Authors: Jason Lowe-Power
         Andreas Sandberg
         Steve Reinhardt

If you've made changes to gem5 that might benefit others, we strongly encourage
you to contribute those changes to the public gem5 repository. There are
several reasons to do this:
 * Share your work with others, so that they can benefit from new functionality.
 * Support the scientific principle by enabling others to evaluate your
   suggestions without having to guess what you did.
 * Once your changes are part of the main repo, you no longer have to merge
   them back in every time you update your local repo. This can be a huge time
   saving!
 * Once your code is in the main repo, other people have to make their changes
   work with your code, and not the other way around.
 * Others may build on your contributions to make them even better, or extend
   them in ways you did not have time to do.
 * You will have the satisfaction of contributing back to the community.

The main method for contributing code to gem5 is via our code review website:
https://gem5-review.googlesource.com/. This documents describes the details of
how to create code changes, upload your changes, have your changes
reviewed, and finally push your changes to gem5. More information can be found
from the following sources:
 * http://gem5.org/Submitting_Contributions
 * https://gerrit-review.googlesource.com/Documentation/index.html
 * https://git-scm.com/book


High-level flow for submitting changes
======================================

    +-------------+
    | Make change |
    +------+------+
           |
           |
           v
    +------+------+
    | Post review |
    +------+------+
           |
           v
    +--------+---------+
    | Wait for reviews | <--------+
    +--------+---------+          |
           |                      |
           |                      |
           v                      |
      +----+----+   No     +------+------+
      |Reviewers+--------->+ Update code |
      |happy?   |          +------+------+
      +----+----+                 ^
           |                      |
           | Yes                  |
           v                      |
      +----+-----+   No           |
      |Maintainer+----------------+
      |happy?    |
      +----+-----+
           |
           | Yes
           v
    +------+------+
    | Submit code |
    +-------------+

After creating your change to gem5, you can post a review on our Gerrit
code-review site: https://gem5-review.googlesource.com. Before being able to
submit your code to the mainline of gem5, the code is reviewed by others in the
community. Additionally, the maintainer for that part of the code must sign off
on it.

Cloning the gem5 repo to contribute
===================================

If you plan on contributing, it is strongly encouraged for you to clone the
repository directly from our gerrit instance at
https://gem5.googlesource.com/.

To clone the master gem5 repository:
 > git clone https://gem5.googlesource.com/public/gem5

Other gem5 repositories
-----------------------

There are a few repositories other than the main gem5 development repository.

 * public/m5threads: The code for a pthreads implementation that works with
   gem5's syscall emulation mode.

Other gem5 branches
-------------------

None right now.

Making changes to gem5
======================

It is strongly encouraged to use git branches when making changes to gem5.
Additionally, keeping changes small and concise and only have a single logical
change per commit.

Unlike our previous flow with Mercurial and patch queues, when using git, you
will be committing changes to your local branch. By using separate branches in
git, you will be able to pull in and merge changes from mainline and simply
keep up with upstream changes.

Requirements for change descriptions
------------------------------------
To help reviewers and future contributors more easily understand and track
changes, we require all change descriptions be strictly formatted.

A canonical commit message consists of three parts:
 * A short summary line describing the change. This line starts with one or
   more keywords (found in the MAINTAINERS file) separated by commas followed
   by a colon and a description of the change. This line should be no more than
   65 characters long since version control systems usually add a prefix that
   causes line-wrapping for longer lines.
 * (Optional, but highly recommended) A detailed description. This describes
   what you have done and why. If the change isn't obvious, you might want to
   motivate why it is needed. Lines need to be wrapped to 75 characters or
   less.
 * Tags describing patch metadata. You are highly recommended to use
   tags to acknowledge reviewers for their work. Gerrit will automatically add
   most tags.

Tags are an optional mechanism to store additional metadata about a patch and
acknowledge people who reported a bug or reviewed that patch. Tags are
generally appended to the end of the commit message in the order they happen.
We currently use the following tags:
 * Signed-off-by: Added by the author and the submitter (if different).
   This tag is a statement saying that you believe the patch to be correct and
   have the right to submit the patch according to the license in the affected
   files. Similarly, if you commit someone else's patch, this tells the rest
   of the world that you have have the right to forward it to the main
   repository. If you need to make any changes at all to submit the change,
   these should be described within hard brackets just before your
   Signed-off-by tag. By adding this line, the contributor certifies the
   contribution is made under the terms of the Developer Certificate of Origin
   (DCO) [https://developercertificate.org/].
 * Reviewed-by: Used to acknowledge patch reviewers. It's generally considered
   good form to add these. Added automatically.
 * Reported-by: Used to acknowledge someone for finding and reporting a bug.
 * Reviewed-on: Link to the review request corresponding to this patch. Added
   automatically.
 * Change-Id: Used by Gerrit to track changes across rebases. Added
   automatically with a commit hook by git.
 * Tested-by: Used to acknowledge people who tested a patch. Sometimes added
   automatically by review systems that integrate with CI systems.

Other than the "Signed-off-by", "Reported-by", and "Tested-by" tags, you
generally don't need to add these manually as they are added automatically by
Gerrit.

It is encouraged for the author of the patch and the submitter to add a
Signed-off-by tag to the commit message. By adding this line, the contributor
certifies the contribution is made under the terms of the Developer Certificate
of Origin (DCO) [https://developercertificate.org/].

It is imperative that you use your real name and your real email address in
both tags and in the author field of the changeset.

For significant changes, authors are encouraged to add copyright information
and their names at the beginning of the file. The main purpose of the author
names on the file is to track who is most knowledgeable about the file (e.g.,
who has contributed a significant amount of code to the file).

Note: If you do not follow these guidelines, the gerrit review site will
automatically reject your patch.
If this happens, update your changeset descriptions to match the required style
and resubmit. The following is a useful git command to update the most recent
commit (HEAD).

 > git commit --amend

Posting a review
================

If you have not signed up for an account on the Gerrit review site
(https://gem5-review.googlesource.com), you first have to create an account.

Setting up an account
---------------------
 1. Go to https://gem5.googlesource.com/
 2. Click "Sign In" in the upper right corner. Note: You will need a Google
 account to contribute.
 3. After signing in, click "Generate Password" and follow the instructions.

Submitting a change
-------------------

In gerrit, to submit a review request, you can simply push your git commits to
a special named branch. For more information on git push see
https://git-scm.com/docs/git-push.

There are three ways to push your changes to gerrit.

Push change to gerrit review
----------------------------

 > git push origin HEAD:refs/for/master

Assuming origin is https://gem5.googlesource.com/public/gem5 and you want to
push the changeset at HEAD, this will create a new review request on top of the
master branch. More generally,

 > git push <gem5 gerrit instance> <changeset>:refs/for/<branch>

See https://gerrit-review.googlesource.com/Documentation/user-upload.html for
more information.

Pushing your first change
--------------------------
The first time you push a change you may get the following error:

 > remote: ERROR: [fb1366b] missing Change-Id in commit message footer
 > ...

Within the error message, there is a command line you should run. For every new
clone of the git repo, you need to run the following command to automatically
insert the change id in the the commit (all on one line).

 > curl -Lo `git rev-parse --git-dir`/hooks/commit-msg
   https://gerrit-review.googlesource.com/tools/hooks/commit-msg ; chmod +x
   `git rev-parse --git-dir`/hooks/commit-msg

If you receive the above error, simply run this command and then amend your
changeset.

 > git commit --amend

Push change to gerrit as a draft
--------------------------------

 > git push origin HEAD:refs/drafts/master

Push change bypassing gerrit
-----------------------------

Only maintainers can bypass gerrit review. This should very rarely be used.

 > git push origin HEAD:refs/heads/master

Other gerrit push options
-------------------------

There are a number of options you can specify when uploading your changes to
gerrit (e.g., reviewers, labels). The gerrit documentation has more
information.
https://gerrit-review.googlesource.com/Documentation/user-upload.html


Reviewing patches
=================

Reviewing patches is done on our gerrit instance at
https://gem5-review.googlesource.com/.

After logging in with your Google account, you will be able to comment, review,
and push your own patches as well as review others' patches. All gem5 users are
encouraged to review patches. The only requirement to review patches is to be
polite and respectful of others.

There are multiple labels in Gerrit that can be applied to each review detailed
below.
 * Code-review: This is used by any gem5 user to review patches. When reviewing
   a patch you can give it a score of -2 to +2 with the following semantics.
   * -2: This blocks the patch. You believe that this patch should never be
     committed. This label should be very rarely used.
   * -1: You would prefer this is not merged as is
   * 0: No score
   * +1: This patch seems good, but you aren't 100% confident that it should be
     pushed.
   * +2: This is a good patch and should be pushed as is.
 * Maintainer: Currently only PMC members are maintainers. At least one
   maintainer must review your patch and give it a +1 before it can be merged.
 * Verified: This is automatically generated from the continuous integrated
   (CI) tests. Each patch must receive at least a +1 from the CI tests before
   the patch can be merged. The patch will receive a +1 if gem5 builds and
   runs, and it will receive a +2 if the stats match.
 * Style-Check: This is automatically generated and tests the patch against the
   gem5 code style (http://www.gem5.org/Coding_Style). The patch must receive a
   +1 from the style checker to be pushed.

Note: Whenever the patch creator updates the patch all reviewers must re-review
the patch. There is no longer a "Fix it, then Ship It" option.

Once you have received reviews for your patch, you will likely need to make
changes. To do this, you should update the original git changeset. Then, you
can simply push the changeset again to the same Gerrit branch to update the
review request.

 > git push origin HEAD:refs/for/master

Note: If you have posted a patch and don't receive any reviews, you may need to
prod the reviewers. You can do this by adding a reply to your changeset review
on gerrit. It is expected that at least the maintainer will supply a review for
your patch.

Committing changes
==================

Each patch must meet the following criteria to be merged:
 * At least one review with +2
 * At least one maintainer with +1
 * At least +1 from the CI tests (gem5 must build and run)
 * At least +1 from the style checker

Once a patch meets the above criteria, the submitter of the patch will be able
to merge the patch by pressing the "Submit" button on Gerrit. When the patch is
submitted, it is merged into the public gem5 branch.