      8 bytes		 Modification time
+c_filesize    8 bytes		 Size of data field
+c_maj	      8 bytes		 Major part of file device number
+c_min	      8 bytes		 Minor part of file device number
+c_rmaj	      8 bytes		 Major part of device node reference
+c_rmin	      8 bytes		 Minor part of device node reference
+c_namesize    8 bytes		 Length of filename, including final \0
+c_chksum      8 bytes		 Checksum of data field if c_magic is 070702;
+				 otherwise zero
+============= ================== ==============================================
+
+The c_mode field matches the contents of st_mode returned by stat(2)
+on Linux, and encodes the file type and file permissions.
+
+The c_filesize should be zero for any file which is not a regular file
+or symlink.
+
+The c_chksum field contains a simple 32-bit unsigned sum of all the
+bytes in the data field.  cpio(1) refers to this as "crc", which is
+clearly incorrect (a cyclic redundancy check is a different and
+significantly stronger integrity check), however, this is the
+algorithm used.
+
+If the filename is "TRAILER!!!" this is actually an end-of-archive
+marker; the c_filesize for an end-of-archive marker must be zero.
+
+
+Handling of hard links
+======================
+
+When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino)
+tuple is looked up in a tuple buffer.  If not found, it is entered in
+the tuple buffer and the entry is created as usual; if found, a hard
+link rather than a second copy of the file is created.  It is not
+necessary (but permitted) to include a second copy of the file
+contents; if the file contents is not included, the c_filesize field
+should be set to zero to indicate no data section follows.  If data is
+present, the previous instance of the file is overwritten; this allows
+the data-carrying instance of a file to occur anywhere in the sequence
+(GNU cpio is reported to attach the data to the last instance of a
+file only.)
+
+c_filesize must not be zero for a symlink.
+
+When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is
+reset.  This permits archives which are generated independently to be
+concatenated.
+
+To combine file data from different sources (without having to
+regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of
+the following techniques can be used:
+
+a) Separate the different file data sources with a "TRAILER!!!"
+   end-of-archive marker, or
+
+b) Make sure c_nlink == 1 for all nondirectory entries.
diff --git a/Documentation/driver-api/early-userspace/early_userspace_support.rst b/Documentation/driver-api/early-userspace/early_userspace_support.rst
new file mode 100644
index 000000000000..3deefb34046b
--- /dev/null
+++ b/Documentation/driver-api/early-userspace/early_userspace_support.rst
@@ -0,0 +1,154 @@
+=======================
+Early userspace support
+=======================
+
+Last update: 2004-12-20 tlh
+
+
+"Early userspace" is a set of libraries and programs that provide
+various pieces of functionality that are important enough to be
+available while a Linux kernel is coming up, but that don't need to be
+run inside the kernel itself.
+
+It consists of several major infrastructure components:
+
+- gen_init_cpio, a program that builds a cpio-format archive
+  containing a root filesystem image.  This archive is compressed, and
+  the compressed image is linked into the kernel image.
+- initramfs, a chunk of code that unpacks the compressed cpio image
+  midway through the kernel boot process.
+- klibc, a userspace C library, currently packaged separately, that is
+  optimized for correctness and small size.
+
+The cpio file format used by initramfs is the "newc" (aka "cpio -H newc")
+format, and is documented in the file "buffer-format.txt".  There are
+two ways to add an early userspace image: specify an existing cpio
+archive to be used as the image or have the kernel build process build
+the image from specifications.
+
+CPIO ARCHIVE method
+-------------------
+
+You can create a cpio archive that contains the early userspace image.
+Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it
+will be used directly.  Only a single cpio file may be specified in
+CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in
+combination with a cpio archive.
+
+IMAGE BUILDING method
+---------------------
+
+The kernel build process can also build an early userspace image from
+source parts rather than supplying a cpio archive.  This method provides
+a way to create images with root-owned files even though the image was
+built by an unprivileged user.
+
+The image is specified as one or more sources in
+CONFIG_INITRAMFS_SOURCE.  Sources can be either directories or files -
+cpio archives are *not* allowed when building from sources.
+
+A source directory will have it and all of its contents packaged.  The
+specified directory name will be mapped to '/'.  When packaging a
+directory, limited user and group ID translation can be performed.
+INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to
+user root (0).  INITRAMFS_ROOT_GID can be set to a group ID that needs
+to be mapped to group root (0).
+
+A source file must be directives in the format required by the
+usr/gen_init_cpio utility (run 'usr/gen_init_cpio -h' to get the
+file format).  The directives in the file will be passed directly to
+usr/gen_init_cpio.
+
+When a combination of directories and files are specified then the
+initramfs image will be an aggregate of all of them.  In this way a user
+can create a 'root-image' directory and install all files into it.
+Because device-special files cannot be created by a unprivileged user,
+special files can be listed in a 'root-files' file.  Both 'root-image'
+and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete
+early userspace image can be built by an unprivileged user.
+
+As a technical note, when directories and files are specified, the
+entire CONFIG_INITRAMFS_SOURCE is passed to
+usr/gen_initramfs_list.sh.  This means that CONFIG_INITRAMFS_SOURCE
+can really be interpreted as any legal argument to
+gen_initramfs_list.sh.  If a directory is specified as an argument then
+the contents are scanned, uid/gid translation is performed, and
+usr/gen_init_cpio file directives are output.  If a directory is
+specified as an argument to usr/gen_initramfs_list.sh then the
+contents of the file are simply copied to the output.  All of the output
+directives from directory scanning and file contents copying are
+processed by usr/gen_init_cpio.
+
+See also 'usr/gen_initramfs_list.sh -h'.
+
+Where's this all leading?
+=========================
+
+The klibc distribution contains some of the necessary software to make
+early userspace useful.  The klibc distribution is currently
+maintained separately from the kernel.
+
+You can obtain somewhat infrequent snapshots of klibc from
+https://www.kernel.org/pub/linux/libs/klibc/
+
+For active users, you are better off using the klibc git
+repository, at http://git.kernel.org/?p=libs/klibc/klibc.git
+
+The standalone klibc distribution currently provides three components,
+in addition to the klibc library:
+
+- ipconfig, a program that configures network interfaces.  It can
+  configure them statically, or use DHCP to obtain information
+  dynamically (aka "IP autoconfiguration").
+- nfsmount, a program that can mount an NFS filesystem.
+- kinit, the "glue" that uses ipconfig and nfsmount to replace the old
+  support for IP autoconfig, mount a filesystem over NFS, and continue
+  system boot using that filesystem as root.
+
+kinit is built as a single statically linked binary to save space.
+
+Eventually, several more chunks of kernel functionality will hopefully
+move to early userspace:
+
+- Almost all of init/do_mounts* (the beginning of this is already in
+  place)
+- ACPI table parsing
+- Insert unwieldy subsystem that doesn't really need to be in kernel
+  space here
+
+If kinit doesn't meet your current needs and you've got bytes to burn,
+the klibc distribution includes a small Bourne-compatible shell (ash)
+and a number of other utilities, so you can replace kinit and build
+custom initramfs images that meet your needs exactly.
+
+For questions and help, you can sign up for the early userspace
+mailing list at http://www.zytor.com/mailman/listinfo/klibc
+
+How does it work?
+=================
+
+The kernel has currently 3 ways to mount the root filesystem:
+
+a) all required device and filesystem drivers compiled into the kernel, no
+   initrd.  init/main.c:init() will call prepare_namespace() to mount the
+   final root filesystem, based on the root= option and optional init= to run
+   some other init binary than listed at the end of init/main.c:init().
+
+b) some device and filesystem drivers built as modules and stored in an
+   initrd.  The initrd must contain a binary '/linuxrc' which is supposed to
+   load these driver modules.  It is also possible to mount the final root
+   filesystem via linuxrc and use the pivot_root syscall.  The initrd is
+   mounted and executed via prepare_namespace().
+
+c) using initramfs.  The call to prepare_namespace() must be skipped.
+   This means that a binary must do all the work.  Said binary can be stored
+   into initramfs either via modifying usr/gen_init_cpio.c or via the new
+   initrd format, an cpio archive.  It must be called "/init".  This binary
+   is responsible to do all the things prepare_namespace() would do.
+
+   To maintain backwards compatibility, the /init binary will only run if it
+   comes via an initramfs cpio archive.  If this is not the case,
+   init/main.c:init() will run prepare_namespace() to mount the final root
+   and exec one of the predefined init binaries.
+
+Bryan O'Sullivan <bos@serpentine.com>
diff --git a/Documentation/driver-api/early-userspace/index.rst b/Documentation/driver-api/early-userspace/index.rst
new file mode 100644
index 000000000000..6f20c3c560d8
--- /dev/null
+++ b/Documentation/driver-api/early-userspace/index.rst
@@ -0,0 +1,16 @@
+===============
+Early Userspace
+===============
+
+.. toctree::
+    :maxdepth: 1
+
+    early_userspace_support
+    buffer-format
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index baa77a666e46..0f281f4f648f 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -16,6 +16,7 @@ available subsections can be seen below.
 
    basics
    infrastructure
+   early-userspace/index
    pm/index
    clk
    device-io