U-Boot Development Process

Management Summary

  • Development happens in Release Cycles of 3 months.

  • The first 3 weeks of the cycle are referred to as the Merge Window, which is followed by a Stabilization Period.

  • Patches with new code get only accepted while the Merge Window is open.

  • A patch that is generally in good shape and that was submitted while the Merge Window was open is eligible to go into the upcoming release, even if changes and resubmits are needed.

  • During the Stabilization Period, only patches that contain bug fixes get applied.

Phases of the Development Process

U-Boot development takes place in Release Cycles. A Release Cycle lasts normally for three months.

The first three weeks of each Release Cycle are called Merge Window.

It is followed by a Stabilization Period.

The end of a Release Cycle is marked by the release of a new U-Boot version.

Merge Window

The Merge Window is the period when new patches get submitted (and hopefully accepted) for inclusion into U-Boot mainline. This period lasts for 21 days (3 weeks) and ends with the release of "-rc1".

This is the only time when new code (like support for new processors or new boards, or other new features or reorganization of code) is accepted.

Twilight Time

Usually patches do not get accepted as they are - the peer review that takes place will usually require changes and resubmissions of the patches before they are considered to be ripe for inclusion into mainline.

Also the review often happens not immediately after a patch was submitted, but only when somebody (usually the responsible custodian) finds time to do this.

The result is that the final version of such patches gets submitted after the merge window has been closed.

It is current practice in U-Boot that such patches are eligible to go into the upcoming release.

The result is that the release of the "-rc1" version and formal closing of the Merge Window does not preclude patches that were already posted from being merged for the upcoming release.

Stabilization Period

During the Stabilization Period only patches containing bug fixes get applied.

Corner Cases

Sometimes it is not clear if a patch contains a bug fix or not. For example, changes that remove dead code, unused macros etc. or that contain Coding Style fixes are not strict bug fixes.

In such situations it is up to the responsible custodian to decide if they apply such patches even when the Merge Window is closed.

Exception: at the end of the Stabilization Period only strict bug fixes my be applied.

Sometimes patches miss the Merge Window slightly - say by a few hours or even a day. Patch acceptance is not as critical as a financial transaction, or such. So if there is such a slight delay, the custodian is free to turn a blind eye and accept it anyway. The idea of the development process is to make it foreseeable, but not to slow down development.

It makes more sense if an engineer spends another day on testing and cleanup and submits the patch a couple of hours late, instead of submitting a green patch which will waste efforts from several people during several rounds of review and reposts.

Differences to the Linux Development Process

  • In Linux, top-level maintainers will collect patches in their trees and send pull requests to Linus as soon as the merge window opens. So far, most U-Boot custodians do not work like that; they send pull requests only at (or even after) the end of the merge window.

  • In Linux, the closing of the merge window is marked by the release of the next "-rc1" In U-Boot, "-rc1" will only be released after all (or at least most of the) patches that were submitted during the merge window have been applied.


The Custodians take responsibility for some area of the U-Boot code. The in-tree MAINTAINERS files list who is responsible for which areas.

It is their responsibility to pick up patches from the mailing list that fall into their responsibility, and to process these.

A very important responsibility of each custodian is to provide feedback to the submitter of a patch about what is going on: if the patch was accepted, or if it was rejected (which exact list of reasons), if it needs to be reworked (with respective review comments). Even a “I have no time now, will look into it later” message is better than nothing. Also, if there are remarks to a patch, these should leave no doubt if they were just comments and the patch will be accepted anyway, or if the patch should be reworked/resubmitted, or if it was rejected.

Review Process, Git Tags

There are a number of git tags that are used to document the origin and the processing of patches on their way into the mainline U-Boot code. The following is an attempt to document how these are usually handled in the U-Boot project.

In general, we try to follow the established procedures from other projects, especially the Linux kernel, but there may be smaller differences. For reference, see the Linux kernel’s Submitting patches document.

  • Signed-off-by: the Signed-off-by: is a line at the end of the commit message by which the signer certifies that they were involved in the development of the patch and that they accept the Developer Certificate of Origin. Following this and adding a Signed-off-by: line that contains the developer’s name and email address is required.

    • Please note that in U-Boot, we do not add a Signed-off-by tag if we just pass on a patch without any changes.

    • Please note that when importing code from other projects you must say where it comes from, and what revision you are importing. You must not however copy Signed-off-by or other tags.

  • Everybody who can is invited to review and test the changes. Typically, we follow the same guidelines as the Linux kernel for Acked-by as well as Reviewed-by and similar additional tags.

  • Reviewed-by: The patch has been reviewed and found acceptable according to the Reviewer’s statement of oversight. A Reviewed-by: tag is a statement of opinion that the patch is an appropriate modification of the code without any remaining serious technical issues. Any interested reviewer (who has done the work) can offer a Reviewed-by: tag for a patch.

  • Acked-by: If a person was not directly involved in the preparation or handling of a patch but wishes to signify and record their approval of it then they can arrange to have an Acked-by: line added to the patch’s changelog.

  • Tested-by: A Tested-by: tag indicates that the patch has been successfully tested (in some environment) by the person named. Andrew Morton: “I think it’s very useful information to have. For a start, it tells you who has the hardware and knows how to build a kernel. So if you’re making a change to a driver and want it tested, you can troll the file’s changelog looking for people who might be able to help.”

  • Reported-by: If this patch fixes a problem reported by somebody else, consider adding a Reported-by: tag to credit the reporter for their contribution. Please note that this tag should not be added without the reporter’s permission, especially if the problem was not reported in a public forum.

  • Cc: If a person should have the opportunity to comment on a patch, you may optionally add a Cc: tag to the patch. Git tools (git send-email) will then automatically arrange that they receives a copy of the patch when you submit it to the mailing list. This is the only tag which might be added without an explicit action by the person it names. This tag documents that potentially interested parties have been included in the discussion. For example, when your change affects a specific board or driver, then makes a lot of sense to put the respective maintainer of this code on Cc:

Work flow of a Custodian

The normal flow of work in the U-Boot development process will look like this:

  1. The responsible custodian inspects this patch, especially for:

    1. The commit message is useful, descriptive and makes correct and appropriate usage of required git tags.

    2. U-Boot Coding Style

    3. Basic logic:

      • The patch fixes a real problem.

      • The patch does not introduce new problems, especially it does not break other boards or architectures

    4. U-Boot Philosophy, as documented in U-Boot Design Principles.

    5. Applies cleanly to the source tree. The custodian is expected to put in a “best effort” if a patch does not apply cleanly, but can be made to apply still. It is up to the custodian to decide how recent of a commit the patch must be against. It is acceptable to request patches against the last officially released version of U-Boot or newer. Of course a custodian can also accept patches against older code. It can be difficult to find the correct balance between putting too much work on the custodian or too much work on an individual submitting a patch when something does not apply cleanly.

    6. Passes Continuous Integration testing as this checks for new warnings and other issues.

  2. Note that in some cases more than one custodian may feel responsible for a particular change. To avoid duplicated efforts, the custodian who starts processing the patch should follow up to the email saying they intend to pick it up.

  3. Commits must show original author in the author field and include all of the Signed-off-by, Reviewed-by, etc, tags that have been submitted.

  4. The final decision to accept or reject a patch comes down to the custodian in question.

  5. If accepted, the custodian adds the patch to their public git repository. Ideally, they will also follow up on the mailing list with some notification that it has been applied. This is not always easy given different custodian workflows and environments however.

  6. Although a custodian is supposed to perform their own tests it is a well-known and accepted fact that they need help from other developers who - for example - have access to the required hardware or other relevant environments. Custodians are expected to ask for assistance with testing when required.

  7. Custodians are expected to submit a timely pull request of their git repository to the main repository. It is strongly encouraged that a CI run has been completed prior to submission, but not required.