How to Get Your Change Into the Linux Kernel
  		or
  	Care And Operation Of Your Linus Torvalds
  
  
  
  For a person or company who wishes to submit a change to the Linux
  kernel, the process can sometimes be daunting if you're not familiar
  with "the system."  This text is a collection of suggestions which
  can greatly increase the chances of your change being accepted.

  Read Documentation/SubmitChecklist for a list of items to check
  before submitting code.  If you are submitting a driver, also read
  Documentation/SubmittingDrivers.

  Many of these steps describe the default behavior of the git version
  control system; if you use git to prepare your patches, you'll find much
  of the mechanical work done for you, though you'll still need to prepare
  and document a sensible set of patches.

  
  --------------------------------------------
  SECTION 1 - CREATING AND SENDING YOUR CHANGE
  --------------------------------------------
  
  
  
  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.

  
  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
  in "unified diff" format, as supplied by the '-u' argument to diff(1).
  Also, please use the '-p' argument which shows which C function each
  change is in - that makes the resultant diff a lot easier to read.
  Patches should be based in the root kernel source directory,
  not in any lower subdirectory.
  
  To create a patch for a single file, it is often sufficient to do:

  	SRCTREE= linux-2.6

  	MYFILE=  drivers/net/mydriver.c
  
  	cd $SRCTREE
  	cp $MYFILE $MYFILE.orig
  	vi $MYFILE	# make your change
  	cd ..
  	diff -up $SRCTREE/$MYFILE{.orig,} > /tmp/patch
  
  To create a patch for multiple files, you should unpack a "vanilla",
  or unmodified kernel source tree, and generate a diff against your
  own source tree.  For example:

  	MYSRC= /devel/linux-2.6

  	tar xvfz linux-2.6.12.tar.gz
  	mv linux-2.6.12 linux-2.6.12-vanilla
  	diff -uprN -X linux-2.6.12-vanilla/Documentation/dontdiff \
  		linux-2.6.12-vanilla $MYSRC > /tmp/patch

  
  "dontdiff" is a list of files which are generated by the kernel during
  the build process, and should be ignored in any diff(1)-generated

  patch.  The "dontdiff" file is included in the kernel tree in

  2.6.12 and later.

  
  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-
  generated it with diff(1), to ensure accuracy.

  If your changes produce a lot of deltas, you need to split them into
  individual patches which modify things in logical stages; see section
  #3.  This will facilitate easier reviewing by other kernel developers,
  very important if you want your patch accepted.

  If you're using git, "git rebase -i" can help you with this process.  If
  you're not using git, quilt <http://savannah.nongnu.org/projects/quilt>
  is another popular alternative.

  
  2) Describe your changes.
  
  Describe the technical detail of the change(s) your patch includes.
  
  Be as specific as possible.  The WORST descriptions possible include
  things like "update driver X", "bug fix for driver X", or "this patch
  includes updates for subsystem X.  Please apply."

  The maintainer will thank you if you write your patch description in a
  form which can be easily pulled into Linux's source code management
  system, git, as a "commit log".  See #15, below.

  If your description starts to get long, that's a sign that you probably
  need to split up your patch.  See #3, next.

  When you submit or resubmit a patch or patch series, include the
  complete patch description and justification for it.  Don't just
  say that this is version N of the patch (series).  Don't expect the
  patch merger to refer back to earlier patch versions or referenced
  URLs to find the patch description and put that into the patch.
  I.e., the patch (series) and its description should be self-contained.
  This benefits both the patch merger(s) and reviewers.  Some reviewers
  probably didn't even receive earlier versions of the patch.

  Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
  instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
  to do frotz", as if you are giving orders to the codebase to change
  its behaviour.

  If the patch fixes a logged bug entry, refer to that bug entry by

  number and URL.  If the patch follows from a mailing list discussion,
  give a URL to the mailing list archive; use the https://lkml.kernel.org/
  redirector with a Message-Id, to ensure that the links cannot become
  stale.
  
  However, try to make your explanation understandable without external
  resources.  In addition to giving a URL to a mailing list archive or
  bug, summarize the relevant points of the discussion that led to the
  patch as submitted.

  If you want to refer to a specific commit, don't just refer to the
  SHA-1 ID of the commit. Please also include the oneline summary of
  the commit, to make it easier for reviewers to know what it is about.
  Example:
  
  	Commit e21d2170f36602ae2708 ("video: remove unnecessary
  	platform_set_drvdata()") removed the unnecessary
  	platform_set_drvdata(), but left the variable "dev" unused,
  	delete it.

  
  3) Separate your changes.

  Separate _logical changes_ into a single patch file.

  
  For example, if your changes include both bug fixes and performance
  enhancements for a single driver, separate those changes into two
  or more patches.  If your changes include an API update, and a new
  driver which uses that new API, separate those into two patches.
  
  On the other hand, if you make a single change to numerous files,
  group those changes into a single patch.  Thus a single logical change
  is contained within a single patch.
  
  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.

  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.

  4) Style check your changes.
  
  Check your patch for basic style violations, details of which can be
  found in Documentation/CodingStyle.  Failure to do so simply wastes

  the reviewers time and will get your patch rejected, probably

  without even being read.
  
  At a minimum you should check your patches with the patch style

  checker prior to submission (scripts/checkpatch.pl).  You should

  be able to justify all violations that remain in your patch.
  
  
  
  5) Select e-mail destination.

  
  Look through the MAINTAINERS file and the source code, and determine
  if your change applies to a specific subsystem of the kernel, with

  an assigned maintainer.  If so, e-mail that person.  The script
  scripts/get_maintainer.pl can be very useful at this step.

  
  If no maintainer is listed, or the maintainer does not respond, send
  your patch to the primary Linux kernel developer's mailing list,
  linux-kernel@vger.kernel.org.  Most kernel developers monitor this
  e-mail list, and can comment on your changes.

  
  Do not send more than 15 patches at once to the vger mailing lists!!!

  Linus Torvalds is the final arbiter of all changes accepted into the

  Linux kernel.  His e-mail address is <torvalds@linux-foundation.org>. 
  He gets a lot of e-mail, so typically you should do your best to -avoid-
  sending him e-mail. 

  
  Patches which are bug fixes, are "obvious" changes, or similarly
  require little discussion should be sent or CC'd to Linus.  Patches
  which require discussion or do not have a clear advantage should
  usually be sent first to linux-kernel.  Only after the patch is
  discussed should the patch then be submitted to Linus.

  6) Select your CC (e-mail carbon copy) list.

  
  Unless you have a reason NOT to do so, CC linux-kernel@vger.kernel.org.
  
  Other kernel developers besides Linus need to be aware of your change,
  so that they may comment on it and offer code review and suggestions.
  linux-kernel is the primary Linux kernel developer mailing list.
  Other mailing lists are available for specific subsystems, such as
  USB, framebuffer devices, the VFS, the SCSI subsystem, etc.  See the
  MAINTAINERS file for a mailing list that relates specifically to
  your change.

  Majordomo lists of VGER.KERNEL.ORG at:
  	<http://vger.kernel.org/vger-lists.html>

  If changes affect userland-kernel interfaces, please send
  the MAN-PAGES maintainer (as listed in the MAINTAINERS file)
  a man-pages patch, or at least a notification of the change,
  so that some information makes its way into the manual pages.

  Even if the maintainer did not respond in step #5, make sure to ALWAYS

  copy the maintainer when you change their code.
  
  For small patches you may want to CC the Trivial Patch Monkey

  trivial@kernel.org which collects "trivial" patches. Have a look
  into the MAINTAINERS file for its current manager.
  Trivial patches must qualify for one of the following rules:

   Spelling fixes in documentation

   Spelling fixes which could break grep(1)

   Warning fixes (cluttering with useless warnings is bad)
   Compilation fixes (only if they are actually correct)
   Runtime fixes (only if they actually fix things)

   Removing use of deprecated functions/macros (eg. check_region)

   Contact detail and documentation fixes
   Non-portable code replaced by portable code (even in arch-specific,
   since people copy, as long as it's trivial)

   Any fix by the author/maintainer of the file (ie. patch monkey

   in re-transmission mode)

  7) No MIME, no links, no compression, no attachments.  Just plain text.

  
  Linus and other kernel developers need to be able to read and comment
  on the changes you are submitting.  It is important for a kernel
  developer to be able to "quote" your changes, using standard e-mail
  tools, so that they may comment on specific portions of your code.
  
  For this reason, all patches should be submitting e-mail "inline".
  WARNING:  Be wary of your editor's word-wrap corrupting your patch,
  if you choose to cut-n-paste your patch.
  
  Do not attach the patch as a MIME attachment, compressed or not.
  Many popular e-mail applications will not always transmit a MIME
  attachment as plain text, making it impossible to comment on your
  code.  A MIME attachment also takes Linus a bit more time to process,
  decreasing the likelihood of your MIME-attached change being accepted.
  
  Exception:  If your mailer is mangling patches then someone may ask
  you to re-send them using MIME.

  See Documentation/email-clients.txt for hints about configuring
  your e-mail client so that it sends your patches untouched.

  8) E-mail size.

  When sending patches to Linus, always follow step #7.

  
  Large changes are not appropriate for mailing lists, and some

  maintainers.  If your patch, uncompressed, exceeds 300 kB in size,

  it is preferred that you store your patch on an Internet-accessible
  server, and provide instead a URL (link) pointing to your patch.

  9) Name your kernel version.

  
  It is important to note, either in the subject line or in the patch
  description, the kernel version to which this patch applies.
  
  If the patch does not apply cleanly to the latest kernel version,
  Linus will not apply it.

  10) Don't get discouraged.  Re-submit.

  
  After you have submitted your change, be patient and wait.  If Linus
  likes your change and applies it, it will appear in the next version
  of the kernel that he releases.
  
  However, if your change doesn't appear in the next version of the
  kernel, there could be any number of reasons.  It's YOUR job to
  narrow down those reasons, correct what was wrong, and submit your
  updated change.
  
  It is quite common for Linus to "drop" your patch without comment.
  That's the nature of the system.  If he drops your patch, it could be
  due to

  * Your patch did not apply cleanly to the latest kernel version.

  * Your patch was not sufficiently discussed on linux-kernel.

  * A style issue (see section 2).
  * An e-mail formatting issue (re-read this section).
  * A technical problem with your change.
  * He gets tons of e-mail, and yours got lost in the shuffle.
  * You are being annoying.

  
  When in doubt, solicit comments on linux-kernel mailing list.

  11) Include PATCH in the subject

  
  Due to high e-mail traffic to Linus, and to linux-kernel, it is common
  convention to prefix your subject line with [PATCH].  This lets Linus
  and other kernel developers more easily distinguish patches from other
  e-mail discussions.

  12) Sign your work

  
  To improve tracking of who did what, especially with patches that can
  percolate to their final resting place in the kernel through several
  layers of maintainers, we've introduced a "sign-off" procedure on
  patches that are being emailed around.
  
  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 an 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 <random@developer.example.org>

  using your real name (sorry, no pseudonyms or anonymous contributions.)

  Some people also put extra tags at the end.  They'll just be ignored for
  now, but you can do this to mark internal company procedures or just
  point out some special detail about the sign-off. 

  If you are a subsystem or branch maintainer, sometimes you need to slightly
  modify patches you receive in order to merge them, because the code is not
  exactly the same in your tree and the submitters'. If you stick strictly to
  rule (c), you should ask the submitter to rediff, but this is a totally
  counter-productive waste of time and energy. Rule (b) allows you to adjust
  the code, but then it is very impolite to change one submitter's code and
  make him endorse your bugs. To solve this problem, it is recommended that
  you add a line between the last Signed-off-by header and yours, indicating
  the nature of your changes. While there is nothing mandatory about this, it
  seems like prepending the description with your mail and/or name, all
  enclosed in square brackets, is noticeable enough to make it obvious that
  you are responsible for last-minute changes. Example :
  
  	Signed-off-by: Random J Developer <random@developer.example.org>
  	[lucky@maintainer.example.org: struct foo moved from foo.c to foo.h]
  	Signed-off-by: Lucky K Maintainer <lucky@maintainer.example.org>
  
  This practise is particularly helpful if you maintain a stable branch and
  want at the same time to credit the author, track changes, merge the fix,
  and protect the submitter from complaints. Note that under no circumstances
  can you change the author's identity (the From header), as it is the one
  which appears in the changelog.
  
  Special note to back-porters: It seems to be a common and useful practise
  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 :
  
      Date:   Tue May 13 19:10:30 2008 +0000
  
          SCSI: libiscsi regression in 2.6.25: fix nop timer handling
  
          commit 4cf1043593db6a337f10e006c23c69e5fc93e722 upstream
  
  And here's what appears in 2.4 :
  
      Date:   Tue May 13 22:12:27 2008 +0200
  
          wireless, airo: waitbusy() won't delay
  
          [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
  tree.

  13) When to use Acked-by: and Cc:

  The Signed-off-by: tag indicates that the signer was involved in the
  development of the patch, or that he/she was in the patch's delivery path.
  
  If a person was not directly involved in the preparation or handling of a
  patch but wishes to signify and record their approval of it then they can
  arrange to have an Acked-by: line added to the patch's changelog.
  
  Acked-by: is often used by the maintainer of the affected code when that
  maintainer neither contributed to nor forwarded the patch.
  
  Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
  has at least reviewed the patch and has indicated acceptance.  Hence patch
  mergers will sometimes manually convert an acker's "yep, looks good to me"
  into an Acked-by:.
  
  Acked-by: does not necessarily indicate acknowledgement of the entire patch.
  For example, if a patch affects multiple subsystems and has an Acked-by: from
  one subsystem maintainer then this usually indicates acknowledgement of just
  the part which affects that maintainer's code.  Judgement should be used here.

  When in doubt people should refer to the original discussion in the mailing

  list archives.

  If a person has had the opportunity to comment on a patch, but has not
  provided such comments, you may optionally add a "Cc:" tag to the patch.
  This is the only tag which might be added without an explicit action by the
  person it names.  This tag documents that potentially interested parties
  have been included in the discussion

  14) Using Reported-by:, Tested-by:, Reviewed-by: and Suggested-by:

  
  If this patch fixes a problem reported by somebody else, consider adding a
  Reported-by: tag to credit the reporter for their contribution.  Please
  note that this tag should not be added without the reporter's permission,
  especially if the problem was not reported in a public forum.  That said,
  if we diligently credit our bug reporters, they will, hopefully, be
  inspired to help us again in the future.

  
  A Tested-by: tag indicates that the patch has been successfully tested (in
  some environment) by the person named.  This tag informs maintainers that
  some testing has been performed, provides a means to locate testers for
  future patches, and ensures credit for the testers.
  
  Reviewed-by:, instead, indicates that the patch has been reviewed and found
  acceptable according to the Reviewer's Statement:
  
  	Reviewer's statement of oversight
  
  	By offering my Reviewed-by: tag, I state that:
  
   	 (a) I have carried out a technical review of this patch to
  	     evaluate its appropriateness and readiness for inclusion into
  	     the mainline kernel.
  
  	 (b) Any problems, concerns, or questions relating to the patch
  	     have been communicated back to the submitter.  I am satisfied
  	     with the submitter's response to my comments.
  
  	 (c) While there may be things that could be improved with this
  	     submission, I believe that it is, at this time, (1) a
  	     worthwhile modification to the kernel, and (2) free of known
  	     issues which would argue against its inclusion.
  
  	 (d) While I have reviewed the patch and believe it to be sound, I
  	     do not (unless explicitly stated elsewhere) make any
  	     warranties or guarantees that it will achieve its stated
  	     purpose or function properly in any given situation.
  
  A Reviewed-by tag is a statement of opinion that the patch is an
  appropriate modification of the kernel without any remaining serious
  technical issues.  Any interested reviewer (who has done the work) can
  offer a Reviewed-by tag for a patch.  This tag serves to give credit to
  reviewers and to inform maintainers of the degree of review which has been
  done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
  understand the subject area and to perform thorough reviews, will normally

  increase the likelihood of your patch getting into the kernel.

  A Suggested-by: tag indicates that the patch idea is suggested by the person
  named and ensures credit to the person for the idea. Please note that this
  tag should not be added without the reporter's permission, especially if the
  idea was not posted in a public forum. That said, if we diligently credit our
  idea reporters, they will, hopefully, be inspired to help us again in the
  future.

  
  15) The canonical patch format

  The canonical patch subject line is:

      Subject: [PATCH 001/123] subsystem: summary phrase

  
  The canonical patch message body contains the following:
  
    - A "from" line specifying the patch author.
  
    - An empty line.
  
    - The body of the explanation, which will be copied to the
      permanent changelog to describe this patch.
  
    - The "Signed-off-by:" lines, described above, which will
      also go in the changelog.
  
    - A marker line containing simply "---".
  
    - Any additional comments not suitable for the changelog.
  
    - The actual patch (diff output).
  
  The Subject line format makes it very easy to sort the emails
  alphabetically by subject line - pretty much any email reader will
  support that - since because the sequence number is zero-padded,
  the numerical and alphabetic sort is the same.

  The "subsystem" in the email's Subject should identify which
  area or subsystem of the kernel is being patched.
  
  The "summary phrase" in the email's Subject should concisely
  describe the patch which that email contains.  The "summary
  phrase" should not be a filename.  Do not use the same "summary

  phrase" for every patch in a whole patch series (where a "patch
  series" is an ordered sequence of multiple, related patches).

  Bear in mind that the "summary phrase" of your email becomes a
  globally-unique identifier for that patch.  It propagates all the way
  into the git changelog.  The "summary phrase" may later be used in
  developer discussions which refer to the patch.  People will want to
  google for the "summary phrase" to read discussion regarding that
  patch.  It will also be the only thing that people may quickly see
  when, two or three months later, they are going through perhaps
  thousands of patches using tools such as "gitk" or "git log
  --oneline".
  
  For these reasons, the "summary" must be no more than 70-75
  characters, and it must describe both what the patch changes, as well
  as why the patch might be necessary.  It is challenging to be both
  succinct and descriptive, but that is what a well-written summary
  should do.
  
  The "summary phrase" may be prefixed by tags enclosed in square
  brackets: "Subject: [PATCH tag] <summary phrase>".  The tags are not
  considered part of the summary phrase, but describe how the patch
  should be treated.  Common tags might include a version descriptor if
  the multiple versions of the patch have been sent out in response to
  comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
  comments.  If there are four patches in a patch series the individual
  patches may be numbered like this: 1/4, 2/4, 3/4, 4/4.  This assures
  that developers understand the order in which the patches should be
  applied and that they have reviewed or applied all of the patches in
  the patch series.

  
  A couple of example Subjects:
  
      Subject: [patch 2/5] ext2: improve scalability of bitmap searching
      Subject: [PATCHv2 001/207] x86: fix eflags tracking

  
  The "from" line must be the very first line in the message body,
  and has the form:
  
          From: Original Author <author@example.com>
  
  The "from" line specifies who will be credited as the author of the
  patch in the permanent changelog.  If the "from" line is missing,
  then the "From:" line from the email header will be used to determine
  the patch author in the changelog.
  
  The explanation body will be committed to the permanent source
  changelog, so should make sense to a competent reader who has long
  since forgotten the immediate details of the discussion that might

  have led to this patch.  Including symptoms of the failure which the
  patch addresses (kernel log messages, oops messages, etc.) is
  especially useful for people who might be searching the commit logs
  looking for the applicable patch.  If a patch fixes a compile failure,
  it may not be necessary to include _all_ of the compile failures; just
  enough that it is likely that someone searching for the patch can find
  it.  As in the "summary phrase", it is important to be both succinct as
  well as descriptive.

  
  The "---" marker line serves the essential purpose of marking for patch
  handling tools where the changelog message ends.
  
  One good use for the additional comments after the "---" marker is for

  a diffstat, to show what files have changed, and the number of
  inserted and deleted lines per file.  A diffstat is especially useful
  on bigger patches.  Other comments relevant only to the moment or the
  maintainer, not suitable for the permanent changelog, should also go
  here.  A good example of such comments might be "patch changelogs"
  which describe what has changed between the v1 and v2 version of the
  patch.
  
  If you are going to include a diffstat after the "---" marker, please
  use diffstat options "-p 1 -w 70" so that filenames are listed from
  the top of the kernel source tree and don't use too much horizontal

  space (easily fit in 80 columns, maybe with some indentation).  (git
  generates appropriate diffstats by default.)

  
  See more details on the proper patch format in the following
  references.

  16) Sending "git pull" requests  (from Linus emails)

  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.
  
  So the proper format is something along the lines of:
  
  	"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).
  
  
  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.
  
  With rename detection, the statistics are rather different [...]
  because git will notice that a fair number of the changes are renames.

  -----------------------------------
  SECTION 2 - HINTS, TIPS, AND TRICKS
  -----------------------------------
  
  This section lists many of the common "rules" associated with code
  submitted to the kernel.  There are always exceptions... but you must
  have a really good reason for doing so.  You could probably call this
  section Linus Computer Science 101.
  
  
  
  1) Read Documentation/CodingStyle
  
  Nuff said.  If your code deviates too much from this, it is likely
  to be rejected without further review, and without comment.

  One significant exception is when moving code from one file to
  another -- in this case you should not modify the moved code at all in

  the same patch which moves it.  This clearly delineates the act of
  moving the code and your changes.  This greatly aids review of the
  actual differences and allows tools to better track the history of
  the code itself.

  Check your patches with the patch style checker prior to submission

  (scripts/checkpatch.pl).  The style checker should be viewed as
  a guide not as the final word.  If your code looks better with
  a violation then its probably best left alone.
  
  The checker reports at three levels:
   - ERROR: things that are very likely to be wrong
   - WARNING: things requiring careful review
   - CHECK: things requiring thought
  
  You should be able to justify all violations that remain in your
  patch.

  
  
  2) #ifdefs are ugly
  
  Code cluttered with ifdefs is difficult to read and maintain.  Don't do
  it.  Instead, put your ifdefs in a header, and conditionally define
  'static inline' functions, or macros, which are used in the code.
  Let the compiler optimize away the "no-op" case.
  
  Simple example, of poor code:
  
  	dev = alloc_etherdev (sizeof(struct funky_private));
  	if (!dev)
  		return -ENODEV;
  	#ifdef CONFIG_NET_FUNKINESS
  	init_funky_net(dev);
  	#endif
  
  Cleaned-up example:
  
  (in header)
  	#ifndef CONFIG_NET_FUNKINESS
  	static inline void init_funky_net (struct net_device *d) {}
  	#endif
  
  (in the code itself)
  	dev = alloc_etherdev (sizeof(struct funky_private));
  	if (!dev)
  		return -ENODEV;
  	init_funky_net(dev);
  
  
  
  3) 'static inline' is better than a macro
  
  Static inline functions are greatly preferred over macros.
  They provide type safety, have no length limitations, no formatting
  limitations, and under gcc they are as cheap as macros.
  
  Macros should only be used for cases where a static inline is clearly

  suboptimal [there are a few, isolated cases of this in fast paths],

  or where it is impossible to use a static inline function [such as
  string-izing].
  
  'static inline' is preferred over 'static __inline__', 'extern inline',
  and 'extern __inline__'.
  
  
  
  4) Don't over-design.
  
  Don't try to anticipate nebulous future cases which may or may not

  be useful:  "Make it as simple as you can, and no simpler."

  
  
  ----------------------
  SECTION 3 - REFERENCES
  ----------------------
  
  Andrew Morton, "The perfect patch" (tpp).

    <http://www.ozlabs.org/~akpm/stuff/tpp.txt>

  Jeff Garzik, "Linux kernel patch submission format".

    <http://linux.yyz.us/patch-format.html>

  Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".

    <http://www.kroah.com/log/linux/maintainer.html>
    <http://www.kroah.com/log/linux/maintainer-02.html>
    <http://www.kroah.com/log/linux/maintainer-03.html>
    <http://www.kroah.com/log/linux/maintainer-04.html>
    <http://www.kroah.com/log/linux/maintainer-05.html>

  NO!!!! No more huge patch bombs to linux-kernel@vger.kernel.org people!

    <https://lkml.org/lkml/2005/7/11/336>

  Kernel Documentation/CodingStyle:

    <http://users.sosdg.org/~qiyong/lxr/source/Documentation/CodingStyle>

  Linus Torvalds's mail on the canonical patch format:

    <http://lkml.org/lkml/2005/4/7/183>

  
  Andi Kleen, "On submitting kernel patches"

    Some strategies to get difficult or controversial changes in.

    http://halobates.de/on-submitting-patches.pdf

  --