4 Contributing Changes to a Component

Contributions to the Yocto Project and OpenEmbedded are very welcome. Because the system is extremely configurable and flexible, we recognize that developers will want to extend, configure or optimize it for their specific uses.

4.1 Contributing through mailing lists — Why not using web-based workflows?

Both Yocto Project and OpenEmbedded have many key components that are maintained by patches being submitted on mailing lists. We appreciate this approach does look a little old fashioned when other workflows are available through web technology such as GitHub, GitLab and others. Since we are often asked this question, we’ve decided to document the reasons for using mailing lists.

One significant factor is that we value peer review. When a change is proposed to many of the core pieces of the project, it helps to have many eyes of review go over them. Whilst there is ultimately one maintainer who needs to make the final call on accepting or rejecting a patch, the review is made by many eyes and the exact people reviewing it are likely unknown to the maintainer. It is often the surprise reviewer that catches the most interesting issues!

This is in contrast to the “GitHub” style workflow where either just a maintainer makes that review, or review is specifically requested from nominated people. We believe there is significant value added to the codebase by this peer review and that moving away from mailing lists would be to the detriment of our code.

We also need to acknowledge that many of our developers are used to this mailing list workflow and have worked with it for years, with tools and processes built around it. Changing away from this would result in a loss of key people from the project, which would again be to its detriment.

The projects are acutely aware that potential new contributors find the mailing list approach off-putting and would prefer a web-based GUI. Since we don’t believe that can work for us, the project is aiming to ensure patchwork is available to help track patch status and also looking at how tooling can provide more feedback to users about patch status. We are looking at improving tools such as patchtest to test user contributions before they hit the mailing lists and also at better documenting how to use such workflows since we recognise that whilst this was common knowledge a decade ago, it might not be as familiar now.

4.2 Preparing Changes for Submission

4.2.1 Set up Git

The first thing to do is to install Git packages. Here is an example on Debian and Ubuntu:

sudo aptitude install git-core git-email

Then, you need to set a name and e-mail address that Git will use to identify your commits:

git config --global user.name "Ada Lovelace"
git config --global user.email "ada.lovelace@gmail.com"

4.2.2 Clone the Git repository for the component to modify

After identifying the component to modify as described in the “Identify the component” section, clone the corresponding Git repository. Here is an example for OpenEmbedded-Core:

git clone https://git.openembedded.org/openembedded-core
cd openembedded-core

4.2.3 Create a new branch

Then, create a new branch in your local Git repository for your changes, starting from the reference branch in the upstream repository (often called master):

$ git checkout <ref-branch>
$ git checkout -b my-changes

If you have completely unrelated sets of changes to submit, you should even create one branch for each set.

4.2.4 Implement and commit changes

In each branch, you should group your changes into small, controlled and isolated ones. Keeping changes small and isolated aids review, makes merging/rebasing easier and keeps the change history clean should anyone need to refer to it in future.

To this purpose, you should create one Git commit per change, corresponding to each of the patches you will eventually submit. See further guidance in the Linux kernel documentation if needed.

For example, when you intend to add multiple new recipes, each recipe should be added in a separate commit. For upgrades to existing recipes, the previous version should usually be deleted as part of the same commit to add the upgraded version.

1. Stage Your Changes: Stage your changes by using the git add command on each file you modified. If you want to stage all the files you modified, you can even use the git add -A command.

2. Commit Your Changes: This is when you can create separate commits. For each commit to create, use the git commit -s command with the files or directories you want to include in the commit:

   $ git commit -s file1 file2 dir1 dir2 ...
   

   To include all staged files:

   $ git commit -sa
   

   • The -s option of git commit adds a “Signed-off-by:” line to your commit message. There is the same requirement for contributing to the Linux kernel. Adding such a line signifies that you, the submitter, have agreed to the Developer’s Certificate of Origin 1.1 as follows:

     Developer's Certificate of Origin 1.1
     
     By making a contribution to this project, I certify that:
     
     (a) The contribution was created in whole or in part by me and I
         have the right to submit it under the open source license
         indicated in the file; or
     
     (b) The contribution is based upon previous work that, to the best
         of my knowledge, is covered under an appropriate open source
         license and I have the right under that license to submit that
         work with modifications, whether created in whole or in part
         by me, under the same open source license (unless I am
         permitted to submit under a different license), as indicated
         in the file; or
     
     (c) The contribution was provided directly to me by some other
         person who certified (a), (b) or (c) and I have not modified
         it.
     
     (d) I understand and agree that this project and the contribution
         are public and that a record of the contribution (including all
         personal information I submit with it, including my sign-off) is
         maintained indefinitely and may be redistributed consistent with
         this project or the open source license(s) involved.
     

   • Provide a single-line summary of the change and, if more explanation is needed, provide more detail in the body of the commit. This summary is typically viewable in the “shortlist” of changes. Thus, providing something short and descriptive that gives the reader a summary of the change is useful when viewing a list of many commits. You should prefix this short description with the recipe name (if changing a recipe), or else with the short form path to the file being changed.

     Note

     To find a suitable prefix for the commit summary, a good idea is to look for prefixes used in previous commits touching the same files or directories:

     git log --oneline <paths>
     

   • For the body of the commit message, provide detailed information that describes what you changed, why you made the change, and the approach you used. It might also be helpful if you mention how you tested the change. Provide as much detail as you can in the body of the commit message.

     Note

     If the single line summary is enough to describe a simple change, the body of the commit message can be left empty.

   • If the change addresses a specific bug or issue that is associated with a bug-tracking ID, include a reference to that ID in your detailed description. For example, the Yocto Project uses a specific convention for bug references — any commit that addresses a specific bug should use the following form for the detailed description. Be sure to use the actual bug-tracking ID from Bugzilla for bug-id:

     Fixes [YOCTO #bug-id]
     
     detailed description of change
     

3. Crediting contributors: By using the git commit --amend command, you can add some tags to the commit description to credit other contributors to the change:

   • Reported-by: name and email of a person reporting a bug that your commit is trying to fix. This is a good practice to encourage people to go on reporting bugs and let them know that their reports are taken into account.

   • Suggested-by: name and email of a person to credit for the idea of making the change.

   • Tested-by, Reviewed-by: name and email for people having tested your changes or reviewed their code. These fields are usually added by the maintainer accepting a patch, or by yourself if you submitted your patches to early reviewers, or are submitting an unmodified patch again as part of a new iteration of your patch series.

   • CC: Name and email of people you want to send a copy of your changes to. This field will be used by git send-email.

   See more guidance about using such tags in the Linux kernel documentation.

4.3 Creating Patches

Here is the general procedure on how to create patches to be sent through email:

1. Describe the Changes in your Branch: If you have more than one commit in your branch, it’s recommended to provide a cover letter describing the series of patches you are about to send.

   For this purpose, a good solution is to store the cover letter contents in the branch itself:

   git branch --edit-description
   

   This will open a text editor to fill in the description for your changes. This description can be updated when necessary and will be used by Git to create the cover letter together with the patches.

   It is recommended to start this description with a title line which will serve a the subject line for the cover letter.

2. Generate Patches for your Branch: The git format-patch command will generate patch files for each of the commits in your branch. You need to pass the reference branch your branch starts from.

   If you branch didn’t need a description in the previous step:

   $ git format-patch <ref-branch>
   

   If you filled a description for your branch, you will want to generate a cover letter too:

   $ git format-patch --cover-letter --cover-from-description=auto <ref-branch>
   

   After the command is run, the current directory contains numbered .patch files for the commits in your branch. If you have a cover letter, it will be in the 0000-cover-letter.patch.

   Note

   The --cover-from-description=auto option makes git format-patch use the first paragraph of the branch description as the cover letter title. Another possibility, which is easier to remember, is to pass only the --cover-letter option, but you will have to edit the subject line manually every time you generate the patches.

   See the git format-patch manual page for details.

3. Review each of the Patch Files: This final review of the patches before sending them often allows to view your changes from a different perspective and discover defects such as typos, spacing issues or lines or even files that you didn’t intend to modify. This review should include the cover letter patch too.

   If necessary, rework your commits as described in “Taking Patch Review into Account”.

4.4 Validating Patches with Patchtest

patchtest is available in openembedded-core as a tool for making sure that your patches are well-formatted and contain important info for maintenance purposes, such as Signed-off-by and Upstream-Status tags. Currently, it only supports testing patches for openembedded-core branches. To setup, perform the following:

pip install -r meta/lib/patchtest/requirements.txt
source oe-init-build-env
bitbake-layers add-layer ../meta-selftest

Once these steps are complete and you have generated your patch files, you can run patchtest like so:

patchtest --patch <patch_name>

Alternatively, if you want patchtest to iterate over and test multiple patches stored in a directory, you can use:

patchtest --directory <directory_name>

By default, patchtest uses its own modules’ file paths to determine what repository and test suite to check patches against. If you wish to test patches against a repository other than openembedded-core and/or use a different set of tests, you can use the --repodir and --testdir flags:

patchtest --patch <patch_name> --repodir <path/to/repo> --testdir <path/to/testdir>

Finally, note that patchtest is designed to test patches in a standalone way, so if your patches are meant to apply on top of changes made by previous patches in a series, it is possible that patchtest will report false failures regarding the “merge on head” test.

Using patchtest in this manner provides a final check for the overall quality of your changes before they are submitted for review by the maintainers.

4.5 Sending the Patches via Email

4.5.1 Using Git to Send Patches

To submit patches through email, it is very important that you send them without any whitespace or HTML formatting that either you or your mailer introduces. The maintainer that receives your patches needs to be able to save and apply them directly from your emails, using the git am command.

Using the git send-email command is the only error-proof way of sending your patches using email since there is no risk of compromising whitespace in the body of the message, which can occur when you use your own mail client. It will also properly include your patches as inline attachments, which is not easy to do with standard e-mail clients without breaking lines. If you used your regular e-mail client and shared your patches as regular attachments, reviewers wouldn’t be able to quote specific sections of your changes and make comments about them.

4.5.2 Setting up Git to Send Email

The git send-email command can send email by using a local or remote Mail Transport Agent (MTA) such as msmtp, sendmail, or through a direct SMTP configuration in your Git ~/.gitconfig file.

Here are the settings for letting git send-email send e-mail through your regular STMP server, using a Google Mail account as an example:

git config --global sendemail.smtpserver smtp.gmail.com
git config --global sendemail.smtpserverport 587
git config --global sendemail.smtpencryption tls
git config --global sendemail.smtpuser ada.lovelace@gmail.com
git config --global sendemail.smtppass = XXXXXXXX

These settings will appear in the .gitconfig file in your home directory.

If you neither can use a local MTA nor SMTP, make sure you use an email client that does not touch the message (turning spaces in tabs, wrapping lines, etc.). A good mail client to do so is Pine (or Alpine) or Mutt. For more information about suitable clients, see Email clients info for Linux in the Linux kernel sources.

If you use such clients, just include the patch in the body of your email.

4.5.3 Finding a Suitable Mailing List

You should send patches to the appropriate mailing list so that they can be reviewed by the right contributors and merged by the appropriate maintainer. The specific mailing list you need to use depends on the location of the code you are changing.

If people have concerns with any of the patches, they will usually voice their concern over the mailing list. If patches do not receive any negative reviews, the maintainer of the affected layer typically takes them, tests them, and then based on successful testing, merges them.

In general, each component (e.g. layer) should have a README file that indicates where to send the changes and which process to follow.

The “poky” repository, which is the Yocto Project’s reference build environment, is a hybrid repository that contains several individual pieces (e.g. BitBake, Metadata, documentation, and so forth) built using the combo-layer tool. The upstream location used for submitting changes varies by component:

• Core Metadata: Send your patches to the openembedded-core mailing list. For example, a change to anything under the meta or scripts directories should be sent to this mailing list.

• BitBake: For changes to BitBake (i.e. anything under the bitbake directory), send your patches to the bitbake-devel mailing list.

• “meta-*” trees: These trees contain Metadata. Use the poky mailing list.

• Documentation: For changes to the Yocto Project documentation, use the docs mailing list.

For changes to other layers and tools hosted in the Yocto Project source repositories (i.e. git.yoctoproject.org), use the yocto general mailing list.

For changes to other layers hosted in the OpenEmbedded source repositories (i.e. git.openembedded.org), use the openembedded-devel mailing list, unless specified otherwise in the layer’s README file.

If you intend to submit a new recipe that neither fits into the core Metadata, nor into meta-openembedded, you should look for a suitable layer in https://layers.openembedded.org. If similar recipes can be expected, you may consider Creating Your Own Layer.

If in doubt, please ask on the yocto general mailing list or on the openembedded-devel mailing list.

4.5.4 Subscribing to the Mailing List

After identifying the right mailing list to use, you will have to subscribe to it if you haven’t done it yet.

If you attempt to send patches to a list you haven’t subscribed to, your email will be returned as undelivered.

However, if you don’t want to be receive all the messages sent to a mailing list, you can set your subscription to “no email”. You will still be a subscriber able to send messages, but you won’t receive any e-mail. If people reply to your message, their e-mail clients will default to including your email address in the conversation anyway.

Anyway, you’ll also be able to access the new messages on mailing list archives, either through a web browser, or for the lists archived on https://lore.kernelorg, through an individual newsgroup feed or a git repository.

4.5.5 Sending Patches via Email

At this stage, you are ready to send your patches via email. Here’s the typical usage of git send-email:

git send-email --to <mailing-list-address> *.patch

Then, review each subject line and list of recipients carefully, and then and then allow the command to send each message.

You will see that git send-email will automatically copy the people listed in any commit tags such as Signed-off-by or Reported-by.

In case you are sending patches for meta-openembedded or any layer other than openembedded-core, please add the appropriate prefix so that it is clear which layer the patch is intended to be applied to:

git format-patch --subject-prefix="meta-oe][PATCH" ...

Note

It is actually possible to send patches without generating them first. However, make sure you have reviewed your changes carefully because git send-email will just show you the title lines of each patch.

Here’s a command you can use if you just have one patch in your branch:

git send-email --to <mailing-list-address> -1

If you have multiple patches and a cover letter, you can send patches for all the commits between the reference branch and the tip of your branch:

git send-email --cover-letter --cover-from-description=auto --to <mailing-list-address> -M <ref-branch>

See the git send-email manual page for details.

4.5.6 Troubleshooting Email Issues

4.5.6.1 Fixing your From identity

We have a frequent issue with contributors whose patches are received through a From field which doesn’t match the Signed-off-by information. Here is a typical example for people sending from a domain name with https://en.wikipedia.org/wiki/DMARC:

From: "Linus Torvalds via lists.openembedded.org <linus.torvalds=kernel.org@lists.openembedded.org>"

This From field is used by git am to recreate commits with the right author name. The following will ensure that your e-mails have an additional From field at the beginning of the Email body, and therefore that maintainers accepting your patches don’t have to fix commit author information manually:

git config --global sendemail.from "linus.torvalds@kernel.org"

The sendemail.from should match your user.email setting, which appears in the Signed-off-by line of your commits.

4.5.7 Streamlining git send-email usage

If you want to save time and not be forced to remember the right options to use with git send-email, you can use Git configuration settings.

• To set the right mailing list address for a given repository:

  git config --local sendemail.to openembedded-devel@lists.openembedded.org
  

• If the mailing list requires a subject prefix for the layer (this only works when the repository only contains one layer):

  git config --local format.subjectprefix "meta-something][PATCH"
  

4.6 Using Scripts to Push a Change Upstream and Request a Pull

For larger patch series it is preferable to send a pull request which not only includes the patch but also a pointer to a branch that can be pulled from. This involves making a local branch for your changes, pushing this branch to an accessible repository and then using the create-pull-request and send-pull-request scripts from openembedded-core to create and send a patch series with a link to the branch for review.

Follow this procedure to push a change to an upstream “contrib” Git repository once the steps in “Preparing Changes for Submission” have been followed:

Note

You can find general Git information on how to push a change upstream in the Git Community Book.

1. Request Push Access to an “Upstream” Contrib Repository: Send an email to helpdesk@yoctoproject.org:

   • Attach your SSH public key which usually named id_rsa.pub.. If you don’t have one generate it by running ssh-keygen -t rsa -b 4096 -C "your_email@example.com".

   • List the repositories you’re planning to contribute to.

   • Include your preferred branch prefix for -contrib repositories.

2. Push Your Commits to the “Contrib” Upstream: Push your changes to that repository:

   $ git push upstream_remote_repo local_branch_name
   

   For example, suppose you have permissions to push into the upstream meta-intel-contrib repository and you are working in a local branch named your_name/README. The following command pushes your local commits to the meta-intel-contrib upstream repository and puts the commit in a branch named your_name/README:

   $ git push meta-intel-contrib your_name/README
   

3. Determine Who to Notify: Determine the maintainer or the mailing list that you need to notify for the change.

   Before submitting any change, you need to be sure who the maintainer is or what mailing list that you need to notify. Use either these methods to find out:

   • Maintenance File: Examine the maintainers.inc file, which is located in the Source Directory at meta/conf/distro/include, to see who is responsible for code.

   • Search by File: Using Git, you can enter the following command to bring up a short list of all commits against a specific file:

     git shortlog -- filename
     

     Just provide the name of the file for which you are interested. The information returned is not ordered by history but does include a list of everyone who has committed grouped by name. From the list, you can see who is responsible for the bulk of the changes against the file.

   • Find the Mailing List to Use: See the “Finding a Suitable Mailing List” section above.

4. Make a Pull Request: Notify the maintainer or the mailing list that you have pushed a change by making a pull request.

   The Yocto Project provides two scripts that conveniently let you generate and send pull requests to the Yocto Project. These scripts are create-pull-request and send-pull-request. You can find these scripts in the scripts directory within the Source Directory (e.g. poky/scripts).

   Using these scripts correctly formats the requests without introducing any whitespace or HTML formatting. The maintainer that receives your patches either directly or through the mailing list needs to be able to save and apply them directly from your emails. Using these scripts is the preferred method for sending patches.

   First, create the pull request. For example, the following command runs the script, specifies the upstream repository in the contrib directory into which you pushed the change, and provides a subject line in the created patch files:

   $ poky/scripts/create-pull-request -u meta-intel-contrib -s "Updated Manual Section Reference in README"
   

   Running this script forms *.patch files in a folder named pull-PID in the current directory. One of the patch files is a cover letter.

   Before running the send-pull-request script, you must edit the cover letter patch to insert information about your change. After editing the cover letter, send the pull request. For example, the following command runs the script and specifies the patch directory and email address. In this example, the email address is a mailing list:

   $ poky/scripts/send-pull-request -p ~/meta-intel/pull-10565 -t meta-intel@lists.yoctoproject.org
   

   You need to follow the prompts as the script is interactive.

   Note

   For help on using these scripts, simply provide the -h argument as follows:

   $ poky/scripts/create-pull-request -h
   $ poky/scripts/send-pull-request -h
   

4.7 Submitting Changes to Stable Release Branches

The process for proposing changes to a Yocto Project stable branch differs from the steps described above. Changes to a stable branch must address identified bugs or CVEs and should be made carefully in order to avoid the risk of introducing new bugs or breaking backwards compatibility. Typically bug fixes must already be accepted into the master branch before they can be backported to a stable branch unless the bug in question does not affect the master branch or the fix on the master branch is unsuitable for backporting.

The list of stable branches along with the status and maintainer for each branch can be obtained from the Releases wiki page.

Note

Changes will not typically be accepted for branches which are marked as End-Of-Life (EOL).

With this in mind, the steps to submit a change for a stable branch are as follows:

1. Identify the bug or CVE to be fixed: This information should be collected so that it can be included in your submission.

   See Checking for Vulnerabilities for details about CVE tracking.

2. Check if the fix is already present in the master branch: This will result in the most straightforward path into the stable branch for the fix.

   1. If the fix is present in the master branch — submit a backport request by email: You should send an email to the relevant stable branch maintainer and the mailing list with details of the bug or CVE to be fixed, the commit hash on the master branch that fixes the issue and the stable branches which you would like this fix to be backported to.

   2. If the fix is not present in the master branch — submit the fix to the master branch first: This will ensure that the fix passes through the project’s usual patch review and test processes before being accepted. It will also ensure that bugs are not left unresolved in the master branch itself. Once the fix is accepted in the master branch a backport request can be submitted as above.

   3. If the fix is unsuitable for the master branch — submit a patch directly for the stable branch: This method should be considered as a last resort. It is typically necessary when the master branch is using a newer version of the software which includes an upstream fix for the issue or when the issue has been fixed on the master branch in a way that introduces backwards incompatible changes. In this case follow the steps in “Preparing Changes for Submission” and in the following sections but modify the subject header of your patch email to include the name of the stable branch which you are targetting. This can be done using the --subject-prefix argument to git format-patch, for example to submit a patch to the “mickledore” branch use:

      git format-patch --subject-prefix='mickledore][PATCH' ...
      

4.8 Taking Patch Review into Account

You may get feedback on your submitted patches from other community members or from the automated patchtest service. If issues are identified in your patches then it is usually necessary to address these before the patches are accepted into the project. In this case you should your commits according to the feedback and submit an updated version to the relevant mailing list.

In any case, never fix reported issues by fixing them in new commits on the tip of your branch. Always come up with a new series of commits without the reported issues.

Note

It is a good idea to send a copy to the reviewers who provided feedback to the previous version of the patch. You can make sure this happens by adding a CC tag to the commit description:

CC: William Shakespeare <bill@yoctoproject.org>

A single patch can be amended using git commit --amend, and multiple patches can be easily reworked and reordered through an interactive Git rebase:

git rebase -i <ref-branch>

See this tutorial for practical guidance about using Git interactive rebasing.

You should also modify the [PATCH] tag in the email subject line when sending the revised patch to mark the new iteration as [PATCH v2], [PATCH v3], etc as appropriate. This can be done by passing the -v argument to git format-patch with a version number:

git format-patch -v2 <ref-branch>

Lastly please ensure that you also test your revised changes. In particular please don’t just edit the patch file written out by git format-patch and resend it.

4.9 Tracking the Status of Patches

The Yocto Project uses a Patchwork instance to track the status of patches submitted to the various mailing lists and to support automated patch testing. Each submitted patch is checked for common mistakes and deviations from the expected patch format and submitters are notified by patchtest if such mistakes are found. This process helps to reduce the burden of patch review on maintainers.

Note

This system is imperfect and changes can sometimes get lost in the flow. Asking about the status of a patch or change is reasonable if the change has been idle for a while with no feedback.

If your patches have not had any feedback in a few days, they may have already been merged. You can run git pull branch to check this. Note that many if not most layer maintainers do not send out acknowledgement emails when they accept patches. Alternatively, if there is no response or merge after a few days the patch may have been missed or the appropriate reviewers may not currently be around. It is then perfectly fine to reply to it yourself with a reminder asking for feedback.

Note

Patch reviews for feature and recipe upgrade patches are likely be delayed during a feature freeze because these types of patches aren’t merged during at that time — you may have to wait until after the freeze is lifted.

Maintainers also commonly use -next branches to test submissions prior to merging patches. Thus, you can get an idea of the status of a patch based on whether the patch has been merged into one of these branches. The commonly used testing branches for OpenEmbedded-Core are as follows:

• openembedded-core “master-next” branch: This branch is part of the openembedded-core repository and contains proposed changes to the core metadata.

• poky “master-next” branch: This branch is part of the poky repository and combines proposed changes to BitBake, the core metadata and the poky distro.

Similarly, stable branches maintained by the project may have corresponding -next branches which collect proposed changes. For example, nanbield-next and mickledore-next branches in both the “openembdedded-core” and “poky” repositories.

Other layers may have similar testing branches but there is no formal requirement or standard for these so please check the documentation for the layers you are contributing to.