prep: preparing your patch series

The first stage of contributor workflow is to prepare your patch series for submission upstream. It generally consists of the following stages:

  1. start a new topical branch using b4 prep -n topical-name

  2. add commits as usual and work with them using git rebase -i

  3. prepare the cover letter using b4 prep --edit-cover

  4. prepare the list of recipients using b4 prep --auto-to-cc

  5. run basic checks using b4 prep --check

Starting a new topical branch

When you are ready to start working on a new submission, the first step is to create a topical branch:

b4 prep -n descriptive-name [-f tagname]

It’s important to give your branch a short descriptive name, because it becomes part of the unique change-id that is used to track your proposal across revisions. In other words, don’t call it “stuff” or “foo”.

This command performs the following operations:

  1. Creates a new branch called b4/descriptive-name and switches to it.

  2. Creates an empty commit with a cover letter template.


Generally, you should fork from some well-defined point in the project history, not from some random tip commit. You can use -f to specify a fork-point for b4 to use, such as a recent tag name.

You can then edit the cover letter using:

b4 prep --edit-cover

This should start a text editor using your defined $EDITOR or core.editor and automatically update the cover letter commit when you save and exit.

Cover letter strategies

By default, b4 keeps the cover letter in an empty commit at the start of your series. This has the following benefits:

  • it’s easy to keep track where your series starts without needing to keep a “tracking base branch” around

  • you can view and edit the cover letter using regular git commands (git log, git rebase -i)

  • you can push the entire branch to a remote and pull it from a different location to continue working on your series from a different system

However, keeping an empty commit in your history can have some disadvantages in some less-common situations:

  • it complicates merging between branches

  • some non-native git tools may drop empty commits

  • editing the cover letter rewrites the commit history of the entire branch

For this reason, b4 supports alternative strategies for storing the cover letter. You can tell b4 which strategy to use using the b4.prep-cover-strategy configuration variable.

commit strategy (default)

This is the default strategy that keeps the cover letter and all tracking information in an empty commit at the start of your series. See the preceding section for upsides and downsides.

This is the recommended strategy for developers who mostly send out patch series and don’t perform actual subsystem tree management tasks, such as merging submissions from sub-maintainers, cherry-picking commits, etc.

branch-description strategy

This keeps the cover letter and all tracking information outside of the git commits by using the branch description configuration value, stored locally in .git/config.


  • this is how git expects you to handle cover letters, see git format-patch --cover-from-description

  • editing the cover letter doesn’t rewrite commit history

  • merging between branches is easiest


  • the cover letter only exists local to your tree – you won’t be able to commit it to the repository and push it remotely

  • you have to rely on the base branch for keeping track of where your series starts

tip-commit strategy

This is similar to the default commit strategy, but instead of keeping the cover letter and all tracking information in an empty commit at the start of your series, it keeps it at the tip of your series.


  • allows you to push the series to a remote and pull it from a different location to continue working on a series

  • editing the cover letter doesn’t rewrite commit history, which may be easier when working in teams


  • adding new commits is a bit more complicated, because you have to immediately rebase them to be in front of the cover letter

  • you have to rely on the base branch for keeping track of where your series starts


At this time, you can’t easily switch from one strategy to the other once you have created the branch with b4 prep -n. This may be supported in the future.

Enrolling an existing branch

If you’ve already started working on a set of commits without first running b4 prep -n, you can enroll your existing branch to make it “prep-tracked.”

For example, if you have a branch called my-topical-branch that was forked from master, you can enroll it with b4:

b4 prep -e master

Once that completes, you should be able to edit the cover letter and use all other b4 contributor-oriented commands.

Creating a branch from a sent series

If you have previously sent a patch series, you can create your new topical branch from that submission by passing the --from-thread parameter to b4 prep -n. All you need is the msgid of the series, e.g.:

b4 prep -n my-topical-branch -F some-msgid@localhost

If you sent the series using b4 send it should even contain all the preserved tracking information, but it works reasonably well with any patch series.

Working with commits

All your commits in a prep-tracked branch are just regular git commits and you can work with them using any regular git tooling:

  • you can rebase them using git rebase

  • you can amend, reword, split, squash commits interactively using git rebase -i; there are many excellent tutorials available online on how to use interactive rebase

Unless you are using an old version of git, your empty cover letter commit should remain preserved through all rebase operations.


You can edit the cover letter using regular git operations, though it isn’t recommended and it’s best to always do it with b4 prep --edit-cover. If you do want to edit it directly using git rebase -i, remember to use git commit --allow-empty to commit it back into the tree.

What if the series only has a single patch?

When you only have a single patch, b4 should “mix-in” the contents of the cover letter into the “under-the-cut” portion of the patch itself, where it serves as a source of additional information for the reviewers, but never makes it into the actual commit.

You can just use the cover letter for extra To/Cc trailers and changelog entries as your patch goes through revisions. If you add more commits at some point in the future, you can fill in the cover letter content with additional information about the intent of your entire series.

Working with series dependencies


new in (v0.14+)

This is an experimental set of features introduced in v0.14. It may have rough edges or unexpected bugs.

If your series depends on another set of patches that aren’t yet merged into the repository, you can specify these patches as prerequisites.

There are multiple ways to indicate a dependency:

  • by the change-id of the series

  • by the message-id of its first patch or cover letter

  • by the patch-id of individual patches

Using change-id is easiest, because this allows an easy way to identify when a newer version of the prerequisite series is available. The change-id is usually included in series created and sent with b4 itself.

To find the change-id of a series, look at the first patch or the cover letter and see if it has a change-id: entry at the bottom.

Here is one such example:

To specify this series as a dependency, run b4 prep --edit-deps and add the following line in the editor that opens:

change-id: 20240402-b4-dts_dxl_audio-74ba02030a72:v5

In addition to the change-id itself, it’s also necessary to add :v5 to the end of that line, to indicate the specific revision of that series. This allows checking if a newer version of the series is available when running b4 prep --check-deps during the next step.

In addition to the change-id, you must also specify the base-commit to use for your combined series. Most of the time it is sufficient to copy-paste the base-commit: line from the same series where you copy-pasted the change-id line, but you can also use a specific tag.

Using the message-id

If a series doesn’t specify a change-id, you can also refer to it using the message-id of the cover letter or the first patch in the series. Enclose the message-id in the brackets, e.g. you can refer to the same series mentioned above by its message-id:

message-id: <>

If you specify a series by its message-id, running --check-deps won’t show if there is a newer version of the series available. For this reason, it is better to always use the series change-id, if it’s available.


Specifying the message-id always pulls in the entire series, not individual patches. To specify individual patches, use the patch-id mechanism described below.

Using the patch-id

If you really want to, you can specify individual patches using their patch-ids, generated with git patch-id --stable. For example, to obtain the patch-id of just the 3rd patch in the example series above, run:

$ curl -sL \
> | git patch-id --stable
29e16de21ac5396e1475bbfe2798ac1ab2c7b7a1 0000000000000000000000000000000000000000

You can now specify the dependency using that patch-id hash:

patch-id: 29e16de21ac5396e1475bbfe2798ac1ab2c7b7a1

Running --check-deps

After you have finished specifying dependencies using b4 prep --edit-deps, you should run --check-deps, which does the following verifications:

  • ensures that it can find all specified series on the server

  • checks if newer versions of the series are available

  • checks that the specified base-commit is present in the tree

  • checks if prerequisite series plus your patches cleanly apply

You should run --check-deps right after editing them, and right before submitting your series for review.

Prepare the list of recipients

When you are getting ready to submit your work, you need to figure out who the recipients of your series should be. By default, b4 should send the series to any address mentioned in the trailers, plus to any other addresses you tell it to use.

For the Linux kernel, a required step is to gather the recipients from the output of, which b4 does for you automatically when you run the auto-to-cc command:

b4 prep --auto-to-cc

B4 should append any discovered recipients to the cover letter as extra To: and Cc: trailers. It’s normal for this list to be pretty large if your change is touching a lot of files. You can add or remove recipients by adding or removing the recipient trailers from the cover letter using b4 prep --edit-cover.

For projects that aren’t using the MAINTAINERS file, there is usually a single list where you should send your changes. You can set that in the repository’s .git/config file as follows:

  send-series-to =

This may also be already set by the project, if they have a .b4-config file in the root of their git repository.

Checking your work


new in (v0.14+)

This is a new feature in version 0.14 and you should consider it experimental.

Once you are getting close to submitting your series, you should run b4 prep --check. This should run a suite of recommended local checks to make sure that your patches do not have some of the more common problems, such as spelling errors, missing Signed-off-by trailers, etc.

For the Linux kernel, this automatically runs scripts/, while other projects may define their own checks as part of the default .b4-config.

Cleaning up old work

Once project maintainers accept your series, you can archive and clean up the prep-managed branch, together with all of its sent tags:

b4 prep --cleanup

This command lists all prep-managed branches in your repository. Pick a branch to clean up, make sure it’s not currently checked out, and run the command again:

b4 prep --cleanup b4/my-topical-branch

After you confirm your action, this should create a tarball with all the patches, cover letters, and tracking information from your series. Afterwards, b4 deletes the branch and all related tags from your local repository.

Prep command flags

Please also see Contributor-oriented settings, which allows setting or modifying defaults for some of these flags.

-c, --auto-to-cc

Automatically populate the cover letter with addresses collected from commit trailers. If b4 finds a MAINTAINERS file, together with scripts/, it runs the recommended query to collect the maintainers and mailing lists where to send your series.

-p OUTPUT_DIR, --format-patch OUTPUT_DIR

This outputs your tracked series as patches similar to what git-format-patch would do.


Lets you edit the cover letter using the editor command defined in git-config as core.editor, the $EDITOR environment var if that isn’t found, or vim – because it’s safe to assume that if you don’t like vim, you would have already set your $EDITOR to use some other command.

--edit-deps (v0.14+)

Lets you edit the series dependencies using the editor command defined in git-config as core.editor, the $EDITOR environment var if that isn’t found, or vim.

--check-deps (v0.14+)

Verifies that b4 can resolve all specified dependencies and that everything cleanly applies to the base-commit specified.

--check (v0.14+)

Runs a set of checks on your series to identify some of the more common problems.

For the Linux kernel, this runs the following command for each of your commits:

./scripts/ --terse --no-summary --mailback --showfile

You can specify your own command by setting the b4.prep-perpatch-check-cmd configuration parameter. For example you can make it more strict:

prep-perpatch-check-cmd = ./scripts/ --terse --no-summary --mailback --strict --showfile

If you want to see a more detailed checkpatch report, you can always run it separately:

./scripts/ --strict --git $(b4 prep --show-info series-range)

Shows the current series revision.

--force-revision N

Forces the revision to a different integer number. This modifies your cover letter and tracking information and makes this change permanent.

--compare-to vN (v0.11+)

This executes a git range-diff command that lets you compare the previously sent version of the series to what is currently in your working branch. This is very useful right before sending off a new revision to make sure that you didn’t forget to include anything into changelogs.

--manual-reroll MSGID

Normally, your patch series should be automatically rerolled to the next version after a successful b4 send operation (see send: sending in your work). However, if you sent it in using some other mechanism, such as git-send-email, you can trigger a manual version reroll using this command. It requires a message-id that can be retrieved from the public-inbox server, so we can properly add the reference to the previously sent series to the cover letter changelog.

--set-prefixes PREFIX [PREFIX ...] (v0.11+)

If you want to mark your patch as RFC, WIP, or add any other subsystem identifiers, you can define them via this command. Do not add PATCH or v1 here, as these are already automatically added to the subject lines. To remove any extra prefixes you previously set, you can run --set-prefixes ''.

Alternatively, you can add any extra prefixes to the cover letter subject line, using the usual square brackets notation, e.g.:

[RFC] Cover letter subject

When b4 sends the message, it should add PATCH, vN, to the subject as necessary.

--add-prefixes PREFIX [PREFIX ...] (v0.14+)

Similar to --set-prefixes, but will add prefixes to any ones currently defined, as opposed to completely replacing them.

--show-info [PARAM] (v0.13+)

Dumps information about the current series in a format suitable for parsing by other tools. Starting with v0.13, the parameter can be one of the following:

  • key name to show just a specific value from the current branch

  • branch name to show all info about a specific branch

  • branch name:key name to show a specific value from a specific branch

For example, if you have a branch called b4/foodrv-bar and you want to display the series-range value, run:

b4 prep --show-info b4/foodrv-bar:series-range

Or, to show all values for branch b4/foodrv-bar:

b4 prep --show-info b4/foodrv-bar

Or, to show series-range for the current branch:

b4 prep --show-info series-range

And, to show all values for the current branch:

b4 prep --show-info
--cleanup [BRANCHNAME] (v0.13+)

Archive and delete obsolete prep-managed branches and all git objects related to them, such as sent tags. Run without parameters to list all known prep-managed branches in the repository. Rerun with the branch name to create an archival tarball with all patches, covers, and tracking information, and then delete all git objects related to that series from the local repository.


Creates a new branch to start work on a new patch series.

-f FORK_POINT, --fork-point FORK_POINT

When creating a new branch, use a specific fork-point instead of whatever commit happens to be at the current HEAD.

-F MSGID, --from-thread MSGID

After creating a new branch, populate it with patches from this pre-existing patch series. Requires a message-id to retrieve from the public-inbox server.


Enrolls your current branch to be b4-prep managed. Requires the name of the branch to use as the fork-point tracking base.