[RFC 0046] Platform Support Tiers

rfcs/0046-platform-support-tiers.md

@@ -0,0 +1,226 @@
+---
+feature: platform_support_tiers
+start-date: 2019-04-28
+author: Michael Raskin
+co-authors: Matthew Bauer
+related-issues:
+---
+
+# Summary
+[summary]: #summary
+
+Document the way to describe the level of support a platform can expect in
+Nixpkgs.
+
+# Motivation
+[motivation]: #motivation
+
+When adding a new platform, be it a new C library option, a new
+cross-compilation target configuration or a new CPU architecture, there is a
+discussion of support expectations and maintenance burden. Having a documented
+vocabulary to describe the expectations and documented precedents should make
+such discussions more efficient.
+
+# Detailed design
+[design]: #detailed-design
+
+## Platform elements
+
+* CPU architecture (possibly: subarchitecture, optional features)
+* OS kernel
+* C compiler
+* C library
+* NixOS/non-NixOS global layout, in case of Linux with glibc
+* Global linking options
+
+## Questions to answer for each platform
+
+* What fraction of packages is expected to work?
+* How many users there are / how much testing one can expect?
+* How much complexity is a platform-specific fix allowed to carry?
+* Is there a binary cache for the platform?
+* What platform-related builds are channel update blockers?
+* Is the platform normally tested by the tools like ofBorg? Is it possible to
+  get something tested with reasonable effort?
+* Is there expectation that updates do not break things for this platform?
+* How many developers are interested in the platform? How easy it it to get a
+  non-trivial fix reviewed?
+* Is Nix supported?
+* Are native bootstrap tools available?
+
+## Platforms
+
+### Tier 1
+
+Aims: all packages should work (unless they don't make sense)
+
+A platform-specific fix is expected to be applied in `master`
+
+Support: good binary cache coverage, full support in tooling
+
+A platform in this tier has a set of build jobs that block a channel update in
+case of failure.
+
+Developer/user base: most of the Nix developers/users
+
+* `x86_64-linux`, `gcc`+`glibc`
+
+### Tier 1.5
+
+Same aims and tooling support, there are channel blocking jobs
+
+Fewer developers and users, less testing — significantly more broken packages
+
+* `aarch64-linux`, `gcc`+`glibc`
+
+* `x86_64-darwin`, `clang`+Darwin/macOS
+
+### Tier 2-ε
+
+Aims: most of the popular packages work
+
+Platform-specific things for arbitrary packages should not be too complicated
+
+Support: native bootstrap tools are available, cross-build toolchains in the
+binary cache, partial tooling support
+
+Native stdenv is a channel-blocking job
+
+Package updates might break build on the platforms of this tier and lower
+
+* `i686-linux`, `gcc`+`glibc` — `ofBorg` builds via `pkgsi686Linux`, binary
+  cache contains `wine` dependencies
+
+### Tier 2
+
+Aims: most of the popular packages work
+
+Support: native bootstrap tools are available, cross-build toolchains in the
+binary cache
+
+This tier and lower doesn't affect channel updates
+
+* `armv{6,7,8}*-linux`, `gcc`+`glibc`
+
+* `mipsel-linux`, `gcc`+`glibc`
+
+* `x86_64-linux`, `gcc`+`musl`
+
+* `x86_64-linux`, `clang`+`glibc`
+
+### Tier 3
+
+Aims: some packages are expected to work
+
+Platform-specific fixes limited to general cleanups of non-standard
+assumptions in the upstream code and basic toolchain fixes
+
+* `aarch64-none`
+
+* `avr`
+
+* `arm-none`
+
+* `i686-none`
+
+* `x86_64-none`
+
+* `powerpc-none`
+
+* `powerpcle-none`
+
+* `x86_64-mingw32`
+
+* `i686-mingw32`
+
+* `x86_64-linux`, `gcc`+`musl` — static
+
+* `x86_64-linux`, `clang`+`glibc` — `llvm` linker
+
+* `x86_64-linux` — Android
+
+* `aarch64-linux` — Android
+
+* `armv{7,8}-linux` — Android
+
+### Tier 4
+
+Aims: none
+
+Support: none or accidental
+
+Platform definitions present
+
+A small amount of packages seems to work, maybe by luck
+
+* `x86_64-linux`, `gcc`+`glibc` — static
+
+* `x86_64-linux`, `gcc`+`glibc` — `llvm` linker
+
+### Tier 4.5
+
+Work ongoing to provide/merge Tier 4 support
+
+* `wasm-wasi`
+
+* `powerpc64le-linux`, `gcc`+`glibc`
+
+### Tier 5
+
+Aims: none
+
+Support: none
+
+No current support, but previous support of clear path to add support
+
+* `aarch64-darwin`
+
+* `i686-darwin`
+
+* `x86_64-freebsd`
+
+## Adding a new platform
+
+It is expected that Tier-4 support can be added freely, and Tier-3 support is
+added once enough packages are tested and sustained development happens.
+Tier-2 support (and higher tolerance to platform-specific fixes in
+non-toolchain packages) is generally linked to higher user interest and
+sustainability of both the platform itself and Nixpkgs development for the
+platform.
+
+Support above Tier-2 (and expectation that platform non-users pay attention to
+the platform support on updates) requires deployment of test infrastructure
+for the platform.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Maintaining the list of platforms (and coordinating agreement on explicit
+support expectations) takes effort, both technical and organisational.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Do nothing; make decisions on platform support trade-offs on case-by-case
+basis without a shared framework.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+The list of currently supported platforms is incomplete.
+
+# Future work
+[future]: #future-work
+
+Clarify what other considerations there are from the point of view of support
+expectations.
+
+Describe what expectations usually appear together.
+
+Support expectations for packages (and package options), NixOS modules, and
+hardware configurations could also be defined.
+
+Levels of desirability for tricks that are sometimes the only way but are not
+generally encouraged could be defined. (Example: when building an FHS
+environment becomes a reasonable strategy to get something running on a NixOS
+machine?)