Add support for Zephyr OS

[tomtom5152]

Proposal

In their own words:

The Zephyr OS is based on a small-footprint kernel designed for use on resource-constrained and embedded systems: from simple embedded environmental sensors and LED wearables to sophisticated embedded controllers, smart watches, and IoT wireless applications.

Notably for embedded development it has achieved Bluetooth qualification making it comparable with the ESPIDF framework that is already supported within rust (see riscv32imc-esp-espidf target)

Unlike ESPIDF however, it supports a huge number of boards and SOCs from a range of manufactures are architectures. This may make offering full support impossible, if not extremely difficult. This proposal is to add the base OS level integration, starting with a few specific targets with architectures already supported by rustc.

Some support for adding Zephyr to rust has already been done by tylerwhall in his zephyr-rust repo. Many of the changes necessary to std have already been completed as a proof of concept.

Initially I would propose adding the following targets, along with a justification for each. Whilst there are more QEMU arches supported by rust, I see these as the primary and most useful for an initial proposal.

Target (names subject to discussion)	Purpose
aarch64-zephyr-elf	QEMU
riscv32imac-zephyr-elf	QEMU
riscv32imc-zephyr-elf	Arch of popular ESP32-C3 already in rust
thumbv7m-zephyr-eabi	QEMU
thumbv7em-zephyr-eabihf	Arch of popular nrf52832 and family as supported by the nrf-hal project

I have already (misguidedly) created rust-lang/libc#3184 with the first round of changes, and now seek advice and guidance on if this is a feature Rust would like to see.

Tier 3 policy

Tier 3 policy:

A tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)

I pledge to do my best maintaining support for Zephyr OS at and OS level, I will not be able to support targets for all possible hardware but will aim to support all QEMU capable targets as a base for others.

Targets must use naming consistent with any existing targets; for instance, a target for the same CPU or OS as an existing Rust target should use the same name for that CPU or OS. Targets should normally use the same names and naming conventions as used elsewhere in the broader ecosystem beyond Rust (such as in other toolchains), unless they have a very good reason to diverge. Changing the name of a target can be highly disruptive, especially once the target reaches a higher tier, so getting the name right is important even for a tier 3 target.

The proposed triples are subject to discussion, however for consistency the unknown vendor portion has been omitted.

Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.

I do not believe the targets cause any confusion.

Tier 3 targets may have unusual requirements to build or use, but must not create legal issues or impose onerous legal terms for the Rust project or for Rust developers or users.

Building flashable binaries for Zephyr requires a multi-stage compilation, which is massively helped by the Zephyr meta tools West, which in turn uses cmake. All these tools are fully open source.

The target must not introduce license incompatibilities.

The Zephyr Project is licensed under Apache-2.0

Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).

Understood.

The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements.

There may be a requirement to install West and checkout the Zephyr project in full. The majority of these are licensed under Apache-2.0, however a few are licensed under BSD-style licenses which are compatible with the Rust MIT license at a minimum (and I would assume the Apache-2.0 license given their current usage).

Compiling, linking, and emitting functional binaries, libraries, or other code for the target (whether hosted on the target itself or cross-compiling from another target) must not depend on proprietary (non-FOSS) libraries. Host tools built for the target itself may depend on the ordinary runtime libraries supplied by the platform and commonly used by other applications built for the target, but those libraries must not be required for code generation for the target; cross-compilation to the target must not require such libraries at all. For instance, rustc built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3.

As previously said it's using open source tools only.

"onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.

To the best of my ability to search there are no such terms present.

Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.

I'm not the reviewer here.

This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.

Again I'm not the reviewer here.

Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate (core for most targets, alloc for targets that can support dynamic memory allocation, std for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions.
The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running binaries, or running tests (even if they do not pass), the documentation must explain how to run such binaries or tests for the target, using emulation if possible or dedicated hardware if necessary.

Instructions for building for Zephyr will need to be developed alongside compiler, core, and std support, as the final flashable executable needs to be manipulated to form a complete binary. This will be similar to how the esp-rs project is currently building images for ESPIDF.

Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on a tier 3 target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.

Understood.

Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.

Understood.

Patches adding or updating tier 3 targets must not break any existing tier 2 or tier 1 target, and must not knowingly break another tier 3 target without approval of either the compiler team or the maintainers of the other tier 3 target.

The inclusion of the zephyr OS should have no bearing on existing targets.

In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.

There should be no such issues here as where hardware support for features is not present (see riscv32imc not supporting hardware atomics), Zephyr already includes C software implementations.

Mentors or Reviewers

This is my first contribution to Rust and as such I do not know who would be the most appropriate reviewer to mention. As such I would very much appreciate a mentor to guide me through the process.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[rftafas]

I have found this: https://github.com/tylerwhall/zephyr-rust and it seems to be Apache licensed.

[wesleywiser]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted