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Architecture

ADR 0002 — No GPL code; clean-room implementation from papers

Accepted · 2026-10-31 · reflex/docs/adr/0002-no-gpl-clean-room-policy.md

Status: Accepted Date: 2026-10-31 Source: the stack’s no-GPL policy, set in cortex’s ADR 0002 Related: ADR-0001

Context

Section titled “Context”

cortex’s ADR 0002 rules out GPL code anywhere in cortex, for any purpose. The reason applies to reflex unchanged: partners redistribute the stack inside their own certified products, and any GPL obligation, even one confined to a test, is something their legal reviews would have to clear.

In flight control, the pull towards GPL code is strong. Several of the most widely used open flight stacks are GPL-licensed:

  • Betaflight, INAV and Cleanflight (GPL-3.0)
  • ArduPilot (GPL-3.0)
  • Paparazzi (GPL-2.0)

PX4 (BSD-3-Clause) is permissive. reflex already follows PX4’s conventions (the Quad X rotor order), and its cascade V&V plan names a code-to-code comparison with PX4’s multicopter controllers.

Decision

Section titled “Decision”

No GPL-licensed code anywhere in reflex, for any purpose. Not as a dependency, not in a test target, not as a build-time tool in what reflex builds, and not as a reference oracle. The scope is ADR-0001’s: what reflex ships, the code that builds it and the tests, and not the docs site’s toolchain.

  • Controllers, filters and models are implemented clean-room from papers, textbooks and specifications, plus permissively licensed (MIT / BSD / Apache-2.0) references.
  • Check a reference’s license before opening its code. Reading GPL source and then writing the same thing “from scratch” is not clean-room. A GPL project’s structure may be consulted, meaning its directory layout and how it splits the problem into modules, and nothing else: never its code, its algorithms’ details, or its data, which includes tuning tables, gains and filter coefficients.
  • Test oracles are in-house or permissive: reflex’s own reference models and recorded traces, closed-form results from the literature, permissive libraries (mp-dsp is the dsp/ tests’ reference), and permissive stacks such as PX4. Never compare against the output of a GPL flight stack (Betaflight, INAV, ArduPilot, …), its SITL or its logs.
  • Conventions are not code. Following a convention that a GPL project also uses (a rotor order, a frame, a unit) is fine, when it is taken from a permissive source or a specification.
  • LGPL, MPL, AGPL and SSPL code is rejected, as a dependency and as a reference, as ADR-0001 rejects it. An exception would need an ADR of its own that supersedes this one.

Consequences

Section titled “Consequences”
  • Each controller costs more to build: the design comes from the literature, and the V&V uses reflex’s own models and permissive references.
  • PX4 is the comparison of record for code-to-code checks, as the cascade V&V plan already assumes.
  • Partners can redistribute reflex under MIT terms with no copyleft review.
  • Current state: reflex’s controllers, filters and models are written in-house. The only external flight stack it refers to is PX4, for conventions and a planned comparison. No GPL code or oracle is used.