linux/Documentation/development-process/5.Posting
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   15: POSTING PATCHES
   2
   3Sooner or later, the time comes when your work is ready to be presented to
   4the community for review and, eventually, inclusion into the mainline
   5kernel.  Unsurprisingly, the kernel development community has evolved a set
   6of conventions and procedures which are used in the posting of patches;
   7following them will make life much easier for everybody involved.  This
   8document will attempt to cover these expectations in reasonable detail;
   9more information can also be found in the files SubmittingPatches,
  10SubmittingDrivers, and SubmitChecklist in the kernel documentation
  11directory.
  12
  13
  145.1: WHEN TO POST
  15
  16There is a constant temptation to avoid posting patches before they are
  17completely "ready."  For simple patches, that is not a problem.  If the
  18work being done is complex, though, there is a lot to be gained by getting
  19feedback from the community before the work is complete.  So you should
  20consider posting in-progress work, or even making a git tree available so
  21that interested developers can catch up with your work at any time.
  22
  23When posting code which is not yet considered ready for inclusion, it is a
  24good idea to say so in the posting itself.  Also mention any major work
  25which remains to be done and any known problems.  Fewer people will look at
  26patches which are known to be half-baked, but those who do will come in
  27with the idea that they can help you drive the work in the right direction.
  28
  29
  305.2: BEFORE CREATING PATCHES
  31
  32There are a number of things which should be done before you consider
  33sending patches to the development community.  These include:
  34
  35 - Test the code to the extent that you can.  Make use of the kernel's
  36   debugging tools, ensure that the kernel will build with all reasonable
  37   combinations of configuration options, use cross-compilers to build for
  38   different architectures, etc.
  39
  40 - Make sure your code is compliant with the kernel coding style
  41   guidelines.
  42
  43 - Does your change have performance implications?  If so, you should run
  44   benchmarks showing what the impact (or benefit) of your change is; a
  45   summary of the results should be included with the patch.
  46
  47 - Be sure that you have the right to post the code.  If this work was done
  48   for an employer, the employer likely has a right to the work and must be
  49   agreeable with its release under the GPL.
  50
  51As a general rule, putting in some extra thought before posting code almost
  52always pays back the effort in short order.
  53
  54
  555.3: PATCH PREPARATION
  56
  57The preparation of patches for posting can be a surprising amount of work,
  58but, once again, attempting to save time here is not generally advisable
  59even in the short term.
  60
  61Patches must be prepared against a specific version of the kernel.  As a
  62general rule, a patch should be based on the current mainline as found in
  63Linus's git tree.  When basing on mainline, start with a well-known release
  64point - a stable or -rc release - rather than branching off the mainline at
  65an arbitrary spot.
  66
  67It may become necessary to make versions against -mm, linux-next, or a
  68subsystem tree, though, to facilitate wider testing and review.  Depending
  69on the area of your patch and what is going on elsewhere, basing a patch
  70against these other trees can require a significant amount of work
  71resolving conflicts and dealing with API changes.
  72
  73Only the most simple changes should be formatted as a single patch;
  74everything else should be made as a logical series of changes.  Splitting
  75up patches is a bit of an art; some developers spend a long time figuring
  76out how to do it in the way that the community expects.  There are a few
  77rules of thumb, however, which can help considerably:
  78
  79 - The patch series you post will almost certainly not be the series of
  80   changes found in your working revision control system.  Instead, the
  81   changes you have made need to be considered in their final form, then
  82   split apart in ways which make sense.  The developers are interested in
  83   discrete, self-contained changes, not the path you took to get to those
  84   changes.
  85
  86 - Each logically independent change should be formatted as a separate
  87   patch.  These changes can be small ("add a field to this structure") or
  88   large (adding a significant new driver, for example), but they should be
  89   conceptually small and amenable to a one-line description.  Each patch
  90   should make a specific change which can be reviewed on its own and
  91   verified to do what it says it does.
  92
  93 - As a way of restating the guideline above: do not mix different types of
  94   changes in the same patch.  If a single patch fixes a critical security
  95   bug, rearranges a few structures, and reformats the code, there is a
  96   good chance that it will be passed over and the important fix will be
  97   lost.
  98
  99 - Each patch should yield a kernel which builds and runs properly; if your
 100   patch series is interrupted in the middle, the result should still be a
 101   working kernel.  Partial application of a patch series is a common
 102   scenario when the "git bisect" tool is used to find regressions; if the
 103   result is a broken kernel, you will make life harder for developers and
 104   users who are engaging in the noble work of tracking down problems.
 105
 106 - Do not overdo it, though.  One developer once posted a set of edits
 107   to a single file as 500 separate patches - an act which did not make him
 108   the most popular person on the kernel mailing list.  A single patch can
 109   be reasonably large as long as it still contains a single *logical*
 110   change.
 111
 112 - It can be tempting to add a whole new infrastructure with a series of
 113   patches, but to leave that infrastructure unused until the final patch
 114   in the series enables the whole thing.  This temptation should be
 115   avoided if possible; if that series adds regressions, bisection will
 116   finger the last patch as the one which caused the problem, even though
 117   the real bug is elsewhere.  Whenever possible, a patch which adds new
 118   code should make that code active immediately.
 119
 120Working to create the perfect patch series can be a frustrating process
 121which takes quite a bit of time and thought after the "real work" has been
 122done.  When done properly, though, it is time well spent.
 123
 124
 1255.4: PATCH FORMATTING AND CHANGELOGS
 126
 127So now you have a perfect series of patches for posting, but the work is
 128not done quite yet.  Each patch needs to be formatted into a message which
 129quickly and clearly communicates its purpose to the rest of the world.  To
 130that end, each patch will be composed of the following:
 131
 132 - An optional "From" line naming the author of the patch.  This line is
 133   only necessary if you are passing on somebody else's patch via email,
 134   but it never hurts to add it when in doubt.
 135
 136 - A one-line description of what the patch does.  This message should be
 137   enough for a reader who sees it with no other context to figure out the
 138   scope of the patch; it is the line that will show up in the "short form"
 139   changelogs.  This message is usually formatted with the relevant
 140   subsystem name first, followed by the purpose of the patch.  For
 141   example:
 142
 143        gpio: fix build on CONFIG_GPIO_SYSFS=n
 144
 145 - A blank line followed by a detailed description of the contents of the
 146   patch.  This description can be as long as is required; it should say
 147   what the patch does and why it should be applied to the kernel.
 148
 149 - One or more tag lines, with, at a minimum, one Signed-off-by: line from
 150   the author of the patch.  Tags will be described in more detail below.
 151
 152The items above, together, form the changelog for the patch.  Writing good
 153changelogs is a crucial but often-neglected art; it's worth spending
 154another moment discussing this issue.  When writing a changelog, you should
 155bear in mind that a number of different people will be reading your words.
 156These include subsystem maintainers and reviewers who need to decide
 157whether the patch should be included, distributors and other maintainers
 158trying to decide whether a patch should be backported to other kernels, bug
 159hunters wondering whether the patch is responsible for a problem they are
 160chasing, users who want to know how the kernel has changed, and more.  A
 161good changelog conveys the needed information to all of these people in the
 162most direct and concise way possible.
 163
 164To that end, the summary line should describe the effects of and motivation
 165for the change as well as possible given the one-line constraint.  The
 166detailed description can then amplify on those topics and provide any
 167needed additional information.  If the patch fixes a bug, cite the commit
 168which introduced the bug if possible (and please provide both the commit ID
 169and the title when citing commits).  If a problem is associated with
 170specific log or compiler output, include that output to help others
 171searching for a solution to the same problem.  If the change is meant to
 172support other changes coming in later patch, say so.  If internal APIs are
 173changed, detail those changes and how other developers should respond.  In
 174general, the more you can put yourself into the shoes of everybody who will
 175be reading your changelog, the better that changelog (and the kernel as a
 176whole) will be.
 177
 178Needless to say, the changelog should be the text used when committing the
 179change to a revision control system.  It will be followed by:
 180
 181 - The patch itself, in the unified ("-u") patch format.  Using the "-p"
 182   option to diff will associate function names with changes, making the
 183   resulting patch easier for others to read.
 184
 185You should avoid including changes to irrelevant files (those generated by
 186the build process, for example, or editor backup files) in the patch.  The
 187file "dontdiff" in the Documentation directory can help in this regard;
 188pass it to diff with the "-X" option.
 189
 190The tags mentioned above are used to describe how various developers have
 191been associated with the development of this patch.  They are described in
 192detail in the SubmittingPatches document; what follows here is a brief
 193summary.  Each of these lines has the format:
 194
 195        tag: Full Name <email address>  optional-other-stuff
 196
 197The tags in common use are:
 198
 199 - Signed-off-by: this is a developer's certification that he or she has
 200   the right to submit the patch for inclusion into the kernel.  It is an
 201   agreement to the Developer's Certificate of Origin, the full text of
 202   which can be found in Documentation/SubmittingPatches.  Code without a
 203   proper signoff cannot be merged into the mainline.
 204
 205 - Acked-by: indicates an agreement by another developer (often a
 206   maintainer of the relevant code) that the patch is appropriate for
 207   inclusion into the kernel.
 208
 209 - Tested-by: states that the named person has tested the patch and found
 210   it to work.
 211
 212 - Reviewed-by: the named developer has reviewed the patch for correctness;
 213   see the reviewer's statement in Documentation/SubmittingPatches for more
 214   detail.
 215
 216 - Reported-by: names a user who reported a problem which is fixed by this
 217   patch; this tag is used to give credit to the (often underappreciated)
 218   people who test our code and let us know when things do not work
 219   correctly.
 220
 221 - Cc: the named person received a copy of the patch and had the
 222   opportunity to comment on it.
 223
 224Be careful in the addition of tags to your patches: only Cc: is appropriate
 225for addition without the explicit permission of the person named.
 226
 227
 2285.5: SENDING THE PATCH
 229
 230Before you mail your patches, there are a couple of other things you should
 231take care of:
 232
 233 - Are you sure that your mailer will not corrupt the patches?  Patches
 234   which have had gratuitous white-space changes or line wrapping performed
 235   by the mail client will not apply at the other end, and often will not
 236   be examined in any detail.  If there is any doubt at all, mail the patch
 237   to yourself and convince yourself that it shows up intact.
 238
 239   Documentation/email-clients.txt has some helpful hints on making
 240   specific mail clients work for sending patches.
 241
 242 - Are you sure your patch is free of silly mistakes?  You should always
 243   run patches through scripts/checkpatch.pl and address the complaints it
 244   comes up with.  Please bear in mind that checkpatch.pl, while being the
 245   embodiment of a fair amount of thought about what kernel patches should
 246   look like, is not smarter than you.  If fixing a checkpatch.pl complaint
 247   would make the code worse, don't do it.
 248
 249Patches should always be sent as plain text.  Please do not send them as
 250attachments; that makes it much harder for reviewers to quote sections of
 251the patch in their replies.  Instead, just put the patch directly into your
 252message.
 253
 254When mailing patches, it is important to send copies to anybody who might
 255be interested in it.  Unlike some other projects, the kernel encourages
 256people to err on the side of sending too many copies; don't assume that the
 257relevant people will see your posting on the mailing lists.  In particular,
 258copies should go to:
 259
 260 - The maintainer(s) of the affected subsystem(s).  As described earlier,
 261   the MAINTAINERS file is the first place to look for these people.
 262
 263 - Other developers who have been working in the same area - especially
 264   those who might be working there now.  Using git to see who else has
 265   modified the files you are working on can be helpful.
 266
 267 - If you are responding to a bug report or a feature request, copy the
 268   original poster as well.
 269
 270 - Send a copy to the relevant mailing list, or, if nothing else applies,
 271   the linux-kernel list.
 272
 273 - If you are fixing a bug, think about whether the fix should go into the
 274   next stable update.  If so, stable@vger.kernel.org should get a copy of
 275   the patch.  Also add a "Cc: stable@vger.kernel.org" to the tags within
 276   the patch itself; that will cause the stable team to get a notification
 277   when your fix goes into the mainline.
 278
 279When selecting recipients for a patch, it is good to have an idea of who
 280you think will eventually accept the patch and get it merged.  While it
 281is possible to send patches directly to Linus Torvalds and have him merge
 282them, things are not normally done that way.  Linus is busy, and there are
 283subsystem maintainers who watch over specific parts of the kernel.  Usually
 284you will be wanting that maintainer to merge your patches.  If there is no
 285obvious maintainer, Andrew Morton is often the patch target of last resort.
 286
 287Patches need good subject lines.  The canonical format for a patch line is
 288something like:
 289
 290        [PATCH nn/mm] subsys: one-line description of the patch
 291
 292where "nn" is the ordinal number of the patch, "mm" is the total number of
 293patches in the series, and "subsys" is the name of the affected subsystem.
 294Clearly, nn/mm can be omitted for a single, standalone patch.
 295
 296If you have a significant series of patches, it is customary to send an
 297introductory description as part zero.  This convention is not universally
 298followed though; if you use it, remember that information in the
 299introduction does not make it into the kernel changelogs.  So please ensure
 300that the patches, themselves, have complete changelog information.
 301
 302In general, the second and following parts of a multi-part patch should be
 303sent as a reply to the first part so that they all thread together at the
 304receiving end.  Tools like git and quilt have commands to mail out a set of
 305patches with the proper threading.  If you have a long series, though, and
 306are using git, please stay away from the --chain-reply-to option to avoid
 307creating exceptionally deep nesting.
 308
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