3.7. Code Review Guidelines

3.7.1. Why do we do code reviews?

The main goal of code reviews is to improve the code quality. By reviewing each other’s code, we can help catch issues that were missed by the author before they are integrated in the source tree. Different people bring different perspectives, depending on their past work, experiences and their current use cases of TF-A in their products.

Code reviews also play a key role in sharing knowledge within the community. People with more expertise in one area of the code base can help those that are less familiar with it.

Code reviews are meant to benefit everyone through team work. It is not about unfairly criticizing or belittling the work of any contributor.

3.7.2. Overview of the code review process

All contributions to Trusted Firmware-A project are reviewed by the community to ensure they meet the project’s expectations before they get merged, according to the Project Maintenance Process defined for all Trusted Firmware projects.

Technical ownership of most parts of the codebase falls on the Code owners. All patches are ultimately merged by the Maintainers.

Approval of a patch is tracked using Gerrit labels. For a patch to be merged, it must get all of the following votes:

  • At least one Code-Owner-Review+1 up-vote, and no Code-Owner-Review-1 down-vote.

  • At least one Maintainer-Review+1 up-vote, and no Maintainer-Review-1 down-vote.

  • Verified+1 vote applied by the automated Continuous Integration (CI) system.

Note that, in some instances, the maintainers might give a waiver for some of the CI failures and manually override the Verified+1 score.

3.7.3. Good practices for all reviewers

To ensure the code review gives the greatest possible benefit, participants in the project should:

  • Be considerate of other people and their needs. Participants may be working to different timescales, and have different priorities. Keep this in mind - be gracious while waiting for action from others, and timely in your actions when others are waiting for you.

  • Review other people’s patches where possible. The more active reviewers there are, the more quickly new patches can be reviewed and merged. Contributing to code review helps everyone in the long run, as it creates a culture of participation which serves everyone’s interests.

3.7.4. Guidelines for patch contributors

In addition to the rules outlined in the Contributor’s Guide, as a patch contributor you are expected to:

  • Answer all comments from people who took the time to review your patches.

  • Be patient and resilient. It is quite common for patches to go through several rounds of reviews and rework before they get approved, especially for larger features.

    In the event that a code review takes longer than you would hope for, you may try the following actions to speed it up:

  • Ping the reviewers on Gerrit or on the mailing list. If it is urgent, explain why. Please remain courteous and do not abuse this.

  • If one code owner has become unresponsive, ask the other code owners for help progressing the patch.

  • If there is only one code owner and they have become unresponsive, ask one of the project maintainers for help.

  • Do the right thing for the project, not the fastest thing to get code merged.

    For example, if some existing piece of code - say a driver - does not quite meet your exact needs, go the extra mile and extend the code with the missing functionality you require - as opposed to copying the code into some other directory to have the freedom to change it in any way. This way, your changes benefit everyone and will be maintained over time.

  • It is the patch-author’s responsibility to respond to review comments within 21 days. In the event that the patch-author does not respond within this timeframe, the maintainer is entitled to abandon the patch(es). Patch author(s) may be busy with other priorities, causing a delay in responding to active review comments after posting patch(es). In such a situation, if the author’s patch(es) is/are abandoned, they can restore their work for review by resolving comments, merge-conflicts, and revising their original submissions.

3.7.5. Guidelines for all reviewers

There are no good or bad review comments. If you have any doubt about a patch or need some clarifications, it’s better to ask rather than letting a potential issue slip. Examples of review comments could be:

  • Questions (“Why do you need to do this?”, “What if X happens?”)

  • Bugs (“I think you need a logical || rather than a bitwise |.”)

  • Design issues (“This won’t scale well when we introduce feature X.”)

  • Improvements (“Would it be better if we did Y instead?”)

3.7.6. Guidelines for code owners

Code owners are listed on the Project Maintenance page, along with the module(s) they look after.

When reviewing a patch, code owners are expected to check the following:

  • The patch looks good from a technical point of view. For example:

  • The structure of the code is clear.

  • It complies with the relevant standards or technical documentation (where applicable).

  • It leverages existing interfaces rather than introducing new ones unnecessarily.

  • It fits well in the design of the module.

  • It adheres to the security model of the project. In particular, it does not increase the attack surface (e.g. new SMCs) without justification.

  • The patch adheres to the TF-A Coding Style. The CI system should help catch coding style violations.

  • (Only applicable to generic code) The code is MISRA-compliant (see MISRA Compliance). The CI system should help catch violations.

  • Documentation is provided/updated (where applicable).

  • The patch has had an appropriate level of testing. Testing details are expected to be provided by the patch author. If they are not, do not hesitate to request this information.

  • All CI automated tests pass.

If a code owner is happy with a patch, they should give their approval through the Code-Owner-Review+1 label in Gerrit. If instead, they have concerns, questions, or any other type of blocking comment, they should set Code-Owner-Review-1.

Code owners are expected to behave professionally and responsibly. Here are some guidelines for them:

  • Once you are engaged in a review, make sure you stay involved until the patch is merged. Rejecting a patch and going away is not very helpful. You are expected to monitor the patch author’s answers to your review comments, answer back if needed and review new revisions of their patch.

  • Provide constructive feedback. Just saying, “This is wrong, you should do X instead.” is usually not very helpful. The patch author is unlikely to understand why you are requesting this change and might feel personally attacked.

  • Be mindful when reviewing a patch. As a code owner, you are viewed as the expert for the relevant module. By approving a patch, you are partially responsible for its quality and the effects it has for all TF-A users. Make sure you fully understand what the implications of a patch might be.

3.7.7. Guidelines for maintainers

Maintainers are listed on the Project Maintenance page.

When reviewing a patch, maintainers are expected to check the following:

  • The general structure of the patch looks good. This covers things like:

    • Code organization.

    • Files and directories, names and locations.

      For example, platform code should be added under the plat/ directory.

    • Naming conventions.

      For example, platform identifiers should be properly namespaced to avoid name clashes with generic code.

    • API design.

  • Interaction of the patch with other modules in the code base.

  • The patch aims at complying with any standard or technical documentation that applies.

  • New files must have the correct license and copyright headers. See this paragraph for more information. The CI system should help catch files with incorrect or no copyright/license headers.

  • There is no third party code or binary blobs with potential IP concerns. Maintainers should look for copyright or license notices in code, and use their best judgement. If they are unsure about a patch, they should ask other maintainers for help.

  • Generally speaking, new driver code should be placed in the generic layer. There are cases where a driver has to stay into the platform layer but this should be the exception, rather than the rule.

  • Existing common drivers (in particular for Arm IPs like the GIC driver) should not be copied into the platform layer to cater for platform quirks. This type of code duplication hurts the maintainability of the project. The duplicate driver is less likely to benefit from bug fixes and future enhancements. In most cases, it is possible to rework a generic driver to make it more flexible and fit slightly different use cases. That way, these enhancements benefit everyone.

  • When a platform specific driver really is required, the burden lies with the patch author to prove the need for it. A detailed justification should be posted via the commit message or on the mailing list.

  • Before merging a patch, verify that all review comments have been addressed. If this is not the case, encourage the patch author and the relevant reviewers to resolve these together.

If a maintainer is happy with a patch, they should give their approval through the Maintainer-Review+1 label in Gerrit. If instead, they have concerns, questions, or any other type of blocking comment, they should set Maintainer-Review-1.

Copyright (c) 2020-2023, Arm Limited. All rights reserved.