16. Support for Multiple Architectures

FreeBSD is a highly portable operating system intended to function on many different types of hardware architectures. Maintaining clean separation of Machine Dependent (MD) and Machine Independent (MI) code, as well as minimizing MD code, is an important part of our strategy to remain agile with regards to current hardware trends. Each new hardware architecture supported by FreeBSD adds substantially to the cost of code maintenance, toolchain support, and release engineering. It also dramatically increases the cost of effective testing of kernel changes. As such, there is strong motivation to differentiate between classes of support for various architectures while remaining strong in a few key architectures that are seen as the FreeBSD target audience.

16.1. Statement of General Intent

The FreeBSD Project targets "production quality commercial off-the-shelf (COTS) workstation, server, and high-end embedded systems". By retaining a focus on a narrow set of architectures of interest in these environments, the FreeBSD Project is able to maintain high levels of quality, stability, and performance, as well as minimize the load on various support teams on the project, such as the ports team, documentation team, security officer, and release engineering teams. Diversity in hardware support broadens the options for FreeBSD consumers by offering new features and usage opportunities (such as support for 64-bit CPUs, use in embedded environments, etc.), but these benefits must always be carefully considered in terms of the real-world maintenance cost associated with additional platform support.

The FreeBSD Project differentiates platform targets into four tiers. Each tier includes a specification of the requirements for an architecture to be in that tier, as well as specifying the obligations of developers with regards to the platform. In addition, a policy is defined regarding the circumstances required to change the tier of an architecture.

16.2. Tier 1: Fully Supported Architectures

Tier 1 platforms are fully supported by the security officer, release engineering, and toolchain maintenance staff. New features added to the operating system must be fully functional across all Tier 1 architectures for every release (features which are inherently architecture-specific, such as support for hardware device drivers, may be exempt from this requirement). In general, all Tier 1 platforms must have build and Tinderbox support either in the FreeBSD.org cluster, or be easily available for all developers. Embedded platforms may substitute an emulator available in the FreeBSD cluster for actual hardware.

Tier 1 architectures are expected to be Production Quality with respects to all aspects of the FreeBSD operating system, including installation and development environments.

Tier 1 architectures are expected to be completely integrated into the source tree and have all features necessary to produce an entire system relevant for that target architecture. Tier 1 architectures generally have at least 6 active developers.

Tier 1 architectures are expected to be fully supported by the ports system. All the ports should build on a Tier 1 platform, or have the appropriate filters to prevent the inappropriate ones from building there. The packaging system must support all Tier 1 architectures. To ensure an architecture's Tier 1 status, proponents of that architecture must show that all relevant packages can be built on that platform.

Tier 1 embedded architectures must be able to cross-build packages on at least one other Tier 1 architecture. The packages must be the most relevant for the platform, but may be a non-empty subset of those that build natively.

Tier 1 architectures must be fully documented. All basic operations need to be covered by the handbook or other documents. All relevant integration documentation must also be integrated into the tree, or readily available.

Current Tier 1 platforms are i386 and amd64.

16.3. Tier 2: Developmental Architectures

Tier 2 platforms are not supported by the security officer and release engineering teams. Platform maintainers are responsible for toolchain support in the tree. The toolchain maintainer is expected to work with the platform maintainers to refine these changes. Major new toolchain components are allowed to break support for Tier 2 architectures if the FreeBSD-local changes have not been incorporated upstream. The toolchain maintainers are expected to provide prompt review of any proposed changes and cannot block, through their inaction, changes going into the tree. New features added to FreeBSD should be feasible to implement on these platforms, but an implementation is not required before the feature may be added to the FreeBSD source tree. New features that may be difficult to implement on Tier 2 architectures should provide a means of disabling them on those architectures. The implementation of a Tier 2 architecture may be committed to the main FreeBSD tree as long as it does not interfere with production work on Tier 1 platforms, or substantially with other Tier 2 platforms. Before a Tier 2 platform can be added to the FreeBSD base source tree, the platform must be able to boot multi-user on actual hardware. Generally, there must be at least three active developers working on the platform.

Tier 2 architectures are usually systems targeted at Tier 1 support, but that are still under development. Architectures reaching end of life may also be moved from Tier 1 status to Tier 2 status as the availability of resources to continue to maintain the system in a Production Quality state diminishes. Well supported niche architectures may also be Tier 2.

Tier 2 architectures may have some support for them integrated into the ports infrastructure. They may have cross compilation support added, at the discretion of portmgr. Some ports must built natively into packages if the package system supports that architecture. If not integrated into the base system, some external patches for the architecture for ports must be available.

Tier 2 architectures can be integrated into the FreeBSD handbook. The basics for how to get a system running must be documented, although not necessarily for every single board or system a Tier 2 architecture supports. The supported hardware list must exist and should be no more than a couple of months old. It should be integrated into the FreeBSD documentation.

Current Tier 2 platforms are arm, ia64, pc98, powerpc, and sparc64.

16.4. Tier 3: Experimental Architectures

Tier 3 platforms are not supported by the security officer and release engineering teams. At the discretion of the toolchain maintainer, they may be supported in the toolchain. Tier 3 platforms are architectures in the early stages of development, for non-mainstream hardware platforms, or which are considered legacy systems unlikely to see broad future use. New Tier 3 systems will not be committed to the base source tree. Support for Tier 3 systems may be worked on in the FreeBSD Perforce Repository, providing source control and easier change integration from the main FreeBSD tree. Platforms that transition to Tier 3 status may be removed from the tree if they are no longer actively supported by the FreeBSD developer community at the discretion of the release engineer.

Tier 3 platforms may have ports support, either integrated or external, but do not require it.

Tier 3 platforms must have the basics documented for how to build a kernel and how to boot it on at least one target hardware or emulation environment. This documentation need not be integrated into the FreeBSD tree.

Current Tier 3 platforms are mips and S/390®.

16.5. Tier 4: Unsupported Architectures

Tier 4 systems are not supported in any form by the project.

All systems not otherwise classified into a support tier are Tier 4 systems.

16.6. Policy on Changing the Tier of an Architecture

Systems may only be moved from one tier to another by approval of the FreeBSD Core Team, which shall make that decision in collaboration with the Security Officer, Release Engineering, and toolchain maintenance teams.

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
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