.. _development_process:
  
  How the development process works
  =================================

  
  Linux kernel development in the early 1990's was a pretty loose affair,
  with relatively small numbers of users and developers involved.  With a
  user base in the millions and with some 2,000 developers involved over the
  course of one year, the kernel has since had to evolve a number of
  processes to keep development happening smoothly.  A solid understanding of
  how the process works is required in order to be an effective part of it.

  The big picture
  ---------------

  
  The kernel developers use a loosely time-based release process, with a new
  major kernel release happening every two or three months.  The recent
  release history looks like this:

  	======  =================

  	5.0	March 3, 2019
  	5.1	May 5, 2019
  	5.2	July 7, 2019
  	5.3	September 15, 2019
  	5.4	November 24, 2019
  	5.5	January 6, 2020

  	======  =================

  Every 5.x release is a major kernel release with new features, internal
  API changes, and more.  A typical release can contain about 13,000
  changesets with changes to several hundred thousand lines of code.  5.x is
  the leading edge of Linux kernel development; the kernel uses a

  rolling development model which is continually integrating major changes.
  
  A relatively straightforward discipline is followed with regard to the
  merging of patches for each release.  At the beginning of each development
  cycle, the "merge window" is said to be open.  At that time, code which is
  deemed to be sufficiently stable (and which is accepted by the development
  community) is merged into the mainline kernel.  The bulk of changes for a
  new development cycle (and all of the major changes) will be merged during
  this time, at a rate approaching 1,000 changes ("patches," or "changesets")
  per day.
  
  (As an aside, it is worth noting that the changes integrated during the
  merge window do not come out of thin air; they have been collected, tested,
  and staged ahead of time.  How that process works will be described in
  detail later on).

  The merge window lasts for approximately two weeks.  At the end of this
  time, Linus Torvalds will declare that the window is closed and release the

  first of the "rc" kernels.  For the kernel which is destined to be 5.6,

  for example, the release which happens at the end of the merge window will

  be called 5.6-rc1.  The -rc1 release is the signal that the time to

  merge new features has passed, and that the time to stabilize the next
  kernel has begun.

  
  Over the next six to ten weeks, only patches which fix problems should be
  submitted to the mainline.  On occasion a more significant change will be
  allowed, but such occasions are rare; developers who try to merge new
  features outside of the merge window tend to get an unfriendly reception.
  As a general rule, if you miss the merge window for a given feature, the
  best thing to do is to wait for the next development cycle.  (An occasional
  exception is made for drivers for previously-unsupported hardware; if they
  touch no in-tree code, they cannot cause regressions and should be safe to
  add at any time).
  
  As fixes make their way into the mainline, the patch rate will slow over
  time.  Linus releases new -rc kernels about once a week; a normal series
  will get up to somewhere between -rc6 and -rc9 before the kernel is

  considered to be sufficiently stable and the final release is made.

  At that point the whole process starts over again.

  As an example, here is how the 5.4 development cycle went (all dates in
  2019):

  	==============  ===============================

  	September 15	5.3 stable release
  	September 30	5.4-rc1, merge window closes
  	October 6	5.4-rc2
  	October 13	5.4-rc3
  	October 20	5.4-rc4
  	October 27	5.4-rc5
  	November 3	5.4-rc6
  	November 10	5.4-rc7
  	November 17	5.4-rc8
  	November 24	5.4 stable release

  	==============  ===============================

  
  How do the developers decide when to close the development cycle and create
  the stable release?  The most significant metric used is the list of
  regressions from previous releases.  No bugs are welcome, but those which
  break systems which worked in the past are considered to be especially
  serious.  For this reason, patches which cause regressions are looked upon
  unfavorably and are quite likely to be reverted during the stabilization

  period.

  
  The developers' goal is to fix all known regressions before the stable
  release is made.  In the real world, this kind of perfection is hard to
  achieve; there are just too many variables in a project of this size.
  There comes a point where delaying the final release just makes the problem
  worse; the pile of changes waiting for the next merge window will grow

  larger, creating even more regressions the next time around.  So most 5.x

  kernels go out with a handful of known regressions though, hopefully, none
  of them are serious.
  
  Once a stable release is made, its ongoing maintenance is passed off to the

  "stable team," currently Greg Kroah-Hartman. The stable team will release
  occasional updates to the stable release using the 5.x.y numbering scheme.
  To be considered for an update release, a patch must (1) fix a significant
  bug, and (2) already be merged into the mainline for the next development
  kernel. Kernels will typically receive stable updates for a little more
  than one development cycle past their initial release. So, for example, the
  5.2 kernel's history looked like this (all dates in 2019):

  	==============  ===============================

  	July 7		5.2 stable release

  	July 14		5.2.1
  	July 21		5.2.2
  	July 26		5.2.3
  	July 28		5.2.4
  	July 31  	5.2.5

  	...		...

  	October 11	5.2.21

  	==============  ===============================

  5.2.21 was the final stable update of the 5.2 release.

  
  Some kernels are designated "long term" kernels; they will receive support
  for a longer period.  As of this writing, the current long term kernels
  and their maintainers are:

  	======  ================================	=======================
  	3.16	Ben Hutchings				(very long-term kernel)
  	4.4	Greg Kroah-Hartman & Sasha Levin	(very long-term kernel)
  	4.9	Greg Kroah-Hartman & Sasha Levin
  	4.14	Greg Kroah-Hartman & Sasha Levin
  	4.19	Greg Kroah-Hartman & Sasha Levin
  	5.4	Greg Kroah-Hartman & Sasha Levin
  	======  ================================	=======================

  
  The selection of a kernel for long-term support is purely a matter of a
  maintainer having the need and the time to maintain that release.  There
  are no known plans for long-term support for any specific upcoming
  release.

  The lifecycle of a patch
  ------------------------

  
  Patches do not go directly from the developer's keyboard into the mainline
  kernel.  There is, instead, a somewhat involved (if somewhat informal)
  process designed to ensure that each patch is reviewed for quality and that
  each patch implements a change which is desirable to have in the mainline.
  This process can happen quickly for minor fixes, or, in the case of large
  and controversial changes, go on for years.  Much developer frustration
  comes from a lack of understanding of this process or from attempts to

  circumvent it.

  
  In the hopes of reducing that frustration, this document will describe how
  a patch gets into the kernel.  What follows below is an introduction which
  describes the process in a somewhat idealized way.  A much more detailed
  treatment will come in later sections.
  
  The stages that a patch goes through are, generally:
  
   - Design.  This is where the real requirements for the patch - and the way
     those requirements will be met - are laid out.  Design work is often
     done without involving the community, but it is better to do this work
     in the open if at all possible; it can save a lot of time redesigning
     things later.
  
   - Early review.  Patches are posted to the relevant mailing list, and
     developers on that list reply with any comments they may have.  This
     process should turn up any major problems with a patch if all goes
     well.
  
   - Wider review.  When the patch is getting close to ready for mainline

     inclusion, it should be accepted by a relevant subsystem maintainer -

     though this acceptance is not a guarantee that the patch will make it
     all the way to the mainline.  The patch will show up in the maintainer's

     subsystem tree and into the -next trees (described below).  When the

     process works, this step leads to more extensive review of the patch and
     the discovery of any problems resulting from the integration of this
     patch with work being done by others.

  -  Please note that most maintainers also have day jobs, so merging
     your patch may not be their highest priority.  If your patch is
     getting feedback about changes that are needed, you should either
     make those changes or justify why they should not be made.  If your
     patch has no review complaints but is not being merged by its
     appropriate subsystem or driver maintainer, you should be persistent
     in updating the patch to the current kernel so that it applies cleanly
     and keep sending it for review and merging.

   - Merging into the mainline.  Eventually, a successful patch will be
     merged into the mainline repository managed by Linus Torvalds.  More
     comments and/or problems may surface at this time; it is important that
     the developer be responsive to these and fix any issues which arise.
  
   - Stable release.  The number of users potentially affected by the patch
     is now large, so, once again, new problems may arise.
  
   - Long-term maintenance.  While it is certainly possible for a developer
     to forget about code after merging it, that sort of behavior tends to
     leave a poor impression in the development community.  Merging code
     eliminates some of the maintenance burden, in that others will fix
     problems caused by API changes.  But the original developer should
     continue to take responsibility for the code if it is to remain useful
     in the longer term.
  
  One of the largest mistakes made by kernel developers (or their employers)
  is to try to cut the process down to a single "merging into the mainline"
  step.  This approach invariably leads to frustration for everybody
  involved.

  How patches get into the Kernel
  -------------------------------

  
  There is exactly one person who can merge patches into the mainline kernel

  repository: Linus Torvalds. But, for example, of the over 9,500 patches
  which went into the 2.6.38 kernel, only 112 (around 1.3%) were directly
  chosen by Linus himself. The kernel project has long since grown to a size
  where no single developer could possibly inspect and select every patch
  unassisted. The way the kernel developers have addressed this growth is
  through the use of a lieutenant system built around a chain of trust.

  
  The kernel code base is logically broken down into a set of subsystems:
  networking, specific architecture support, memory management, video
  devices, etc.  Most subsystems have a designated maintainer, a developer
  who has overall responsibility for the code within that subsystem.  These
  subsystem maintainers are the gatekeepers (in a loose way) for the portion
  of the kernel they manage; they are the ones who will (usually) accept a
  patch for inclusion into the mainline kernel.
  
  Subsystem maintainers each manage their own version of the kernel source
  tree, usually (but certainly not always) using the git source management
  tool.  Tools like git (and related tools like quilt or mercurial) allow
  maintainers to track a list of patches, including authorship information
  and other metadata.  At any given time, the maintainer can identify which
  patches in his or her repository are not found in the mainline.
  
  When the merge window opens, top-level maintainers will ask Linus to "pull"
  the patches they have selected for merging from their repositories.  If
  Linus agrees, the stream of patches will flow up into his repository,
  becoming part of the mainline kernel.  The amount of attention that Linus
  pays to specific patches received in a pull operation varies.  It is clear
  that, sometimes, he looks quite closely.  But, as a general rule, Linus
  trusts the subsystem maintainers to not send bad patches upstream.
  
  Subsystem maintainers, in turn, can pull patches from other maintainers.
  For example, the networking tree is built from patches which accumulated
  first in trees dedicated to network device drivers, wireless networking,
  etc.  This chain of repositories can be arbitrarily long, though it rarely
  exceeds two or three links.  Since each maintainer in the chain trusts
  those managing lower-level trees, this process is known as the "chain of

  trust."

  
  Clearly, in a system like this, getting patches into the kernel depends on
  finding the right maintainer.  Sending patches directly to Linus is not
  normally the right way to go.

  Next trees
  ----------

  
  The chain of subsystem trees guides the flow of patches into the kernel,
  but it also raises an interesting question: what if somebody wants to look
  at all of the patches which are being prepared for the next merge window?
  Developers will be interested in what other changes are pending to see
  whether there are any conflicts to worry about; a patch which changes a
  core kernel function prototype, for example, will conflict with any other
  patches which use the older form of that function.  Reviewers and testers
  want access to the changes in their integrated form before all of those
  changes land in the mainline kernel.  One could pull changes from all of
  the interesting subsystem trees, but that would be a big and error-prone
  job.

  The answer comes in the form of -next trees, where subsystem trees are

  collected for testing and review.  The older of these trees, maintained by
  Andrew Morton, is called "-mm" (for memory management, which is how it got
  started).  The -mm tree integrates patches from a long list of subsystem

  trees; it also has some patches aimed at helping with debugging.

  
  Beyond that, -mm contains a significant collection of patches which have
  been selected by Andrew directly.  These patches may have been posted on a
  mailing list, or they may apply to a part of the kernel for which there is
  no designated subsystem tree.  As a result, -mm operates as a sort of
  subsystem tree of last resort; if there is no other obvious path for a
  patch into the mainline, it is likely to end up in -mm.  Miscellaneous
  patches which accumulate in -mm will eventually either be forwarded on to
  an appropriate subsystem tree or be sent directly to Linus.  In a typical

  development cycle, approximately 5-10% of the patches going into the
  mainline get there via -mm.

  The current -mm patch is available in the "mmotm" (-mm of the moment)
  directory at:

  	https://www.ozlabs.org/~akpm/mmotm/

  
  Use of the MMOTM tree is likely to be a frustrating experience, though;
  there is a definite chance that it will not even compile.

  The primary tree for next-cycle patch merging is linux-next, maintained by

  Stephen Rothwell.  The linux-next tree is, by design, a snapshot of what
  the mainline is expected to look like after the next merge window closes.
  Linux-next trees are announced on the linux-kernel and linux-next mailing
  lists when they are assembled; they can be downloaded from:

  	https://www.kernel.org/pub/linux/kernel/next/

  Linux-next has become an integral part of the kernel development process;
  all patches merged during a given merge window should really have found
  their way into linux-next some time before the merge window opens.

  Staging trees
  -------------

  The kernel source tree contains the drivers/staging/ directory, where

  many sub-directories for drivers or filesystems that are on their way to
  being added to the kernel tree live.  They remain in drivers/staging while
  they still need more work; once complete, they can be moved into the
  kernel proper.  This is a way to keep track of drivers that aren't
  up to Linux kernel coding or quality standards, but people may want to use
  them and track development.

  Greg Kroah-Hartman currently maintains the staging tree.  Drivers that
  still need work are sent to him, with each driver having its own
  subdirectory in drivers/staging/.  Along with the driver source files, a
  TODO file should be present in the directory as well.  The TODO file lists
  the pending work that the driver needs for acceptance into the kernel
  proper, as well as a list of people that should be Cc'd for any patches to
  the driver.  Current rules require that drivers contributed to staging
  must, at a minimum, compile properly.
  
  Staging can be a relatively easy way to get new drivers into the mainline
  where, with luck, they will come to the attention of other developers and
  improve quickly.  Entry into staging is not the end of the story, though;
  code in staging which is not seeing regular progress will eventually be
  removed.  Distributors also tend to be relatively reluctant to enable
  staging drivers.  So staging is, at best, a stop on the way toward becoming
  a proper mainline driver.

  Tools
  -----

  
  As can be seen from the above text, the kernel development process depends
  heavily on the ability to herd collections of patches in various
  directions.  The whole thing would not work anywhere near as well as it
  does without suitably powerful tools.  Tutorials on how to use these tools
  are well beyond the scope of this document, but there is space for a few
  pointers.
  
  By far the dominant source code management system used by the kernel
  community is git.  Git is one of a number of distributed version control
  systems being developed in the free software community.  It is well tuned
  for kernel development, in that it performs quite well when dealing with
  large repositories and large numbers of patches.  It also has a reputation
  for being difficult to learn and use, though it has gotten better over
  time.  Some sort of familiarity with git is almost a requirement for kernel
  developers; even if they do not use it for their own work, they'll need git
  to keep up with what other developers (and the mainline) are doing.
  
  Git is now packaged by almost all Linux distributions.  There is a home

  page at:

  	https://git-scm.com/

  That page has pointers to documentation and tutorials.

  
  Among the kernel developers who do not use git, the most popular choice is
  almost certainly Mercurial:

  	https://www.selenic.com/mercurial/

  
  Mercurial shares many features with git, but it provides an interface which
  many find easier to use.
  
  The other tool worth knowing about is Quilt:

  	https://savannah.nongnu.org/projects/quilt/

  
  Quilt is a patch management system, rather than a source code management
  system.  It does not track history over time; it is, instead, oriented
  toward tracking a specific set of changes against an evolving code base.
  Some major subsystem maintainers use quilt to manage patches intended to go
  upstream.  For the management of certain kinds of trees (-mm, for example),
  quilt is the best tool for the job.

  Mailing lists
  -------------

  
  A great deal of Linux kernel development work is done by way of mailing
  lists.  It is hard to be a fully-functioning member of the community
  without joining at least one list somewhere.  But Linux mailing lists also
  represent a potential hazard to developers, who risk getting buried under a
  load of electronic mail, running afoul of the conventions used on the Linux
  lists, or both.
  
  Most kernel mailing lists are run on vger.kernel.org; the master list can
  be found at:
  
  	http://vger.kernel.org/vger-lists.html
  
  There are lists hosted elsewhere, though; a number of them are at

  redhat.com/mailman/listinfo.

  
  The core mailing list for kernel development is, of course, linux-kernel.
  This list is an intimidating place to be; volume can reach 500 messages per
  day, the amount of noise is high, the conversation can be severely
  technical, and participants are not always concerned with showing a high
  degree of politeness.  But there is no other place where the kernel
  development community comes together as a whole; developers who avoid this
  list will miss important information.
  
  There are a few hints which can help with linux-kernel survival:
  
  - Have the list delivered to a separate folder, rather than your main
    mailbox.  One must be able to ignore the stream for sustained periods of
    time.
  
  - Do not try to follow every conversation - nobody else does.  It is
    important to filter on both the topic of interest (though note that
    long-running conversations can drift away from the original subject
    without changing the email subject line) and the people who are

    participating.

  
  - Do not feed the trolls.  If somebody is trying to stir up an angry
    response, ignore them.
  
  - When responding to linux-kernel email (or that on other lists) preserve
    the Cc: header for all involved.  In the absence of a strong reason (such
    as an explicit request), you should never remove recipients.  Always make
    sure that the person you are responding to is in the Cc: list.  This
    convention also makes it unnecessary to explicitly ask to be copied on
    replies to your postings.
  
  - Search the list archives (and the net as a whole) before asking
    questions.  Some developers can get impatient with people who clearly
    have not done their homework.
  
  - Avoid top-posting (the practice of putting your answer above the quoted
    text you are responding to).  It makes your response harder to read and
    makes a poor impression.
  
  - Ask on the correct mailing list.  Linux-kernel may be the general meeting
    point, but it is not the best place to find developers from all
    subsystems.
  
  The last point - finding the correct mailing list - is a common place for
  beginning developers to go wrong.  Somebody who asks a networking-related
  question on linux-kernel will almost certainly receive a polite suggestion
  to ask on the netdev list instead, as that is the list frequented by most
  networking developers.  Other lists exist for the SCSI, video4linux, IDE,
  filesystem, etc. subsystems.  The best place to look for mailing lists is
  in the MAINTAINERS file packaged with the kernel source.

  Getting started with Kernel development
  ---------------------------------------

  
  Questions about how to get started with the kernel development process are
  common - from both individuals and companies.  Equally common are missteps
  which make the beginning of the relationship harder than it has to be.
  
  Companies often look to hire well-known developers to get a development
  group started.  This can, in fact, be an effective technique.  But it also
  tends to be expensive and does not do much to grow the pool of experienced
  kernel developers.  It is possible to bring in-house developers up to speed
  on Linux kernel development, given the investment of a bit of time.  Taking
  this time can endow an employer with a group of developers who understand
  the kernel and the company both, and who can help to train others as well.
  Over the medium term, this is often the more profitable approach.
  
  Individual developers are often, understandably, at a loss for a place to
  start.  Beginning with a large project can be intimidating; one often wants
  to test the waters with something smaller first.  This is the point where
  some developers jump into the creation of patches fixing spelling errors or
  minor coding style issues.  Unfortunately, such patches create a level of
  noise which is distracting for the development community as a whole, so,
  increasingly, they are looked down upon.  New developers wishing to
  introduce themselves to the community will not get the sort of reception
  they wish for by these means.
  
  Andrew Morton gives this advice for aspiring kernel developers

  ::

  	The #1 project for all kernel beginners should surely be "make sure
  	that the kernel runs perfectly at all times on all machines which
  	you can lay your hands on".  Usually the way to do this is to work
  	with others on getting things fixed up (this can require
  	persistence!) but that's fine - it's a part of kernel development.

  (https://lwn.net/Articles/283982/).

  
  In the absence of obvious problems to fix, developers are advised to look
  at the current lists of regressions and open bugs in general.  There is
  never any shortage of issues in need of fixing; by addressing these issues,
  developers will gain experience with the process while, at the same time,
  building respect with the rest of the development community.