misc: Add a CONTRIBUTING document

This document details how to contribute to gem5 based on our new
contribution flow with git and gerrit.

Change-Id: I0a7e15fd83a3ee3ab6c85c1192f46f1e1d33b7c2
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: http://reviews.gem5.org/r/3814/
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Pierre-Yves Peneau <pierre-yves.peneau@lirmm.fr>
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>
Reviewed-by: Brad Beckmann <brad.beckmann@amd.com>
Reviewed-by: Ali Saidi <Ali.Saidi@ARM.com>

CONTRIBUTING.md

@@ -0,0 +1,332 @@
+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 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.
+
+The keyword should be one or more of the following separated by commas:
+ * Architecture name in lower case (e.g., arm or x86): Anything that is
+   target-architecture specific.
+ * base
+ * ext
+ * stats
+ * sim
+ * syscall_emul
+ * config:
+ * mem: Classic memory system. Ruby uses its own keyword.
+ * ruby: Ruby memory models.
+ * cpu: CPU-model specific (except for kvm)
+ * kvm: KVM-specific. Changes to host architecture specific components should
+   include an architecture keyword (e.g., arm or x86) as well.
+ * gpu-compute
+ * energy
+ * dev
+ * arch: General architecture support (src/arch/)
+ * scons: Build-system related. Trivial changes as a side effect of doing
+   something unrelated (e.g., adding a source file to a SConscript) don't
+   require this.
+ * tests
+ * style: Changes to the style checkers of style fixes.
+ * misc
+
+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.
+
+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.