(and a short version for the impatient):
I started reading over the SubmittingPatches document for git, primarily because I wanted to have a document similar to it for my projects to make sure people understand what they are doing when they write "Signed-off-by" line.
But the patch submission requirements are a lot more relaxed here on the technical/contents front, because my projects are thousand times smaller ;-). So here is only the relevant bits.
In general, always base your work on the oldest branch that your change is relevant to.
To find the tip of a topic branch, run "git log --first-parent master..pu" and look for the merge commit. The second parent of this commit is the tip of the topic branch.
Unless your patch is really trivial, you should not be sending out a patch that was generated between your working tree and your commit head. Instead, always make a commit with complete commit message and generate a series of patches from your repository. It is a good discipline.
Describe the technical detail of the change(s).
If your description starts to get too long, that's a sign that you probably need to split up your commit to finer grained pieces. That being said, patches which plainly describe the things that help reviewers check the patch, and future maintainers understand the code, are the most beautiful patches. Descriptions that summarise the point in the subject well, and describe the motivation for the change, the approach taken by the change, and if relevant how this differs substantially from the prior version, can be found on Usenet archives back into the late 80's. Consider it like good Netiquette, but for code.
Oh, another thing. I am picky about whitespaces. Make sure your changes do not trigger errors with the sample pre-commit hook shipped in templates/hooks--pre-commit. To help ensure this does not happen, run git diff --check on your changes before you commit.
git based diff tools (git, Cogito, and StGIT included) generate unidiff which is the preferred format.
You do not have to be afraid to use -M option to "git diff" or "git format-patch", if your patch involves file renames. The receiving end can handle them just fine.
Please make sure your patch does not include any extra files which do not belong in a patch submission. Make sure to review your patch after generating it, to ensure accuracy. Before sending out, please make sure it cleanly applies to the "master" branch head. If you are preparing a work based on "next" branch, that is fine, but please mark it as such.
People need to be able to read and comment on the changes you are submitting. Do not cut-n-paste your patch; you can lose tabs that way if you are not careful.
It is a common convention to prefix your subject line with [PATCH]. This lets people easily distinguish patches from other e-mail discussions. Use of additional markers after PATCH and the closing bracket to mark the nature of the patch is also encouraged. E.g. [PATCH/RFC] is often used when the patch is not ready to be applied but it is for discussion, [PATCH v2], [PATCH v3] etc. are often seen when you are sending an update to what you have previously sent.
You often want to add additional explanation about the patch, other than the commit message itself. Place such "cover letter" material between the three dash lines and the diffstat.
Do not PGP sign your patch, at least for now. Most likely, your maintainer or other people on the list would not have your PGP key and would not bother obtaining it anyway. Your patch is not judged by who you are; a good patch from an unknown origin has a far better chance of being accepted than a patch from a known, respected origin that is done poorly or does incorrect things.
If you really really really really want to do a PGP signed patch, format it as "multipart/signed", not a text/plain message that starts with '-----BEGIN PGP SIGNED MESSAGE-----'. That is not a text/plain, it's something else.
Unless your patch is a very trivial and an obviously correct one, first send it with "To:" set to the RT email (or mailing list), with "cc:" listing people who are involved in the area you are touching (the output from "git blame $path" and "git shortlog --no-merges $path" would help to identify them), to solicit comments and reviews. After the list reached a consensus that it is a good idea to apply the patch, re-send it with "To:" set to the maintainer and optionally "cc:" the list for inclusion. Do not forget to add trailers such as "Acked-by:", "Reviewed-by:" and "Tested-by:" after your "Signed-off-by:" line as necessary.
To improve tracking of who did what, we've borrowed the "sign-off" procedure from the Linux kernel project on patches that are being emailed around. Although this project is a lot smaller it is a good discipline to follow it.
The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as a open-source patch. The rules are pretty simple: if you can certify the below:
Developer's Certificate of Origin 1.1 By making a contribution to this project, I certify that: (a) The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or (b) The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or (c) The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it. (d) I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it, including my sign-off) is maintained indefinitely and may be redistributed consistent with this project or the open source license(s) involved.
then you just add a line saying
Signed-off-by: Random J Developer <firstname.lastname@example.org>
This line can be automatically added by git if you run the git-commit command with the -s option.
Notice that you can place your own Signed-off-by: line when forwarding somebody else's patch with the above rules for D-C-O. Indeed you are encouraged to do so.
Also notice that a real name is used in the Signed-off-by: line. Please don't hide your real name.
Some people also put extra tags at the end.
"Acked-by:" says that the patch was reviewed by the person who is more familiar with the issues and the area the patch attempts to modify. "Tested-by:" says the patch was tested by the person and found to have the desired effect.
Here is an ideal patch flow for this project the current maintainer suggests to the contributors:
The people who may need to know are the ones whose code you are butchering. These people happen to be the ones who are most likely to be knowledgeable enough to help you, but they have no obligation to help you (i.e. you ask for help, don't demand). "git log -p -- $area_you_are_modifying" would help you find out who they are.
In any time between the (2)-(3) cycle, the maintainer may pick it up from the list and queue it to 'pu', in order to make it easier for people play with it without having to pick up and apply the patch to their trees themselves.