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:

start a new topical branch using b4 prep -n topical-name
add commits as usual and work with them using git rebase -i
prepare the cover letter using b4 prep --edit-cover
prepare the list of recipients using b4 prep --auto-to-cc

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

This command will do the following:

Create a new branch called b4/descriptive-name and switch to it.
Create an empty commit with a cover letter template.

Note

Generally, you will want to fork from some known point in the history, not from some random HEAD 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 will fire up a text editor using your defined $EDITOR or core.editor and automatically update the cover letter commit when you are done.

Cover letter strategies

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

it is 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, which can be set 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 above for upsides and downsides.

This strategy is recommended for developers who mostly send out patch series and do not handle actual subsystem tree management (merging submissions from sub-maintainers, cherry-picking, 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).

Upsides:

this is how git expects you to handle cover letters (see git format-patch --cover-from-description)
editing the cover letter does not rewrite commit history
merging between branches is easiest

Downsides:

the cover letter cannot be pushed to a remote and only exists local to your tree
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 end (“tip”) of your series.

Upsides:

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 does not rewrite commit history, which may be easier when working in teams

Downsides:

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

Note

At this time, you cannot 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 the series was submitted using b4 send it will even contain all the preserved tracking information, but it’s not a requirement and should work reasonably well with most 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 on a different (or an updated) branch using git rebase
you can amend (reword, split, squash, etc) commits interactively using git rebase -i; there are many excellent tutorials available online on how to use interactive rebase

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

Note

You can edit the cover letter using regular git operations, though it is not recommended (best to 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 I only have a single patch?

When you only have a single patch, the contents of the cover letter will be mixed into the “under-the-cut” portion of the patch. 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 in the future version, you can fill in the cover letter content with additional information about the intent of your entire series.

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 will send the series to any address mentioned in the trailers (and 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 get_maintainer.pl, which b4 will do for you automatically when you run the auto-to-cc command:

b4 prep --auto-to-cc

The recipients will be added to the cover letter as extra To: and Cc: trailers. It is normal for this list to be very 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 are not 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:

[b4]
  send-series-to = some@list.name

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

Prep command flags

Please also see Contributor-oriented settings, which allow 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 a MAINTAINERS file is found, together with scripts/get_maintainer.pl, b4 will automatically perform the query to collect the maintainers and lists that should be notified of the change.

-p OUTPUT_DIR, --format-patch OUTPUT_DIR

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

--edit-cover

Lets you edit the cover letter using whatever editor is defined in git-config for core.editor, $EDITOR if that is not found, or vim because we’re pretty sure that if you don’t like vim, you would have already set your $EDITOR to not be vim.

--show-revision

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 will be automatically rerolled to the next version after a successful b4 send (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 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 will already be 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 will be expanded with the usual PATCH, vN, etc.

--show-info (v0.11+)

Dumps information about the current series that can be parsed by other tools.

-n NEW_SERIES_NAME, --new NEW_SERIES_NAME

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 that can be retrieved from the public-inbox server.

-e ENROLL_BASE, --enroll ENROLL_BASE

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