Git

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The wiki is being retired!

Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!

Gerrit

As part of our move to gerrit, git was introduced as primary SCM.

Register with gerrit

For authenticated access (to submit patches) you'll need a gerrit account which you can register at http://review.coreboot.org/. You also need to add your ssh key(s) (used for authenticating your connections to the repo) and your email address(es) (used to match up Signed-off-by: statements) to your gerrit user data at http://review.coreboot.org/#settings.

OpenID

Gerrit uses OpenID for authentication. Besides many large web services that provide OpenID identity services (eg. Google, Yahoo), you can run your own, if you want.

It seems that gerrit is picky about the OpenID format. Always provide a full URL, including protocol (ie. http:// or https:// prefix). Unfortunately the error messages are non-intuitive (but will improve in the future)

Gerrit workflow

Gerrit interprets each Git commit as an individual change. Changes are autobuilt by Jenkins, and can be reviewed by developers. Once a change has gotten a positive review and has no build issues, it is applied to the master branch. Thus, no developer directly pushes to master.

Reviews grant points on a scale from -2 to 2. The meaning is:

  • -2: Do not merge (blocks gerrit from merging)
  • -1: I'd prefer you don't merge it
  • 0: neutral
  • +1: Looks good, but I won't make the last call on it
  • +2: Looks good, go ahead and merge (gerrit provides a "submit" function once it has a +2 vote)

-2 and +2 are only available to core developers as it's comparable to commit rights in SVN.

Gerrit and CLI

Reviews normally happens through the website.

Since gerrit exposes an interface through its ssh daemon, it's also possible to do reviews from CLI or mail. Unfortunately there doesn't seem to be any standing tradition on how to build a workflow around these parts, so we'll document our best practices here once they settled. You can find the official Gerrit documentation about its CLI tools here.

Gerrit and Email

Gerrit has poor email integration (in fact, it doesn't really have any at all). We send a couple of notifications to the mailing list, but that is a coreboot specific extension. Peter intends to build a mail-to-gerrit gateway should the need arise.

This gateway will provide:

  • no patch submission mechanism ("git push" is CLI friendly)
  • patch review (maybe openpgp signed "Acked-by" mails)
  • patch submission (automatically with Acked-by?)
  • maybe patch rejection? (openpgp signed "Nacked-by" mails)

Accessing the repository

The repository can be accessed using ssh (with public key authentication) or http (anonymous read-only or read-write using user/password authentication). The latter is particularily interesting for people behind firewalls, but requires git to be version 1.6.6 or newer (for "Smart HTTP" transfer). The http password can be generated (and regenerated if necessary) at http://review.coreboot.org/#settings,http-password. 

git clone ssh://<username>@review.coreboot.org:29418/coreboot

git clone http://[<username>:<password>@]review.coreboot.org/p/coreboot.git

Inside the checkout you should install a commit-msg hook that adds a Change-Id into commit messages, which uniquely identifies the logical change in Gerrit even across multiple iterations of the commit. The hook only needs to be installed once per clone, and installation can be done with 

wget -O .git/hooks/commit-msg http://review.coreboot.org/tools/hooks/commit-msg && \
  chmod +x .git/hooks/commit-msg

Or you can also just run 

make gitconfig

Working with Git

Git is a distributed version control system. This means that you can manage commits and branches completely without restriction in your local clone of the coreboot repository. Peter wrote a Git introduction after the switch to Git had been announced on the mailing list.

Commit messages

Git does not enforce a commit message style, although perhaps it should. For all aspects of Git to work the best, it's important to follow these simple guidelines for commit messages:

  1. The first line of the commit message has a short (less than 65 characters, absolute maximum is 75) summary
  2. The second line is empty (no whitespace at all)
  3. The third and any number of following lines contain a longer description of the commit as is neccessary, including relevant background information and quite possibly rationale for why the issue was solved in this particular way. These lines should never be longer than 75 characters.
Shell syntax is not relevant background information!
  1. The next line is empty (no whitespace at all)
  2. A Change-Id: line to let gerrit track this logical change
  3. A Signed-off-by: line according to the development guidelines

Please do not create Change-Id: and Signed-off-by: manually because it is boring and error-prone. Instead, please install the commit-msg hook as described above, and remember to always use git commit -s to have git add your Signed-off-by: automatically.

Here is an example of a well-formatted commit message: 

examplecomponent: Refactor duplicated setup into a function

Setting up the demo device correctly requires the exact same register
values to be written into each of the PCI device functions. Moving the
writes into a function allows also otherexamplecomponent to use them.

Signed-off-by: Joe Hacker <joe@hacker.email>

The example is missing a Change-Id: line. This is OK because Joe Hacker has set up the commit-msg hook as mentioned above, which adds a Change-Id: automatically when the commit message is saved.

Guidelines on commit message content

  • If anyone involved in coreboot reads your comment in a year, she/he shall still be able to understand what your commit is about, without analyzing the code.
  • Double-check that you're really committing what you think you are, e.g. by typing the following in the top-level coreboot directory:
git show

Pushing changes

First ensure that the git remote you want to use for pushing refers to an ssh:// URL (see Authenticated read/write access above). If you need to change this after the fact, ie. if you registered on gerrit only after having cloned anonymously, you can. Assuming that your remote is called origin (this is the default) you can run: 

git config remote.origin.url ssh://<username>@review.coreboot.org:29418/coreboot

Then run the following command once, to tell git that by default you want to submit all commits in the currently checked-out branch for review on gerrit: 

git config remote.origin.push HEAD:refs/for/master

After this, the command to push your changes is: 

git push origin

If you always push from the same or a few branches the workflow can be simplified further by running once for each branch: 

git config branch.<particularbranchname>.remote origin

...after which you then push changes with any of the configured branches checked out with a simple: 

git push

Pushing several commits not yet in the coreboot repository at once will create one review request on gerrit per commit.

NB! If you have applied patches from gerrit on a branch and you later push that branch, gerrit will think that you are submitting new versions of the patches that you had applied. This may or may not be what you intend. You can always run 

git log origin/master..

before git push to verify which commits you are about to send for review.

For automating patch submission further (ie. more ways of simplifying the command line), see the last paragraph of this gerrit documentation.

Pushing Drafts

git push origin HEAD:refs/drafts/master

Pushing to Topics

git push origin HEAD:refs/for/master/i915-kernel-x60

will push to the i915-kernel-x60 topic.

Evolving patches

Often it might happen that the patch you sent for approval is not good enough from the first attempt. Gerrit and git make it easy to track patch evolution during the review process until patches meet our quality standards and are ready for approval.

You can easily modify a patch sent to gerrit by you or even by someone else. Just apply it locally using one of the possible ways to do it, make a new local commit that fixes the issues reported by the reviewers, then rebase the change by preserving the same Change-ID. We recommend you to use the git rebase command in interactive mode, 

git rebase -i master

then commit and push the updated patch.

Alternatively, you may amend your local commit and push the updated patch to gerrit: 

git add <path/to/updated/files>
git commit --amend

then push the updated patch.

Further Git reading

There are tons of git tutorials out there. Take a look at some of these documents:

Please also feel free to ask Git questions in the coreboot IRC channel or on the mailing list.

Browsing

See http://review.coreboot.org/gitweb?p=coreboot.git

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