[RFC 0046] Platform Support Tiers

rfcs/0046-platform-support-tiers.md

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+---
+feature: platform_support_tiers
+start-date: 2019-04-28
+author: Michael Raskin
+shepherd-team: Ryan Mulligan, Jonas Pfenniger, Graham Christensen, John Ericson
+shepherd-leader: John Ericson
+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 a 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?
+
+## Main dimensions
+
+Note that Tolerance tier is never higher than Tooling tier (and normally not
+higher than Package coverage tier).
+
+### Tooling
+
+#### Tier 1
+
+A lot of packages built by Hydra, full ofBorg support, some ordinary packages
+are channel-blockers on Hydra.
+
+#### Tier 2
+
+Native bootstrap tools available, cross-build toolchains in the binary cache,
+no channel-blocking jobs on Hydra.
+
+#### Tier 3
+
+None
+
+### Tolerance/impact
+
+#### Tier 1
+
+Problems on these platforms can block updates.
+
+#### Tier 1.5
+
+Platform-specific patches expected to be applied as needed; updates expected
+not to break the build on these platforms, problems should be investigated
+(and reported to the platform maintenance team if no solution was found).
+
+#### Tier 2
+
+Platform-specific fixes are expected to be rare and non-intrusive. Updates
+might break builds.
+
+#### Tier 3
+
+Fixes necessary for this platforms must be either limited to compilation
+toolchains, or general cleanups of non-standard assumptions (e.g. «everything
+that is no x86 is a kind of ARM» or «malloc(0) behaviour is a reliable
+indicator of other malloc features»). These fixes must be generic: there
+should be a reasonable expectation that other exotic platforms would equally
+benefit from the exact same fix.
+
+#### Tier 4
+
+It is recommended to keep platform-specific patches to the toolchain in a
+separate package. Cleanups not necessary on any tier-3 platforms can be
+rejected.
+
+### The number of working packages
+
+#### Tier 1
+
+Almost everything that is not explicitly platform specific and that is not
+abandoned (in Nixpkgs) works.
+
+#### Tier 1.5
+
+Most packages work, credible ambition to reach Tier 1 at some point.
+
+#### Tier 2
+
+Most of the popular packages work.
+
+#### Tier 3
+
+Some packages work.
+
+#### Tier 4
+
+Platform definitions present, a small number of packages might be working.
+
+## Current platforms
+
+### Tier 1
+
+Developer/user base: most of the Nix developers/users
+
+* `x86_64-linux`, `gcc`+`glibc`
+
+### Tier 1.5
+
+Fewer developers and users, less testing; tier-1 tooling.
+
+* `aarch64-linux`, `gcc`+`glibc`
+
+A team @NixOS/aarch64-maintainers shall be created to include people who
+understand the platform and use it.
+
+If there is a complicated problem on this platform when updating a package
+that was previously built succesfully on Aarch64, @NixOS/aarch64-maintainers
+team should be informed.
+
+* `x86_64-darwin`, `clang`+Darwin/macOS
+
+If there is a complicated problem on this platform when updating a package
+that was previously built succesfully on macOS, @NixOS/darwin-maintainers team
+should be informed.
+
+### Tier 2-ε
+
+Pure stdenv for native builds is a channel-blocking job; `wine` dependencies
+are available in the binary cache
+
+* `i686-linux`, `gcc`+`glibc` — `ofBorg` builds via `pkgsi686Linux`, binary
+  cache contains `wine` dependencies
+
+### Tier 2
+
+* `armv{6,7,8}*-linux`, `gcc`+`glibc`
+
+* `mipsel-linux`, `gcc`+`glibc`
+
+* `x86_64-linux`, `gcc`+`musl`
+
+### Tier 3
+
+* `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`
+
+* `x86_64-linux`, `clang`+`glibc` — `llvm` linker
+
+* `x86_64-linux` — Android
+
+* `aarch64-linux` — Android
+
+* `armv{7,8}-linux` — Android
+
+### Tier 4
+
+* `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
+
+No current support, but previous support or 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?)