Separate Kernel Trees
Frodo Looijaard, frodol@dds.nl
v1.0, 11 July 1998


1. Introduction

This document describes a setup which allows the installation and use
of several distinct kernel trees, even when they have the same release
number. 

When you are working on some kernel-related development, trying out
a new compiler version or installing pre-versions, this document might
be useful to you.

Note that some distributions may already have something comparable
to the setup described below. If the directory trees in /lib/modules
are named 2.0.34 for kernel versions 2.0.34, this is probably not the
case. If they are named 2.0.34-something, you will probably not need
this document.

Note that I have only tested this for 2.0.x kernels. I would appreciate
to hear whether it works out for 2.1.x kernels too.


2. Setup

To allow the use of several kernels with the same release number, we
will refer to kernel versions as a concatenation of release number and
compilation number, separated by a dash.

Example:
  Kernel version 2.0.32, third compilation, would be called 2.0.32-3

During execution of /etc/rc (at system start-up), several steps must be
taken to ensure this will work (VERSION is a version number as explained 
above):
  * /usr/src/linux must be linked to /usr/src/VERSION
  * /lib/modules/current must be linked to /lib/modules/VERSION

You can determine VERSION with the following code snippet:

---Cut Here---
kernel_version ()
{
  local IFS_safe="$IFS"
  IFS="#-$IFS"
  set -- `uname -v`
  IFS="$IFS_safe"
  echo `uname -r`-$2
}
---Cut Here---

Using this function, you can add something like this to your rc-files
(this code is a bit paranoid, but better be safe than sorry):

---Cut Here---
echo "Selecting the current modules..."
KERNELD=yes
VERSION=`kernel_version`
if [ -d "/lib/modules/$VERSION" ] ; then
  if [ -L /lib/modules/current ] ; then
    rm -f /lib/modules/current
  fi
  if [ ! -e /lib/modules/current ] ; then
    ln -s "$VERSION" /lib/modules/current
  else
    echo "ERROR: /lib/modules/current exists and is not a symbolic link."
    echo "       Module loading may produce unexpected results, perhaps even"
    echo "       resulting in data loss. We will not load kerneld. Expect"
    echo "       many errors below."
    KERNELD=no
  fi
else
  echo "ERROR: /lib/modules/$VERSION not found."
  echo "       Kerneld can not be started, modules will not be loaded"
  echo "       automatically. Expect many errors below."
  KERNELD=no
fi
---Cut Here---


Moreover, the file /etc/conf.modules must be present and contain at least the 
following information (sometimes this file is called modules.conf):

---Cut Here---
# Where to find the dependency file
depfile=/lib/modules/current/modules.dep

# These are the paths to the kernel version-specific modules
path[fs]=/lib/modules/current/fs
path[misc]=/lib/modules/current/misc
path[net]=/lib/modules/current/net
path[scsi]=/lib/modules/current/scsi
path[block]=/lib/modules/current/block
path[ipv4]=/lib/modules/current/ipv4

# These are the paths to additional modules which are kernel independent
path[fs]=/lib/modules/extra/fs
path[misc]=/lib/modules/extra/misc
path[net]=/lib/modules/extra/net
path[scsi]=/lib/modules/extra/scsi
path[block]=/lib/modules/extra/block
path[ipv4]=/lib/modules/extra/ipv4

# These are the paths to modules which are inserted at boot-time for all
# kernel versions. Note that by making symbolic links to ../current/...
# you can still load different modules for different kernels.
path[boot]=/lib/modules/boot

# These are the paths to modules which are inserted at boot-time for
# specific kernels
path[boot]=/lib/modules/current/boot
---Cut Here---

To ensure this scheme works, you have to be careful when compiling a
new kernel:
  * Make sure the combination of release and compilation numbers is
    unique. You can set the compilation number by entering the number
    _minus one_ in <Kernel-base>/.version (delete this file to use
    number 1).
  * After calling module_install, rename /lib/modules/RELEASE to
    /lib/modules/VERSION (where RELEASE is the kernel release number),
    or use the small patch below:

---Cut Here---
*** /tmp/linux/Makefile Wed Feb  4 19:41:45 1998
--- linux/Makefile      Wed Feb  4 20:04:45 1998
***************
*** 275,279 ****
  modules_install:
        @( \
!       MODLIB=/lib/modules/$(VERSION).$(PATCHLEVEL).$(SUBLEVEL); \
        cd modules; \
        MODULES=""; \
--- 275,279 ----
  modules_install:
        @( \
!       MODLIB=/lib/modules/$(VERSION).$(PATCHLEVEL).$(SUBLEVEL)-`cat .version`; \
        cd modules; \
        MODULES=""; \
---Cut Here---


3. Copyright and disclaimer

This document is copyright (c) 1998 by Frodo Looijaard (frodol@dds.nl).
You may freely copy and distribute it, as long as you recognize me as the 
author, and mark any changes you make as being your own, and distribute this 
notice with it.

This document is distributed in the hope it is useful, but without any 
warranty. If following the guidelines herein blows up your system, I will 
not be held responsible.