使用initramfs最简单的方式，莫过于用已经做好的cpio.gz把kernel里面那个空的给换掉。这是2.6 kernel天生支持的，所以，你不用做什么特殊的设置。

kernel的config option里面有一项CONFIG_INITRAMFS_SOURCE(I.E. General setup—>Initramfs source file(s) in menuconfig)。这个选项指向放着内核打包initramfs需要的所有文件。默认情况下，这个选项是留空的，所以内核编译出来之后initramfs也就是空的，也就是前面提到的rootfs什么都不做的情形。

CONFIG_INITRAMFS_SOURCE 可以是一个绝对路径，也可以是一个从kernel’s top build dir(你敲入build或者是make的地方)开始的相对路径。而指向的目标可以有以下三种：一个已经做好的cpio.gz，或者一个已经为制作cpio.gz准备好所有内容的文件夹，或者是一个text的配置文件。第三种方式是最灵活的，我们先依次来介绍这三种方法。

1)使用一个已经做好的cpio.gz档案

If you already have your own initramfs_data.cpio.gz file (because you created it yourself, or saved the cpio.gz file produced by a previous kernel build), you can point CONFIG_INITRAMFS_SOURCE at it and the kernel build will autodetect the file type and link it into the resulting kernel image.

You can also leave CONFIG_INITRAMFS_SOURCE empty, and instead copy your cpio.gz file to usr/initramfs_data.cpio.gz in your kernel’s build directory. The kernel’s makefile won’t generate a new archive if it doesn’t need to.

Either way, if you build a kernel like this you can boot it without supplying an external initrd image, and it’ll still finish its boot by running your init program out of rootfs. This is packaging method #2, if you’d like to try it now.

2)指定给内核一个文件或者文件夹

If CONFIG_INITRAMFS_SOURCE points to a directory, the kernel will archive it up for you. This is a very easy way to create an initramfs archive, and is method #3.

Interestingly, the kernel build doesn’t use the standard cpio command to create initramfs archives. You don’t even need to have any cpio tools installed on your build system. Instead the kernel build (in usr/Makefile) generates a text file describing the directory with the script “gen_initramfs_list.sh”, and then feeds that descript to a program called “gen_init_cpio” (built from C source in the kernel’s usr directory), which create the cpio archive. This looks something like the following:

scripts/gen_initramfs_list.sh $CONFIG_INITRAMFS_SOURCE > usr/initramfs_list

usr/gen_init_cpio usr/initramfs_list > usr/initramfs_data.cpio

gzip usr/initramfs_data.cpio

To package up our hello world program, you could simply copy it into its own directory, name it “init”, point CONFIG_INITRAMFS_SOURCE at that directory, and rebuild the kernel. The resulting kernel should end its boot by printing “hello world”. And if you need to tweak the contents of that directory, rebuilding the kernel will re-package the contents of that directory if anything has changed.

The downside of this method is that it if your initramfs has device nodes, or cares about file ownership and permissions, you need to be able to create those things in a directory for it to copy. This is hard to do if you haven’t got root access, or are using a cross-compile environment like cygwin. That’s where the fourth and final method comes in.

3)使用configuration文件initramfs_list来告诉内核initramfs在哪里

This is the most flexible method. The kernel’s gen_initramfs_list.sh script creates a text description file listing the contents of initramfs, and gen_init_cpio uses this file to produce an archive. This file is a standard text file, easily editable, containing one line per file. Each line starts with a keyword indicating what type of entry it describes.

The config file to create our “hello world” initramfs only needs a single line:

file /init usr/hello 500 0 0

This takes the file “hello” and packages it so it shows up as /init in rootfs, with permissions 500, with uid and gid 0. It expects to find the source file “hello” in a “usr” subdirectory under the kernel’s build directory. (If you’re building the kernel in a different directory than the source directory, this path would be relative to the build directory, not the source directory.)

To try it yourself, copy “hello” into usr in the kernel’s build directory, copy the above configuration line to its own file, use “make menuconfig” to point CONFIG_INITRAMFS_SOURCE to that file, run the kernel build, and test boot the new kernel. Alternately, you can put the “hello” file in its own directory and use “scripts/gen_initramfs_list.sh dirname” to create a configuration file (where dirname is the path to your directory, from the kernel’s build directory). For large projects, you may want to generate a starting configuration with the script, and then customize it with any text editor.

This configuration file can also specify device nodes (with the “nod” keyword), directories (“dir”), symbolic links (“slink”), named FIFO pipes (“pipe”), and unix domain sockets (“sock”). Full documentation on this file’s format is available by running “usr/gen_init_cpio” (with no arguments) after a kernel build.

A more complicated example containing device nodes and symlinks could look like this:

dir /dev 755 0 0

nod /dev/console 644 0 0 c 5 1

nod /dev/loop0 644 0 0 b 7 0

dir /bin 755 1000 1000

slink /bin/sh busybox 777 0 0

file /bin/busybox initramfs/busybox 755 0 0

dir /proc 755 0 0

dir /sys 755 0 0

dir /mnt 755 0 0

file /init initramfs/init.sh 755 0 0

One significant advantage of the configuration file method is that any regular user can create one, specifying ownership and permissions and the creation of device nodes in initramfs, without any special permissions on the build system. Creating a cpio archive using the cpio command line tool, or pointing the kernel build at a directory, requires a directory that contains everything initramfs will contain. The configuration file method merely requires a few source files to get data from, and a description file.

This also comes in handy cross-compiling from other environments such as cygwin, where the local filesystem may not even be capable of reproducing everything initramfs should have in it.