From 1c15a9405df33b3dd0400b25a3d63a42867a5447 Mon Sep 17 00:00:00 2001
From: P. J. McDermott <pjm@nac.net>
Date: Wed, 30 May 2012 00:36:14 -0400
Subject: Describe multiarch work.

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 <p>An obvious boot sequencing tool is <a href="http://svn.savannah.nongnu.org/viewvc/insserv/trunk/?root=sysvinit">"insserv"</a> maintained by Werner Fink and used by Debian and openSUSE.  However, this C program (in compliance with the LSB) assumes the use of runlevels.  This operating system uses the init daemon of BusyBox, which <a href="http://git.busybox.net/busybox/tree/init/init.c?id=3da46c81#n1221">doesn't support runlevels</a>.  Therefore, we'll need to either modify insserv to work without runlevels or write our own tool for installing symbolic links to init scripts.</p>
 <p>Additionally, we need to decide how completely we'll conform, if at all, with the LSB in this area.</p>
 <h3>Multiarch</h3>
+<p>Multiarch refers to the ability to install and use packages built for non-native architectures.  It is currently being <a href="http://wiki.debian.org/Multiarch">documented and implemented</a> in Debian and Ubuntu.  Multiarch is useful for this distribution because it makes cross compiling easy (see "Package Cross Building Tool" and "Multiarch Toolchain Packages" below).</p>
+<p>Simply speaking, there are six <a href="https://wiki.ubuntu.com/MultiarchSpec#Design">aspects of a multiarch implementation</a>:</p>
+<ul>
+	<li>
+		<h4>Filesystem Hierarchy</h4>
+		<p>To accomodate simultaneous installation of shared libraries built for different architectures, library paths are suffixed with a directory name that identifies a particular architecture.  For this distribution, that directory name will be the name of the binary architecture; for example, library paths for the <code>cortexa8-linux-eglibc</code> architecture will be <code>/lib/cortexa8-linux-eglibc</code> and <code>/usr/lib/cortexa8-linux-eglibc</code>.</p>
+		<p>The toolchain (especially the dynamic linker) needs to be configured to use these library paths.</p>
+	</li>
+	<li>
+		<h4>A <code>Multi-Arch</code> Control Field</h4>
+		<p>A <code>Multi-Arch</code> control field specifies how a package can satisfy dependencies of other packages.  A value of <code>same</code> means that a package can only satisfy dependencies of packages built for the architecture for which it was built.  This is useful for shared libraries, which can only be dynamically linked against binary objects built for the same architecture.  A value of <code>foreign</code> means that a package can satisfy dependencies of packages built for any architecture.  This is useful for utility programs, which can be used by software built for any architecture.</p>
+		<p>The package manager (opkg in our case) and package building tools (specifically oh-checkbuilddeps of opkhelper) must be able to understand and use this field in resolving package dependencies.</p>
+	</li>
+	<li>
+		<h4>Architecture Specification in Package Relationships</h4>
+		<p>Control fields like <code>Depends</code> are extended to support an architecture specification of either <code>any</code> or <code>same</code>, allowing a package to specify whether or not it needs a package of the same architecture.  The dependent package has a <code>Multi-Arch</code> field with value <code>allowed</code>.</p>
+		<p>The package manager and package building tools must be able to understand and use this architecture syntax in relationship fields.</p>
+	</li>
+	<li>
+		<h4>Handling of Packages with <code>Architecture: all</code></h4>
+		<p>Dependence on packages installable and usable on any architecture (especially considering opkg's ability to install packages of multiple architectures) must be researched.</p>
+	</li>
+	<li>
+		<h4>Non-Library Files in Library Packages</h4>
+		<p>Library packages provide files outside of system library paths, such as configuration and package documentation files.  A solution must be designed to allow a package built for one architecture to be co-installable with the same package built for a different architecture (or to determine if they are co-installable at all).</p>
+	</li>
+	<li>
+		<h4>Ability to Install Packages Built for Non-Native Architectures</h4>
+		<p>The package manager must be able to install packages built for multiple architectures (the whole point of multiarch).  Fortunately for us, opkg already handles this.  The <code>arch</code> option in <code>opkg.conf</code> <a href="http://wiki.openwrt.org/doc/techref/opkg#adjust.architectures">allows the user to specify architectures</a> for which packages can be installed.</p>
+	</li>
+</ul>
+<p>In summary, there is much design work to be done, opkg and opkhelper must be modified to support multiarch, and certain packages will need to be built to handle multiarch library paths.  Of course Debian is a great reference implementation, but there still remains much original work to be done.</p>
 <h3>Installation Bootstrap Tool</h3>
 <h3>Package Cross Building Tool</h3>
 <h3>Multiarch Toolchain Packages</h3>
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