Bering-uClibc 4.x - User Guide - Appendices - Overview of the Startup Sequence

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Appendices - Overview of the Startup Sequence
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TODO Expand this skeleton. Mention:

  • [SYS|ISO|PXE]LINUX and the relevant configuration file
  • Linux kernel, kernel arguments and initrd
  • Bering-uClibc LINUXRC (/init), LEAFCFG, PKGPATH
  • inittab (especially for serial consoles)
  • /etc/rc?.d/*


/init

Shell script /init (actually a symbolic link to /var/lib/lrpkg/root.linuxrc) is a critical file with a huge influence on the startup of a Bering-uClibc 4.x machine.

In summary, this script:

  1. Creates the /proc/ directory and mounts the "proc" filesystem at that location.
  2. Creates the /sys/ directory and mounts the "sysfs" filesystem at that location.
  3. Creates the symbolic links for all of the commands provided by the "busybox" executable.
  4. Loads all of the kernel Modules listed in file /boot/etc/modules/
    • Currently just isofs and vfat.
    • The modules are loaded using "modprobe" so any pre-requisite modules are loaded too, provided they are present in directory /boot/lib/modules/.
  5. Loads any modules specified in a comma-separated list against kernel command-line argument "MODULES".
    • For example, if the kernel command line includes "MODULES=e100,natsemi" then modules mii and e100 will be loaded, provided they are present in directory /boot/lib/modules/.
    • Again, the kernel Modules are loaded using "modprobe" so any pre-requisite modules are loaded too.
  6. Loads any kernel Modules which are available in directory /boot/lib/modules/ and which implement device drivers for installed hardware.
    • This is done by looking for files called modalias within the directory structure under /sys/devices/, examining the contents of those files and checking for a match in file /lib/modules/`uname -r`/modules.alias. Any kernel Modules which match are loaded.
    • File /lib/modules/`uname -r`/modules.alias is generated by the "depmod" command.
  7. Mounts the disk device specified by kernel command-line argument "LEAFCFG".
  8. "Sources" file leaf.cfg in the current shell, to set variables required by later processing.
  9. Creates a standard set of empty directories
    • For example /mnt/, /var/run/.
  10. Mounts the disk device specified by variable "PKGPATH".
    • This variable is specified in leaf.cfg.
  11. Loads all of the Packages listed against variable "LRP", provided the corresponding .lrp file can be located in "PKGPATH".
    • This variable is specified in leaf.cfg.
  12. Unmounts the devices for "LEAFCFG" and "PKGPATH".
  13. Mounts the "future" root filesystem (type "tmpfs") in a dummy directory.
  14. Mounts separate "tmpfs" file systems under directories /tmp/ and /var/log/.
  15. Recursively copies the files and directories created earlier (e.g. by loading the Packages) across to the "future" root directory.
  16. Uses the "switch_root" utility to start using the "future" root directory in place of the original (initrd) one.


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