Docs: Bring SubmittingPatches more into the git era

Much of the information in SubmittingPatches shows its pre-git history.
Clean that up a bit and rephrase things with the assumption that developers
will be using git.  Also rewrite the "pull requests" section and include
information on using signed tags.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/SubmittingPatches

@@ -24,13 +24,30 @@ SECTION 1 - CREATING AND SENDING YOUR CHANGE
 --------------------------------------------
 
 
+0) Obtain a current source tree
+-------------------------------
+
+If you do not have a repository with the current kernel source handy, use
+git to obtain one.  You'll want to start with the mainline repository,
+which can be grabbed with:
+
+  git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 
+
+Note, however, that you may not want to develop against the mainline tree
+directly.  Most subsystem maintainers run their own trees and want to see
+patches prepared against those trees.  See the "T:" entry for the subsystem
+in the MAINTAINERS file to find that tree, or simply ask the maintainer if
+the tree is not listed there.
+
+It is still possible to download kernel releases via tarballs (as described
+in the next section), but that is the hard way to do kernel development.
 
 1) "diff -up"
 ------------
 
-Use "diff -up" or "diff -uprN" to create patches.  git generates patches
-in this form by default; if you're using git, you can skip this section
-entirely.
+If you must generate your patches by hand, use "diff -up" or "diff -uprN"
+to create patches.  Git generates patches in this form by default; if
+you're using git, you can skip this section entirely.
 
 All changes to the Linux kernel occur in the form of patches, as
 generated by diff(1).  When creating your patch, make sure to create it
@@ -156,10 +173,15 @@ Example:
 	platform_set_drvdata(), but left the variable "dev" unused,
 	delete it.
 
+You should also be sure to use at least the first twelve characters of the
+SHA-1 ID.  The kernel repository holds a *lot* of objects, making
+collisions with shorter IDs a real possibility.  Bear in mind that, even if
+there is no collision with your six-character ID now, that condition may
+change five years from now.
+
 If your patch fixes a bug in a specific commit, e.g. you found an issue using
 git-bisect, please use the 'Fixes:' tag with the first 12 characters of the
-SHA-1 ID, and the one line summary.
-Example:
+SHA-1 ID, and the one line summary.  For example:
 
 	Fixes: e21d2170f366 ("video: remove unnecessary platform_set_drvdata()")
 
@@ -188,6 +210,12 @@ If one patch depends on another patch in order for a change to be
 complete, that is OK.  Simply note "this patch depends on patch X"
 in your patch description.
 
+When dividing your change into a series of patches, take special care to
+ensure that the kernel builds and runs properly after each patch in the
+series.  Developers using "git bisect" to track down a problem can end up
+splitting your patch series at any point; they will not thank you if you
+introduce bugs in the middle.
+
 If you cannot condense your patch set into a smaller set of patches,
 then only post say 15 or so at a time and wait for review and integration.
 
@@ -445,15 +473,15 @@ which appears in the changelog.
 Special note to back-porters: It seems to be a common and useful practice
 to insert an indication of the origin of a patch at the top of the commit
 message (just after the subject line) to facilitate tracking. For instance,
-here's what we see in 2.6-stable :
+here's what we see in a 3.x-stable release:
 
-    Date:   Tue May 13 19:10:30 2008 +0000
+Date:   Tue Oct 7 07:26:38 2014 -0400
 
-        SCSI: libiscsi regression in 2.6.25: fix nop timer handling
+    libata: Un-break ATA blacklist
 
-        commit 4cf1043593db6a337f10e006c23c69e5fc93e722 upstream
+    commit 1c40279960bcd7d52dbdf1d466b20d24b99176c8 upstream.
 
-And here's what appears in 2.4 :
+And here's what might appear in an older kernel once a patch is backported:
 
     Date:   Tue May 13 22:12:27 2008 +0200
 
@@ -462,7 +490,7 @@ And here's what appears in 2.4 :
         [backport of 2.6 commit b7acbdfbd1f277c1eb23f344f899cfa4cd0bf36a]
 
 Whatever the format, this information provides a valuable help to people
-tracking your trees, and to people trying to trouble-shoot bugs in your
+tracking your trees, and to people trying to troubleshoot bugs in your
 tree.
 
 
@@ -558,6 +586,12 @@ method for indicating a bug fixed by the patch. See #2 above for more details.
 
 
 15) The canonical patch format
+------------------------------
+
+This section describes how the patch itself should be formatted.  Note
+that, if you have your patches stored in a git repository, proper patch
+formatting can be had with "git format-patch".  The tools cannot create
+the necessary text, though, so read the instructions below anyway.
 
 The canonical patch subject line is:
 
@@ -672,33 +706,57 @@ See more details on the proper patch format in the following
 references.
 
 
-16) Sending "git pull" requests  (from Linus emails)
+16) Sending "git pull" requests
+-------------------------------
 
-Please write the git repo address and branch name alone on the same line
-so that I can't even by mistake pull from the wrong branch, and so
-that a triple-click just selects the whole thing.
+If you have a series of patches, it may be most convenient to have the
+maintainer pull them directly into the subsystem repository with a
+"git pull" operation.  Note, however, that pulling patches from a developer
+requires a higher degree of trust than taking patches from a mailing list.
+As a result, many subsystem maintainers are reluctant to take pull
+requests, especially from new, unknown developers.
 
-So the proper format is something along the lines of:
+A pull request should have [GIT] or [PULL] in the subject line.  The
+request itself should include the repository name and the branch of
+interest on a single line; it should look something like:
 
-	"Please pull from
+  Please pull from
 
       git://jdelvare.pck.nerim.net/jdelvare-2.6 i2c-for-linus
 
   to get these changes:"
 
-so that I don't have to hunt-and-peck for the address and inevitably
-get it wrong (actually, I've only gotten it wrong a few times, and
-checking against the diffstat tells me when I get it wrong, but I'm
-just a lot more comfortable when I don't have to "look for" the right
-thing to pull, and double-check that I have the right branch-name).
+A pull request should also include an overall message saying what will be
+included in the request, a "git shortlog" listing of the patches
+themselves, and a diffstat showing the overall effect of the patch series.
+The easiest way to get all this information together is, of course, to let
+git do it for you with the "git request-pull" command.
 
+Some maintainers (including Linus) want to see pull requests from signed
+commits; that increases their confidence that the request actually came
+from you.  Linus, in particular, will not pull from public hosting sites
+like GitHub in the absence of a signed tag.
 
-Please use "git diff -M --stat --summary" to generate the diffstat:
-the -M enables rename detection, and the summary enables a summary of
-new/deleted or renamed files.
+The first step toward creating such tags is to make a GNUPG key and get it
+signed by one or more core kernel developers.  This step can be hard for
+new developers, but there is no way around it.  Attending conferences can
+be a good way to find developers who can sign your key.
 
-With rename detection, the statistics are rather different [...]
-because git will notice that a fair number of the changes are renames.
+Once you have prepared a patch series in git that you wish to have somebody
+pull, create a signed tag with "git tag -s".  This will create a new tag
+identifying the last commit in the series and containing a signature
+created with your private key.  You will also have the opportunity to add a
+changelog-style message to the tag; this is an ideal place to describe the
+effects of the pull request as a whole.
+
+If the tree the maintainer will be pulling from is not the repository you
+are working from, don't forget to push the signed tag explicitly to the
+public tree.
+
+When generating your pull request, use the signed tag as the target.  A
+command like this will do the trick:
+
+  git request-pull master git://my.public.tree/linux.git my-signed-tag
 
 
 ----------------------