Introduction
  differences between initramfs and initrd:
    ramfs vs block
      - less memory
      - dynamic resizing, no limits
      - rootfs always mounted for same reason pid1 unkillable.
    archive vs image
      - no filesystem driver
        - ramfs tiny, always loaded in 2.6.  Wrapper around vfs cache.
    bundled vs unbundled
      - both available to ramfs
    init vs linuxrc
      - linuxrc sucky.
        - Not PID 1.
          - unpredictable PID, killable, zombies, etc.
        - strange flow control, unnecessarily complicated.
          - setup then return to kernel so it can mount root?
          - you're right there already, just do it yourself.
  advantages of initramfs
    - fundamental sequencing problem: mount / before running init.
      - Device enumeration complicated.  (USB, anyone?)
      - NFS mounts require dhcp and login.
      - encrypt, compress, 
      - modular filesystem driver, build into kernel.  Magic format.  Special.
        - root ext3.  Initrd ext2 _not_ ext3, journal on ramdisk!  Build in
          ext2 driver then can't get rid of it.
      - Binary only firmware for some devices (network cards).
        - Too cheap to spring for ram or flash.
          - Require a cross-compiler for mips to build your kernel?
      - initramfs has no chicken and egg problems ala nfs mounts
    - very simple implementation.  (Small, cross-platform.)
    - One file.  Bootloader simple to set up.
    - actually init
      - no assumptions about what you're going to do.  Stay running.
        - root mount not "magic".  Built-in root, ignorable.
    - moves more control to userspace
      - laptops, embedded systems, clusters, mainframes

Build a "hello world" program in a chroot environment.

Build a kernel, check out the toolchain.

  - Grab 2.6.17, apply miniconfig patch, use miniconfig, build,
     boot with hostfs.  Demonstrate basic User Mode Linux.

Building a "hello world" initramfs with User Mode Linux.

  - Build statically linked hello.c.  Put in initramfs with directory
     method.  Show it.  Put in initramfs with script method.  Wheee.

  - Build a statically linked busybox, run it as standalone shell.
    wget http://busybox.net/downloads/busybox-1.2.0.tar.bz2
    tar xvjf busybox-1.2.0.tar.bz2
    cd busybox-1.2.0
    make defconfig
    make

    Now run it as standalone shell under UML with rdinit=

  - Transfer to new RFS (hostfs) with switch_root, by hand.

Now boot a real kernel under QEMU.

  - Build a real kernel, run under QEMU.  Package external initramfs with cpio,
    show initrd=.  Show builtin too.  Build second initrd with data and
    show initrd overlay on builtin base.

  - Cross-compile for ARM (kernel, busybox, uClibc)

Get the size down:

  - Build dynamically linked, copy shared libraries, boot as #3.
    - Script to do this

  - Build against uClibc.  Talk about klibc.
    - uClibc full fledged replacement for glibc.  Build anything against it.
      klibc isn't.  Doesn't even build BusyBox.

  - In many ways, gcc sucks.
      The uClibc guys recommend building gcc from source.
        (I run sed against the source code.)
      There's a wrapper too, and various ways it can break.  Newer gccs break
      less, but can still break.  (libgcc_s) Klibc also uses the wrapper.

Download http://crossdev.timesys.com
PATH=/opt/timesys/toolchains/i686-linux/bin:$PATH make 

Do real things with it:

  - shared libraries, directory layout, /dev, /proc, /sys

  - Setup quick init script.  mdev -s, reproduce root=

  - The firmware Firmware Linux initramfs.

  - encfs

Bonus:
  Extracting initramfs image from existing kernel.