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Emdebian Bootstrapping
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======================
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Cross-building can get very confusing at times. The confusion only increases
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when dealing with a chroot. This document tries to cover some of the reasons,
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some of the problems and tries to provide some clarity, at least as far as how
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chroots can be used in Emdebian.
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There are multiple ways that Emdebian can relate to bootstrapping and chroots.
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The most common method of using a chroot within Debian is pbuilder and the
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pdebuild wrapper and this is the primary inspiration for using chroots in
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Emdebian. Other methods revolve around QEMU or scratchbox and whilst this code
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may be able to support such uses later, there are key differences in how and
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why this method is separate.
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EMDEBIAN PBUILDER CHROOT MODEL
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==============================
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The Debian pbuilder package exists to ensure package dependencies are complete
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and to provide an environment to automate package building on all supported
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Debian architectures without cluttering the build machine with every library
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and dev package in the entire archive. This is particularly useful when
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building GUI packages that can have two dozen dependencies or more. In
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Emdebian, this separation between the build system packages and the chroot
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packages becomes even more useful because cross-building often involves not
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only installing the library and -dev package for the build architecture (e.g.
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i386/amd64) but also building and installing the same library and -dev package
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(with dependencies) in the TARGET architecture, e.g. arm, with apt-cross and
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dpkg-cross. The chaos that could result from trying to upgrade the packages on
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an Emdebian buildd system with a duplicate set of cross dependencies is truly
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scary.
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In the pbuilder model, an Emdebian chroot is the SAME architecture as the
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buildd - i386 on i386 etc. This mimics how emdebian-tools normally work.
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Packages are then downloaded, patched and cross-built for the target
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architecture, just as outside the chroot. This model, the pbuilder model of an
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emdebian chroot, concentrates on dependencies rather than the actual
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cross-building process. Inside such a chroot, cache files are used as normal to
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avoid having to run the cross-compiled code during the build.
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The Emdebian chroot is similar in size to a normal Debian pbuilder chroot,
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mainly because it is a full build environment, not an installation environment.
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This directory contains my experiments with debootstrap, using this pbuilder
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model, trying to make a complete Emdebian cross-building environment that can
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be compressed to a base.tgz. Changes to the chroot can be discarded, just as
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with pbuilder, allowing dependencies to be tested. The main benefit is that
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packages can be built without having to install long chains of cross
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dependencies via apt-cross or dpkg-cross on the main system. To me, it's the
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only way to cross-build GUI suites like GPE against a background of changing
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dependencies.
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In the pbuilder model, Emdebian builds a chroot for the build architecture
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(amd64/i386 etc.) - except that it will run emdebian-tools instead of pbuilder
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to create the arm binaries (or whichever architecture is the dpkg-cross
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default). I expect the same code to work for arm chroots on amd64 etc, but
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mechanisms already exist for those so it's not the priority.
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In order to make best use of the existing pbuilder code, these are shell scripts
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whereas the rest of emdebian-tools uses perl.
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DEPENDENCIES
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------------
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To emdebianise and cross-build packages, the chroot needs to install
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emdebian-tools. This brings in the majority of dependencies to support building
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Debian and Emdebian packages. This (current sample) code uses part of the
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pbuilder source so that the eventual chroots behave as similarly as possible to
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pbuilder and pdebuild in Debian. Therefore, the emdebian pbuilder chroot is NOT
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designed or intended to be installed on any embedded device - it is intended for
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cross-building packages for Emdebian only. See the SLIND installer for more
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information or the emsandbox script in emdebian-tools.
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empdebuild depends on pbuilder and installing emdebian-tools within a chroot
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causes pbuilder to be installed inside the chroot. Whilst empdebuild and emsandbox
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could be split out into a separate package to prevent pbuilder being needed inside
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the chroot, there are corner cases where this can be useful providing sufficient
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care is taken.
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DIFFERENCES TO PBUILDER
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-----------------------
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1. emdebian-tools handles the 'installaptlines' subroutine.
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2. emdebian-tools dependencies handle the installation of build-essential dpkg-dev
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and apt that pbuilder normally does separately.
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3. emdebian-tools handles the BUILDPLACE via debconf.
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4. Other embootstrap options are hardcoded - BUILDRESULT is placed in BUILDPLACE,
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APTCACHE also and APTCACHEHARDLINK is set to yes.
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5. emdebian-tools provides the emdebianised source package and patches.
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EMDEBIAN NATIVE CHROOT MODEL
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============================
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The same code should also be able to create a native chroot so that the
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cross-built packages can be installed alongside each other for more testing.
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These chroots have already been tested and instructions exist in the Wiki:
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http://wiki.debian.org/EmDebian/CrossDebootstrap
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Trying to cross-build inside scratchbox limits you to where scratchbox will
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install which is only certain architectures. emdebian-tools is about
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cross-building Debian packages using normal Debian tools on a normal Debian
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system. Emdebian and emdebian-tools are designed to make building inside
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scratchbox redundant because it simply isn't sufficiently flexible. A
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scratchbox2 has been mooted but nothing has happened yet.
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Cross-build using normal tools (maybe using the chroot as above) and then test
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either on a native device or via scratchbox if you really have to.
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An Emdebian native chroot would be an arm chroot running on an amd64 system -
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indeed it would need to support creation of a chroot *for* any supported Debian
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architecture and be able to create and run that chroot *on* any supported
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Debian architecture.
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This is only the first draft - if there are areas that need further clarification
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or updating, please email the debian-embedded mailing list.
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