This is a general procedure for improving Linux boot times. It only deals
with Linux drivers and userspace.

NOTE: Everything before Linux (hardware, bootloader) have very important
      roles also. They also must be considered for a complete fast boot
      concept.

1. Add "poweroff -f" to the beginning of the boot script /etc/init.d/rcS
   - this allows us to measure time with "time ./start-vm.sh"

2. Do not enable audio in qemu: QEMU_AUDIO_DRV=none
   - this reduces qemu startup time (we want to measure Linux, not qemu)

3. Switch from NFS-Root to initramfs.
   - this avoids ethernet setup and DHCP at boot
   - this avoids network latencies during filesystem interaction
   - initramfs uses /init as default start program, so we add a soft link
     from /init to bin/busybox in the root filesystem
   - removed "ip=dhcp root=... nfsroot=..." from kernel arguments

4. Strip all libraries and binaries.
   - this makes the initramfs *much* smaller
   - smaller initramfs means faster "populate rootfs" on boot

5. Remove libm dependency from busybox.
   - the busybox features using libm are not needed for fast boot requirements
   - removing libm means smaller initramfs
   - less dependencies means faster busybox start time

6. Add "printk.time=1 loglevel=8" to kernel arguments.
   - ./start-vm.sh > boot.log
   - browse boot.log to find any errors, unused features, slow non-important
     features.
   - disable or compile the drivers/features as modules

7. Add "initcall_debug" to kernel arguments.
   - now we can see the time consumed by each driver's init function
   - if the device is in the device-tree, this time will also include
     the device's probe time
   - ./start-vm.sh > boot.log
   - cat boot.log | linux-4.6/scripts/bootgraph.pl > boot.svg

8. Remove (or change to modules) drivers not needed for fast boot requirements
   - search kernel code to find init function
   - look at the Makefile in that directory to see the corresponding
     kernel config option
   - remove amba_clcdfd_init (as example)
   - change smsc911x_init to module (as example)

9. Remove initcall_debug, consoles and set loglevel=0.
   - we are done evaluating individual driver times
   - the console is slow, so don't output anything to it
   - maybe even disable the console in the kernel config

10. Pre-configure loops per jiffy calibration.
    - ./start-vm.sh | grep lpj=
    - add lpj=xyz to kernel parameters

11. Choose optimal kernel compression
    - General setup -> Kernel compression mode
    - set kernel compression to LZO

12. Create fastpoweroff script to run instead of busybox-init.
    - add kernel parameter "rdinit=/bin/fastpoweroff"
    - now init is not started (since init did much more than just
      run /etc/init.d/rcS)

13. Create binary to run instead of script or busybox-init.
    - change kernel parameter to "rdinit=/bin/poweroff"
    - (busybox was hacked so "poweroff" automatically uses "-f")
    - in #12 first a shell is loaded, then the script is parsed,
      then poweroff is called
    - now poweroff is directly called by kernel

14. Add ld.so.cache for initramfs libraries.
    - the dynamic loader now does not search for dependencies

15. For systemd-based userspace, use systemd tools.
    - systemd-analyze blame
    - systemd-analyze plot > systemd.svg
    - systemd-analyze critcal-chain app-qt