Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. _development_advancedtopics:
+
+Advanced topics
+===============
+
+At this point, hopefully, you have a handle on how the development process
+works.  There is still more to learn, however!  This section will cover a
+number of topics which can be helpful for developers wanting to become a
+regular part of the Linux kernel development process.
+
+Managing patches with git
+-------------------------
+
+The use of distributed version control for the kernel began in early 2002,
+when Linus first started playing with the proprietary BitKeeper
+application.  While BitKeeper was controversial, the approach to software
+version management it embodied most certainly was not.  Distributed version
+control enabled an immediate acceleration of the kernel development
+project.  In current times, there are several free alternatives to
+BitKeeper.  For better or for worse, the kernel project has settled on git
+as its tool of choice.
+
+Managing patches with git can make life much easier for the developer,
+especially as the volume of those patches grows.  Git also has its rough
+edges and poses certain hazards; it is a young and powerful tool which is
+still being civilized by its developers.  This document will not attempt to
+teach the reader how to use git; that would be sufficient material for a
+long document in its own right.  Instead, the focus here will be on how git
+fits into the kernel development process in particular.  Developers who
+wish to come up to speed with git will find more information at:
+
+	http://git-scm.com/
+
+	http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
+
+and on various tutorials found on the web.
+
+The first order of business is to read the above sites and get a solid
+understanding of how git works before trying to use it to make patches
+available to others.  A git-using developer should be able to obtain a copy
+of the mainline repository, explore the revision history, commit changes to
+the tree, use branches, etc.  An understanding of git's tools for the
+rewriting of history (such as rebase) is also useful.  Git comes with its
+own terminology and concepts; a new user of git should know about refs,
+remote branches, the index, fast-forward merges, pushes and pulls, detached
+heads, etc.  It can all be a little intimidating at the outset, but the
+concepts are not that hard to grasp with a bit of study.
+
+Using git to generate patches for submission by email can be a good
+exercise while coming up to speed.
+
+When you are ready to start putting up git trees for others to look at, you
+will, of course, need a server that can be pulled from.  Setting up such a
+server with git-daemon is relatively straightforward if you have a system
+which is accessible to the Internet.  Otherwise, free, public hosting sites
+(Github, for example) are starting to appear on the net.  Established
+developers can get an account on kernel.org, but those are not easy to come
+by; see http://kernel.org/faq/ for more information.
+
+The normal git workflow involves the use of a lot of branches.  Each line
+of development can be separated into a separate "topic branch" and
+maintained independently.  Branches in git are cheap, there is no reason to
+not make free use of them.  And, in any case, you should not do your
+development in any branch which you intend to ask others to pull from.
+Publicly-available branches should be created with care; merge in patches
+from development branches when they are in complete form and ready to go -
+not before.
+
+Git provides some powerful tools which can allow you to rewrite your
+development history.  An inconvenient patch (one which breaks bisection,
+say, or which has some other sort of obvious bug) can be fixed in place or
+made to disappear from the history entirely.  A patch series can be
+rewritten as if it had been written on top of today's mainline, even though
+you have been working on it for months.  Changes can be transparently
+shifted from one branch to another.  And so on.  Judicious use of git's
+ability to revise history can help in the creation of clean patch sets with
+fewer problems.
+
+Excessive use of this capability can lead to other problems, though, beyond
+a simple obsession for the creation of the perfect project history.
+Rewriting history will rewrite the changes contained in that history,
+turning a tested (hopefully) kernel tree into an untested one.  But, beyond
+that, developers cannot easily collaborate if they do not have a shared
+view of the project history; if you rewrite history which other developers
+have pulled into their repositories, you will make life much more difficult
+for those developers.  So a simple rule of thumb applies here: history
+which has been exported to others should generally be seen as immutable
+thereafter.
+
+So, once you push a set of changes to your publicly-available server, those
+changes should not be rewritten.  Git will attempt to enforce this rule if
+you try to push changes which do not result in a fast-forward merge
+(i.e. changes which do not share the same history).  It is possible to
+override this check, and there may be times when it is necessary to rewrite
+an exported tree.  Moving changesets between trees to avoid conflicts in
+linux-next is one example.  But such actions should be rare.  This is one
+of the reasons why development should be done in private branches (which
+can be rewritten if necessary) and only moved into public branches when
+it's in a reasonably advanced state.
+
+As the mainline (or other tree upon which a set of changes is based)
+advances, it is tempting to merge with that tree to stay on the leading
+edge.  For a private branch, rebasing can be an easy way to keep up with
+another tree, but rebasing is not an option once a tree is exported to the
+world.  Once that happens, a full merge must be done.  Merging occasionally
+makes good sense, but overly frequent merges can clutter the history
+needlessly.  Suggested technique in this case is to merge infrequently, and
+generally only at specific release points (such as a mainline -rc
+release).  If you are nervous about specific changes, you can always
+perform test merges in a private branch.  The git "rerere" tool can be
+useful in such situations; it remembers how merge conflicts were resolved
+so that you don't have to do the same work twice.
+
+One of the biggest recurring complaints about tools like git is this: the
+mass movement of patches from one repository to another makes it easy to
+slip in ill-advised changes which go into the mainline below the review
+radar.  Kernel developers tend to get unhappy when they see that kind of
+thing happening; putting up a git tree with unreviewed or off-topic patches
+can affect your ability to get trees pulled in the future.  Quoting Linus:
+
+::
+
+	You can send me patches, but for me to pull a git patch from you, I
+	need to know that you know what you're doing, and I need to be able
+	to trust things *without* then having to go and check every
+	individual change by hand.
+
+(http://lwn.net/Articles/224135/).
+
+To avoid this kind of situation, ensure that all patches within a given
+branch stick closely to the associated topic; a "driver fixes" branch
+should not be making changes to the core memory management code.  And, most
+importantly, do not use a git tree to bypass the review process.  Post an
+occasional summary of the tree to the relevant list, and, when the time is
+right, request that the tree be included in linux-next.
+
+If and when others start to send patches for inclusion into your tree,
+don't forget to review them.  Also ensure that you maintain the correct
+authorship information; the git "am" tool does its best in this regard, but
+you may have to add a "From:" line to the patch if it has been relayed to
+you via a third party.
+
+When requesting a pull, be sure to give all the relevant information: where
+your tree is, what branch to pull, and what changes will result from the
+pull.  The git request-pull command can be helpful in this regard; it will
+format the request as other developers expect, and will also check to be
+sure that you have remembered to push those changes to the public server.
+
+
+Reviewing patches
+-----------------
+
+Some readers will certainly object to putting this section with "advanced
+topics" on the grounds that even beginning kernel developers should be
+reviewing patches.  It is certainly true that there is no better way to
+learn how to program in the kernel environment than by looking at code
+posted by others.  In addition, reviewers are forever in short supply; by
+looking at code you can make a significant contribution to the process as a
+whole.
+
+Reviewing code can be an intimidating prospect, especially for a new kernel
+developer who may well feel nervous about questioning code - in public -
+which has been posted by those with more experience.  Even code written by
+the most experienced developers can be improved, though.  Perhaps the best
+piece of advice for reviewers (all reviewers) is this: phrase review
+comments as questions rather than criticisms.  Asking "how does the lock
+get released in this path?" will always work better than stating "the
+locking here is wrong."
+
+Different developers will review code from different points of view.  Some
+are mostly concerned with coding style and whether code lines have trailing
+white space.  Others will focus primarily on whether the change implemented
+by the patch as a whole is a good thing for the kernel or not.  Yet others
+will check for problematic locking, excessive stack usage, possible
+security issues, duplication of code found elsewhere, adequate
+documentation, adverse effects on performance, user-space ABI changes, etc.
+All types of review, if they lead to better code going into the kernel, are
+welcome and worthwhile.