Contributor Expectations

The Zephyr project encourages contributors to submit changes as smaller pull requests. Smaller pull requests (PRs) have the following benefits:

• Reviewed more quickly and reviewed more thoroughly. It’s easier for reviewers to set aside a few minutes to review smaller changes several times than it is to allocate large blocks of time to review a large PR.

• Less wasted work if reviewers or maintainers reject the direction of the change.

• Easier to rebase and merge. Smaller PRs are less likely to conflict with other changes in the tree.

• Easier to revert if the PR breaks functionality.

Note

This page does not apply to draft PRs which can have any size, any number of commits and any combination of smaller PRs for testing and preview purposes. Draft PRs have no review expectation and PRs created as drafts from the start do not notify anyone by default.

Defining Smaller PRs

• Smaller PRs should encompass one self-contained logical change.

• When adding a new large feature or API, the PR should address only one part of the feature. In this case create an RFC proposal to describe the additional parts of the feature for reviewers.

• PRs should include tests or samples under the following conditions:

  • Adding new features or functionality.

  • Modifying a feature, especially for API behavior contract changes.

  • Fixing a hardware agnostic bug. The test should fail without the bug fixed and pass with the fix applied.

• PRs must update any documentation affected by a functional code change.

• If introducing a new API, the PR must include an example usage of the API. This provides context to the reviewer and prevents submitting PRs with unused APIs.

Multiple Commits on a Single PR

Contributors are further encouraged to break up PRs into multiple commits. Keep in mind each commit in the PR must still build cleanly and pass all the CI tests.

For example, when introducing an extension to an API, contributors can break up the PR into multiple commits targeting these specific changes:

1. Introduce the new APIs, including shared devicetree bindings

2. Update driver implementation X, with driver specific devicetree bindings

3. Update driver implementation Y

4. Add tests for the new API

5. Add a sample using the API

6. Update the documentation

Large Changes

Large changes to the Zephyr project must submit an RFC proposal describing the full scope of change and future work. The RFC proposal provides the required context to reviewers, but allows for smaller, incremental, PRs to get reviewed and merged into the project. The RFC should also define the minimum viable implementation.

Changes which require an RFC proposal include:

• Submitting a new feature.

• Submitting a new API.

• Treewide Changes.

• Other large changes that can benefit from the RFC proposal process.

Maintainers have the discretion to request that contributors create an RFC for PRs that are too large or complicated.

PR Requirements

• Each commit in the PR must provide a commit message following the Commit Message Guidelines.

• The PR description must include a summary of the changes and their rationales.

• All files in the PR must comply with Licensing Requirements.

• Follow the Zephyr Coding Style and Coding Guidelines.

• PRs must pass all CI checks. This is a requirement to merge the PR. Contributors may mark a PR as draft and explicitly request reviewers to provide early feedback, even with failing CI checks.

• When breaking a PR into multiple commits, each commit must build cleanly. The CI system does not enforce this policy, so it is the PR author’s responsibility to verify.

• When major new functionality is added, tests for the new functionality shall be added to the automated test suite. All API functions should have test cases and there should be tests for the behavior contracts of the API. Maintainers and reviewers have the discretion to determine if the provided tests are sufficient. The examples below demonstrate best practices on how to test APIs effectively.

  • Kernel timer tests provide around 85% test coverage for the kernel timer , measured by lines of code.

  • Emulators for off-chip peripherals are an effective way to test driver APIs. The fuel gauge tests use the smart battery emulator , providing test coverage for the fuel gauge API and the smart battery driver .

  • Code coverage reports for the Zephyr project are available on Codecov.

• Incompatible changes to APIs must also update the release notes for the next release detailing the change. APIs marked as experimental are excluded from this requirement.

• Changes to APIs must increment the API version number according to the API version rules.

• PRs must also satisfy all Merge Criteria before a member of the release engineering team merges the PR into the zephyr tree.

Maintainers may request that contributors break up a PR into smaller PRs and may request that they create an RFC proposal.

Workflow Suggestions That Help Reviewers

• Unless they applied the reviewer’s recommendation exactly, authors must not resolve and hide comments, they must let the initial reviewer do it. The Zephyr project does not require all comments to be resolved before merge. Leaving some completed discussions open can sometimes be useful to understand the greater picture.

• Respond to comments using the “Start Review” and “Add Review” green buttons in the “Files changed” view. This allows responding to multiple comments and publishing the responses in bulk. This reduces the number of emails sent to reviewers.

• As GitHub does not implement git range-diff, try to minimize rebases in the middle of a review. If a rebase is required, push this as a separate update with no other changes since the last push of the PR. When pushing a rebase only, add a comment to the PR indicating which commit is the rebase.

PR Review Escalation

The Zephyr community is a diverse group of individuals, with different levels of commitment and priorities. As such, reviewers and maintainers may not get to a PR right away.

The Zephyr Dev Meeting performs a triage of PRs missing reviewer approval, following this process:

1. Identify and update PRs missing an Assignee.

2. Identify PRs without any comments or reviews, ping the PR Assignee to start a review or assign to a different maintainer.

3. For PRs that have otherwise stalled, the Zephyr Dev Meeting pings the Assignee and any reviewers that have left comments on the PR.

Contributors may escalate PRs outside of the Zephyr Dev Meeting triage process as follows:

• After 1 week of inactivity, ping the Assignee or reviewers on the PR by adding a comment to the PR.

• After 2 weeks of inactivity, post a message on the #pr-help channel on Discord linking to the PR.

• After 2 weeks of inactivity, add the dev-review label to the PR. This explicitly adds the PR to the agenda for the next Zephyr Dev Meeting independent of the triage process. Not all contributors have the required privileges to add labels to PRs, in this case the contributor should request help on Discord or send an email to the Zephyr devel mailing list.

Note that for new PRs, contributors should generally wait for at least one Zephyr Dev Meeting before escalating the PR themselves.

PR Technical Escalation

In cases where a contributor objects to change requests from reviewers, Zephyr defines the following escalation process for resolving technical disagreements.

Before escalation of technical disagreements, follow the steps below:

• Resolve in the PR among assignee, maintainers and reviewer.

  • Assignee to act as moderator if applicable.

• Optionally resolve in the next Zephyr Dev Meeting meeting with more Maintainers and project stakeholders.

  • The involved parties and the Assignee to be present when the issue is discussed.

• If no progress is made, the assignee (maintainer) has the right to dismiss stale, unrelated or irrelevant change requests by reviewers giving the reviewers a minimum of 1 business day to respond and revisit their initial change requests or start the escalation process.

  The assignee has the responsibility to document the reasoning for dismissing any reviews in the PR and should notify the reviewer about their review being dismissed.

  To give the reviewers time to respond and escalate, the assignee is expected to block the PR from being merged either by not approving the PR or by setting the DNM label.

Escalation can be triggered by any party participating in the review process (assignee, reviewers or the original author of the change) following the steps below:

• Escalate to the Architecture Working Group by adding the Architecture Review label on the PR. Beside the weekly meeting where such escalations are processed, the Architecture Working Group shall facilitate an offline review of the escalation if requested, especially if any of the parties can’t attend the meeting.

• If all avenues of resolution and escalation have failed, assignees can escalate to the TSC and get a binding resolution in the TSC by adding the TSC label on the PR.

• The Assignee is expected to ensure the resolution of the escalation and the outcome is documented in the related pull request or issues on Github.

Reviewer Expectations

• Be respectful when commenting on PRs. Refer to the Zephyr Code of Conduct for more details.

• The Zephyr Project recognizes that reviewers and maintainers have limited bandwidth. As a reviewer, prioritize review requests in the following order:

  1. PRs related to items in the Zephyr Release Plan or those targeting the next release during the stabilization period (after RC1).

  2. PRs where the reviewer has requested blocking changes.

  3. PRs assigned to the reviewer as the area maintainer.

  4. All other PRs.

• Reviewers shall strive to advance the PR to a mergeable state with their feedback and engagement with the PR author.

• Try to provide feedback on the entire PR in one shot. This provides the contributor an opportunity to address all comments in the next PR update.

• Partial reviews are permitted, but the reviewer must add a comment indicating what portion of the PR they reviewed. Examples of useful partial reviews include:

  • Domain specific reviews (e.g. Devicetree).

  • Code style changes that impact the readability of the PR.

  • Reviewing commits separately when the requested changes cascade into the later commits.

• Avoid increasing scope of the PR by requesting new features, especially when there is a corresponding RFC associated with the PR. Instead, reviewers should add suggestions as a comment to the RFC. This also encourages more collaboration as it is easier for multiple contributors to work on a feature once the minimum implementation has merged.

• When using the “Request Changes” option, mark trivial, non-functional, requests as “Non-blocking” in the comment. Reviewers should approve PRs once only non-blocking changes remain. The PR author has discretion as to whether they address all non-blocking comments. PR authors should acknowledge every review comment in some way, even if it’s just with an emoticon.

• Reviewers shall be clear and concise what changes they are requesting when the “Request Changes” option is used. Requested changes shall be in the scope of the PR in question and following the contribution and style guidelines of the project.

• Reviewers shall not close a PR due to technical or structural disagreement. If requested changes cannot be resolved within the review process, the PR Technical Escalation path shall be used for any potential resolution path, which may include closing the PR.