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U-Boot Development Process

Management Summary:

  • Development happens in Release Cycles of 2 months.
  • The first 2 weeks are called 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 two months.

The first two 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 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 resubmits 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.

In the result, 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.

In the result, the release of the "-rc1" version does not immediately follow the closing of the Merge Window.

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 he applies 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 the Merge Window slightly - say by 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 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.

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.

Work flow of a Custodian

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

  1. A developer submits a patch via e-mail to the u-boot-users mailing list. U-Boot has adopted the Linux kernel signoff policy, so the submitter must include a "Signed-off-by:" line.
  2. Everybody who can is invited to review and test the changes. Reviews should reply on the mailing list with 'Acked-by' lines.
  3. The responsible custodian (1) inspects this patch, especially for:
    • Coding Style (2)
    • Basic logic:
      • The patch fixes a real problem.
      • The patch does not introduce new problems, especially it does not break other boards or architectures
    • U-Boot Philosophy (3)
    • Applies cleanly to the source tree (4)
    • passes a MAKEALL compile test without creating new warnings
  4. Notes:
    (1): In some cases more than one custodian may be affected or feel responsible. To avoid duplicated efforts, the custodian who starts processing the patch should send a short ACK to the mailing list.
    (2): We should create some tool to automatically do this.
    (3): This is well documented here.
    (4): The custodian decides himself how recent the code must be. 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.
    (5) Commits should show original author in the 'author' field and include all sign off/ack lines.
  5. The custodian decides to accept or to reject the patch.
  6. If accepted, the custodian adds the patch to his public git repository and notifies the mailing list. This note should include:
    • a short description of the changes
    • the list of the affected boards / architectures etc.
    • suggested tests Although the custodian is supposed to perform his own tests it is a well-known and accepted fact that he needs help from other developers who - for example - have access to the required hardware or tool chains. The custodian request help for tests and feedback from specific maintainers and U-Boot users.
  7. Once tests are passed, some agreed time limit expires, the custodian requests that the changes in his public git repository be merged into the main tree. If necessary, the custodian may have to adapt his changes to allow for a clean merge.
    Todo: define a reasonable time limit. 3 weeks?
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