Development Environment and Tools

Code Review

GitHub is intended to provide a framework for reviewing every commit before it is accepted into the code base. Changes, in the form of Pull Requests (PR) are uploaded to GitHub but don’t actually become a part of the project until they’ve been reviewed, passed a series of checks (CI), and are approved by maintainers. GitHub is used to support the standard open source practice of submitting patches, which are then reviewed by the project members before being applied to the code base.

Pull requests should be appropriately labeled, and linked to any relevant bug or feature tracking issues .

The Zephyr project uses GitHub for code reviews and Git tree management. When submitting a change or an enhancement to any Zephyr component, a developer should use GitHub. GitHub automatically assigns a responsible reviewer on a component basis, as defined in the CODEOWNERS file stored with the code tree in the Zephyr project repository. A limited set of release managers are allowed to merge a pull request into the master branch once reviews are complete.

Give reviewers time to review before code merge

The Zephyr project is a global project that is not tied to a certain geography or timezone. We have developers and contributors from across the globe. When changes are proposed using pull request, we need to allow for a minimal review time to give developers and contributors the opportunity to review and comment on changes. There are different categories of changes and we know that some changes do require reviews by subject matter experts and owners of the subsystem being changed. Many changes fall under the “trivial” category that can be addressed with general reviews and do not need to be queued for a maintainer or code-owner review. Additionally, some changes might require further discussions and a decision by the TSC or the Security working group. To summarize the above, the diagram below proposes minimal review times for each category:

Pull request classes

Pull request classes


  • An author of a change can suggest in his pull-request which category a change should belong to. A project maintainers or TSC member monitoring the inflow of changes can change the label of a pull request by adding a comment justifying why a change should belong to another category.
  • The project will use the label system to categorize the pull requests.
  • Changes should not be merged before the minimal time has expired.



Any change that is a fix to an issue that blocks developers from doing their daily work, for example CI breakage, Test breakage, Minor documentation fixes that impact the user experience.

Such fixes can be merged at any time after they have passed CI checks. Depending on the fix, severity, and availability of someone to review them (other than the author) they can be merged with justification without review by one of the project owners.


Trivial changes are those that appear obvious enough and do not require maintainer or code-owner involvement. Such changes should not change the logic or the design of a subsystem or component. For example a trivial change can be:

  • Documentation changes
  • Configuration changes
  • Minor Build System tweaks
  • Minor optimization to code logic without changing the logic
  • Test changes and fixes
  • Sample modifications to support additional configuration or boards etc.

Any changes that touch the logic or the original design of a subsystem or component will need to be reviewed by the code owner or the designated subsystem maintainer. If the code changes is initiated by a contributor or developer other than the owner the pull request needs to be assigned to the code owner who will have to drive the pull request to a mergeable state by giving feedback to the author and asking for more reviews from other developers.


Changes that appear to have an impact to the overall security of the system need to be reviewed by a security expert from the security working group.


Changes that introduce new features or functionality or change the way the overall system works need to be reviewed by the TSC or the responsible Working Group. For example for API changes, the API working group needs to be consulted and made aware of such changes.

A Pull-Request should have an Assignee

  • An assignee to a pull request should not be the same as the author of the pull-request
  • An assignee to a pull request is responsible for driving the pull request to a mergeable state
  • An assignee is responsible for dismissing stale reviews and seeking reviews from additional developers and contributors
  • Pull requests should not be merged without an approval by the assignee.

Pull Request should not be merged by author without review

All pull requests need to be reviewed and should not be merged by the author without a review. The following exceptions apply:

  • Hot fixes: Fixing CI issues, reverts, and system breakage
  • Release related changes: Changing version file, applying tags and release related activities without any code changes.

Developers and contributors should always seek review, however there are cases when reviewers are not available and there is a need to get a code change into the tree as soon as possible.

Reviewers shall not ‘Request Changes’ without comments or justification

Any change requests (-1) on a pull request have to be justified. A reviewer should avoid blocking a pull-request with no justification. If a reviewer feels that a change should not be merged without their review, then: Request change of the category: for example:

  • Trivial -> Maintainer
  • Assign Pull Request to yourself, this will mean that a pull request should not be merged without your approval.

Pull Requests should have at least 2 approvals before they are merged

A pull-request shall be merged only with two positive reviews (approval). Beside the person merging the pull-request (merging != approval), two additional approvals are required to be able to merge a pull request. The person merging the request can merge without approving or approve and merge to get to the 2 approvals required.

Reviewers should keep track of pull requests they have provided feedback to

If a reviewer has requested changes in a pull request, he or she should monitor the state of the pull request and/or respond to mention requests to see if his feedback has been addressed. Failing to do so, negative reviews shall be dismissed by the assignee or an owner of the repository. Reviews will be dismissed following the criteria below:

  • The feedback or concerns were visibly addressed by the author
  • The reviewer did not revisit the pull request after 1 week and multiple pings by the author
  • The review is unrelated to the code change or asking for unjustified structural changes such as:
    • Split the PR
    • Split the commits
    • Can you fix this unrelated code that happens to appear in the diff
    • Can you fix unrelated issues
    • Etc.

Closing Stale Issues and Pull Requests

  • The Pull requests and issues sections on Github are NOT discussion forums. They are items that we need to execute and drive to closure. Use the mailing lists for discussions.
  • In case of both issues and pull-requests the original poster needs to respond to questions and provide clarifications regarding the issue or the change. Failing to do so, an issue or a pull request will be closed automatically after 6 months.

Continuous Integration

All changes submitted to GitHub are subject to sanity tests that are run on emulated platforms and architectures to identify breakage and regressions that can be immediately identified. Sanity testing additionally performs build tests of all boards and platforms. Documentation changes are also verified through review and build testing to verify doc generation will be successful.

Any failures found during the CI test run will result in a negative review assigned automatically by the CI system. Developers are expected to fix issues and rework their patches and submit again.

The CI infrastructure currently runs the following tests:

  • Run ‘’checkpatch’’ for code style issues (can vote -1 on errors)
  • Gitlint: Git commit style based on project requirements
  • License Check: Check for conflicting licenses
  • Run ‘’sanitycheck’’ script
    • Run kernel tests in QEMU (can vote -1 on errors)
    • Build various samples for different boards (can vote -1 on errors)
  • Verify documentation builds correctly.

Labeling issues and pull requests in GitHub

The project uses GitHub issues and pull requests (PRs) to track and manage daily and long-term work and contributions to the Zephyr project. We use GitHub labels to classify and organize these issues and PRs by area, type, priority, and more, making it easier to find and report on relevant items.

All GitHub issues or pull requests must be appropriately labeled. Issues and PRs often have multiple labels assigned, to help classify them in the different available categories. When reviewing a PR, if it has missing or incorrect labels, maintainers shall fix it.

This saves us all time when searching, reduces the chances of the PR or issue being forgotten, speeds up reviewing, avoids duplicate issue reports, etc.

These are the labels we currently have, grouped by type:


Labels Area:*
Applicable to PRs and issues
Description Indicates subsystems (e.g., Kernel, I2C, Memory Management), project functions (e.g., Debugging, Documentation, Process), or other categories (e.g., Coding Style, MISRA-C) affected by the bug or pull request.

An area maintainer should be able to filter by an area label and find all issues and PRs which relate to that area.


Labels Platform:*
Applicable to PRs and issues
Description An issue or PR which affects only a particular platform

To be discussed in a meeting

Labels API, dev-review, TSC
Applicable to PRs and issues
Description The issue is to be discussed in the following API/dev-review/TSC meeting if time permits

Minimum PR review time

Labels Hot Fix, Trivial, Maintainer, Security Review, TSC
Applicable to PRs only
Description Depending on the PR complexity, an indication of how long a merge should be held to ensure proper review. See review process

Issue priority labels

Labels priority:{high|medium|low}
Applicable to Issues only
Description To classify the impact and importance of a bug or feature

Note: Issue priorities are generally set or changed during the bug-triage or TSC meetings.

Miscellaneous labels

For both PRs and issues

Bug The issue is a bug, or the PR is fixing a bug
Coverity A Coverity detected issue or its fix
Waiting for response The Zephyr developers are waiting for the submitter to respond to a question, or address an issue.
Blocked Blocked by another PR or issue
In progress For PRs: is work in progress and should not be merged yet. For issues: Is being worked on
RFC The author would like input from the community. For a PR it should be considered a draft
LTS Long term release branch related
EXT Related to an external component (in ext/)

PR only labels

DNM This PR should not be merged (Do Not Merge). For work in progress, GitHub “draft” PRs are preferred
Stale PR PR which seems abandoned, and requires attention by the author
Needs review The PR needs attention from the maintainers
Backport The PR is a backport or should be backported
Licensing The PR has licensing issues which require a licensing expert to review it

Issue only labels

Regression Something, which was working, but does not anymore (bug subtype)
Question This issue is a question to the Zephyr developers
Enhancement Changes/Updates/Additions to existing features
Feature request A request for a new feature
Feature A planned feature with a milestone
Duplicate This issue is a duplicate of another issue (please specify)
Good first issue Good for a first time contributor to take
Release Notes Issues that need to be mentioned in release notes as known issues with additional information

Any issue must be classified and labeled as either Bug, Question, Enhancement, Feature, or Feature Request. More information on how feature requests are handled and become features can be found in Feature Tracking.