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+[[!meta title="Multiarch Design"]]
+
+There are three high-level design aspects to this multiarch implementation: the
+filesystem hierarchy that primarily enables package coinstallability, control
+information documenting package coinstallability and dependency satisfaction,
+and coinstallability considerations in package management.
+
+For reference, see the [specification][wuc-mas] and [other
+documentation][wdo-ma] for multiarch in Debian and Ubuntu.
+
+
+Filesystem Hierarchy
+====================
+
+For packages to be coninstallable with themselves (such that a user can have
+multiple architecture builds of one package installed simultaneously), all
+architecture-dependent files must be installed in architecture-dependent
+locations. The most straightforward way to do this is to embed architecture
+strings into filesystem paths.
+
+Debian and Ubuntu do something similar to this. In a Debian library package
+that supports multiarch, shared object files are installed in
+`/usr/lib/<triplet>`, where `<triplet>` is a [GNU system type][ac-systemtype].
+
+The scope of Debian's and Ubuntu's multiarch work [does not include][wdo-ma-fut]
+coinstallable executable programs.
+
+Such work, however, would help enable cross installation of packages. So a goal
+of this multiarch implementation is to support coinstallation of shared object
+files, header files, and executable programs.
+
+There are two proposed filesystem hierarchies (see below) to support this.
+
+Parts of the toolchain (especially the dynamic linker) need to be configured
+and/or patched to use multiarch library paths. Additionally, at least the
+native-architecture executable program directories have to be added to the PATH
+environment variable.
+
+Proposal 1: `/usr` Organized Primarily by Architecture
+------------------------------------------------------
+
+It may be useful to install all architecture-dependent files under
+`/usr/<arch>`, where `<arch>` is a distribution architecture string. The
+filesystem hierarchy would then look something like this:
+
+ /
+ +- bin/
+ | +- core-linux-eglibc/
+ | \- cortexa8-linux-eglibc/
+ +- lib/
+ | +- core-linux-eglibc/
+ | \- cortexa8-linux-eglibc/
+ +- sbin/
+ | +- core-linux-eglibc/
+ | \- cortexa8-linux-eglibc/
+ \- usr/
+ +- core-linux-eglibc/
+ | +- bin/
+ | +- games/
+ | +- include/
+ | +- lib/
+ | +- sbin/
+ +- cortexa8-linux-eglibc/
+ | +- bin/
+ | +- games/
+ | +- include/
+ | +- lib/
+ | +- sbin/
+ +- bin/
+ +- games/
+ +- include/
+ +- local/
+ +- sbin/
+ +- share/
+ \- src/
+
+Note that `/usr/lib` doesn't exist, as no architecture-independent files should
+be installed there.
+
+Proposal 2: `/usr` Organized Secondarily by Architecture
+--------------------------------------------------------
+
+It may be cleaner and slightly more efficient to install architecture-dependent
+files within an architecture-dependent directory under otherwise standard paths.
+This is similar to how Debian and Ubuntu have designed their multiarch
+filesystem hierarchy. Such an organized filesystem hierarchy would look
+something like this:
+
+ /
+ +- bin/
+ | +- core-linux-eglibc/
+ | \- cortexa8-linux-eglibc/
+ +- lib/
+ | +- core-linux-eglibc/
+ | \- cortexa8-linux-eglibc/
+ +- sbin/
+ | +- core-linux-eglibc/
+ | \- cortexa8-linux-eglibc/
+ \- usr/
+ +- bin/
+ +- core-linux-eglibc/
+ +- cortexa8-linux-eglibc/
+ +- games/
+ +- core-linux-eglibc/
+ +- cortexa8-linux-eglibc/
+ +- include/
+ +- core-linux-eglibc/
+ +- cortexa8-linux-eglibc/
+ +- lib/
+ +- core-linux-eglibc/
+ +- cortexa8-linux-eglibc/
+ +- local/
+ +- sbin/
+ +- core-linux-eglibc/
+ +- cortexa8-linux-eglibc/
+ +- share/
+ \- src/
+
+
+Control Information
+===================
+
+New package control information will be needed to indicate whether a package is
+coinstallable with itself and to mark its purpose.
+
+During build-time, there are two possible architectures for any dependent package:
+
+ * Host-architecture (architecture for which packages are built). Packages for
+ this architecture provide things like libraries and headers. An example is
+ libexpat.1-dev.
+ * Build-architecture (architecture on which packages are built). Packages for
+ this architecture provide things like build utilities An example is
+ pkg-config.
+
+During install-time, there are also two possible architectures for any dependent
+package:
+
+ * Host-architecture (architecture for which packages are installed). Packages
+ for this architecture provide things like libraries and utilities. Examples
+ include libz.1, gcc-4.7-<arch> (dependency of gcc-<arch>), and fakeroot
+ (recommendation of opkhelper-1.0).
+ * Install-architecture (architecture on which packages are installed).
+ Packages for this architecture provide things like utilities used by
+ maintainer scripts. An example is insserv (or a similar tool).
+
+Host-architecture packages should be coinstallable with themselves, and they
+should satisfy dependencies of the same architecture. For example, the
+dependency of libexpat.1-dev:cortexa8-linux-eglibc on libexpat.1 should resolve
+to a dependency on libexpat.1:cortexa8-linux-eglibc.
+
+Build- and install-architecture packages need not be coinstallable with
+themselves, and they should satisfy dependencies of any architecture. For
+example, the build dependency of glib (built on core-linux-eglibc for
+cortexa8-linux-eglibc) on pkg-config should resolve to a build dependency on
+pkg-config:core-linux-eglibc.
+
+Debian and Ubuntu specify such properties in a `Multi-Arch` control field. A
+value of `same` indicates that a package satisfies dependencies of the same
+architecture; a value of `foreign` indicates that a package satisfies
+dependencies of any architecture. We can implement something similar, perhaps
+even using the same terminology.
+
+Logic to handle the new control information will need to be added to opkg and
+opkhelper (specifically oh-checkbuilddeps).
+
+Special design considerations need to be given to install-time dependencies.
+Specific use case analysis can help here.
+
+
+Architecture-Independent Files
+==============================
+
+Many packages provide architecture-independent files (configuration files,
+documentation, data, etc.). To the extent possible and feasible, these should
+simply be provided by architecture-independent packages (with names like
+"*-common", "*-data", or "*-base").
+
+If this is not possible in all cases, then some modifications to opkg will be
+necessary (reference counting, implicit "Breaks" relationships, etc.).
+
+
+[wdo-ma]: http://wiki.debian.org/Multiarch
+[wuc-mas]: https://wiki.ubuntu.com/MultiarchSpec
+[ac-systemtype]: https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/System-Type.html
+[wdo-ma-fut]: http://wiki.debian.org/Multiarch#Future_development