1.. _development_posting:
2
3Posting patches
4===============
5
6Sooner or later, the time comes when your work is ready to be presented to
7the community for review and, eventually, inclusion into the mainline
8kernel.  Unsurprisingly, the kernel development community has evolved a set
9of conventions and procedures which are used in the posting of patches;
10following them will make life much easier for everybody involved.  This
11document will attempt to cover these expectations in reasonable detail;
12more information can also be found in the files
13:ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
14and :ref:`Documentation/process/submit-checklist.rst <submitchecklist>`.
15
16
17When to post
18------------
19
20There is a constant temptation to avoid posting patches before they are
21completely "ready."  For simple patches, that is not a problem.  If the
22work being done is complex, though, there is a lot to be gained by getting
23feedback from the community before the work is complete.  So you should
24consider posting in-progress work, or even making a git tree available so
25that interested developers can catch up with your work at any time.
26
27When posting code which is not yet considered ready for inclusion, it is a
28good idea to say so in the posting itself.  Also mention any major work
29which remains to be done and any known problems.  Fewer people will look at
30patches which are known to be half-baked, but those who do will come in
31with the idea that they can help you drive the work in the right direction.
32
33
34Before creating patches
35-----------------------
36
37There are a number of things which should be done before you consider
38sending patches to the development community.  These include:
39
40 - Test the code to the extent that you can.  Make use of the kernel's
41   debugging tools, ensure that the kernel will build with all reasonable
42   combinations of configuration options, use cross-compilers to build for
43   different architectures, etc.
44
45 - Make sure your code is compliant with the kernel coding style
46   guidelines.
47
48 - Does your change have performance implications?  If so, you should run
49   benchmarks showing what the impact (or benefit) of your change is; a
50   summary of the results should be included with the patch.
51
52 - Be sure that you have the right to post the code.  If this work was done
53   for an employer, the employer likely has a right to the work and must be
54   agreeable with its release under the GPL.
55
56As a general rule, putting in some extra thought before posting code almost
57always pays back the effort in short order.
58
59
60Patch preparation
61-----------------
62
63The preparation of patches for posting can be a surprising amount of work,
64but, once again, attempting to save time here is not generally advisable
65even in the short term.
66
67Patches must be prepared against a specific version of the kernel.  As a
68general rule, a patch should be based on the current mainline as found in
69Linus's git tree.  When basing on mainline, start with a well-known release
70point - a stable or -rc release - rather than branching off the mainline at
71an arbitrary spot.
72
73It may become necessary to make versions against -mm, linux-next, or a
74subsystem tree, though, to facilitate wider testing and review.  Depending
75on the area of your patch and what is going on elsewhere, basing a patch
76against these other trees can require a significant amount of work
77resolving conflicts and dealing with API changes.
78
79Only the most simple changes should be formatted as a single patch;
80everything else should be made as a logical series of changes.  Splitting
81up patches is a bit of an art; some developers spend a long time figuring
82out how to do it in the way that the community expects.  There are a few
83rules of thumb, however, which can help considerably:
84
85 - The patch series you post will almost certainly not be the series of
86   changes found in your working revision control system.  Instead, the
87   changes you have made need to be considered in their final form, then
88   split apart in ways which make sense.  The developers are interested in
89   discrete, self-contained changes, not the path you took to get to those
90   changes.
91
92 - Each logically independent change should be formatted as a separate
93   patch.  These changes can be small ("add a field to this structure") or
94   large (adding a significant new driver, for example), but they should be
95   conceptually small and amenable to a one-line description.  Each patch
96   should make a specific change which can be reviewed on its own and
97   verified to do what it says it does.
98
99 - As a way of restating the guideline above: do not mix different types of
100   changes in the same patch.  If a single patch fixes a critical security
101   bug, rearranges a few structures, and reformats the code, there is a
102   good chance that it will be passed over and the important fix will be
103   lost.
104
105 - Each patch should yield a kernel which builds and runs properly; if your
106   patch series is interrupted in the middle, the result should still be a
107   working kernel.  Partial application of a patch series is a common
108   scenario when the "git bisect" tool is used to find regressions; if the
109   result is a broken kernel, you will make life harder for developers and
110   users who are engaging in the noble work of tracking down problems.
111
112 - Do not overdo it, though.  One developer once posted a set of edits
113   to a single file as 500 separate patches - an act which did not make him
114   the most popular person on the kernel mailing list.  A single patch can
115   be reasonably large as long as it still contains a single *logical*
116   change.
117
118 - It can be tempting to add a whole new infrastructure with a series of
119   patches, but to leave that infrastructure unused until the final patch
120   in the series enables the whole thing.  This temptation should be
121   avoided if possible; if that series adds regressions, bisection will
122   finger the last patch as the one which caused the problem, even though
123   the real bug is elsewhere.  Whenever possible, a patch which adds new
124   code should make that code active immediately.
125
126Working to create the perfect patch series can be a frustrating process
127which takes quite a bit of time and thought after the "real work" has been
128done.  When done properly, though, it is time well spent.
129
130
131Patch formatting and changelogs
132-------------------------------
133
134So now you have a perfect series of patches for posting, but the work is
135not done quite yet.  Each patch needs to be formatted into a message which
136quickly and clearly communicates its purpose to the rest of the world.  To
137that end, each patch will be composed of the following:
138
139 - An optional "From" line naming the author of the patch.  This line is
140   only necessary if you are passing on somebody else's patch via email,
141   but it never hurts to add it when in doubt.
142
143 - A one-line description of what the patch does.  This message should be
144   enough for a reader who sees it with no other context to figure out the
145   scope of the patch; it is the line that will show up in the "short form"
146   changelogs.  This message is usually formatted with the relevant
147   subsystem name first, followed by the purpose of the patch.  For
148   example:
149
150   ::
151
152	gpio: fix build on CONFIG_GPIO_SYSFS=n
153
154 - A blank line followed by a detailed description of the contents of the
155   patch.  This description can be as long as is required; it should say
156   what the patch does and why it should be applied to the kernel.
157
158 - One or more tag lines, with, at a minimum, one Signed-off-by: line from
159   the author of the patch.  Tags will be described in more detail below.
160
161The items above, together, form the changelog for the patch.  Writing good
162changelogs is a crucial but often-neglected art; it's worth spending
163another moment discussing this issue.  When writing a changelog, you should
164bear in mind that a number of different people will be reading your words.
165These include subsystem maintainers and reviewers who need to decide
166whether the patch should be included, distributors and other maintainers
167trying to decide whether a patch should be backported to other kernels, bug
168hunters wondering whether the patch is responsible for a problem they are
169chasing, users who want to know how the kernel has changed, and more.  A
170good changelog conveys the needed information to all of these people in the
171most direct and concise way possible.
172
173To that end, the summary line should describe the effects of and motivation
174for the change as well as possible given the one-line constraint.  The
175detailed description can then amplify on those topics and provide any
176needed additional information.  If the patch fixes a bug, cite the commit
177which introduced the bug if possible (and please provide both the commit ID
178and the title when citing commits).  If a problem is associated with
179specific log or compiler output, include that output to help others
180searching for a solution to the same problem.  If the change is meant to
181support other changes coming in later patch, say so.  If internal APIs are
182changed, detail those changes and how other developers should respond.  In
183general, the more you can put yourself into the shoes of everybody who will
184be reading your changelog, the better that changelog (and the kernel as a
185whole) will be.
186
187Needless to say, the changelog should be the text used when committing the
188change to a revision control system.  It will be followed by:
189
190 - The patch itself, in the unified ("-u") patch format.  Using the "-p"
191   option to diff will associate function names with changes, making the
192   resulting patch easier for others to read.
193
194You should avoid including changes to irrelevant files (those generated by
195the build process, for example, or editor backup files) in the patch.  The
196file "dontdiff" in the Documentation directory can help in this regard;
197pass it to diff with the "-X" option.
198
199The tags already briefly mentioned above are used to provide insights how
200the patch came into being. They are described in detail in the
201:ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
202document; what follows here is a brief summary.
203
204One tag is used to refer to earlier commits which introduced problems fixed by
205the patch::
206
207	Fixes: 1f2e3d4c5b6a ("The first line of the commit specified by the first 12 characters of its SHA-1 ID")
208
209Another tag is used for linking web pages with additional backgrounds or
210details, for example a report about a bug fixed by the patch or a document
211with a specification implemented by the patch::
212
213	Link: https://example.com/somewhere.html  optional-other-stuff
214
215Many maintainers when applying a patch also add this tag to link to the
216latest public review posting of the patch; often this is automatically done
217by tools like b4 or a git hook like the one described in
218'Documentation/maintainer/configure-git.rst'.
219
220A third kind of tag is used to document who was involved in the development of
221the patch. Each of these uses this format::
222
223	tag: Full Name <email address>  optional-other-stuff
224
225The tags in common use are:
226
227 - Signed-off-by: this is a developer's certification that he or she has
228   the right to submit the patch for inclusion into the kernel.  It is an
229   agreement to the Developer's Certificate of Origin, the full text of
230   which can be found in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
231   Code without a proper signoff cannot be merged into the mainline.
232
233 - Co-developed-by: states that the patch was co-created by several developers;
234   it is a used to give attribution to co-authors (in addition to the author
235   attributed by the From: tag) when multiple people work on a single patch.
236   Every Co-developed-by: must be immediately followed by a Signed-off-by: of
237   the associated co-author.  Details and examples can be found in
238   :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
239
240 - Acked-by: indicates an agreement by another developer (often a
241   maintainer of the relevant code) that the patch is appropriate for
242   inclusion into the kernel.
243
244 - Tested-by: states that the named person has tested the patch and found
245   it to work.
246
247 - Reviewed-by: the named developer has reviewed the patch for correctness;
248   see the reviewer's statement in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
249   for more detail.
250
251 - Reported-by: names a user who reported a problem which is fixed by this
252   patch; this tag is used to give credit to the (often underappreciated)
253   people who test our code and let us know when things do not work
254   correctly.
255
256 - Cc: the named person received a copy of the patch and had the
257   opportunity to comment on it.
258
259Be careful in the addition of tags to your patches, as only Cc: is appropriate
260for addition without the explicit permission of the person named; using
261Reported-by: is fine most of the time as well, but ask for permission if
262the bug was reported in private.
263
264
265Sending the patch
266-----------------
267
268Before you mail your patches, there are a couple of other things you should
269take care of:
270
271 - Are you sure that your mailer will not corrupt the patches?  Patches
272   which have had gratuitous white-space changes or line wrapping performed
273   by the mail client will not apply at the other end, and often will not
274   be examined in any detail.  If there is any doubt at all, mail the patch
275   to yourself and convince yourself that it shows up intact.
276
277   :ref:`Documentation/process/email-clients.rst <email_clients>` has some
278   helpful hints on making specific mail clients work for sending patches.
279
280 - Are you sure your patch is free of silly mistakes?  You should always
281   run patches through scripts/checkpatch.pl and address the complaints it
282   comes up with.  Please bear in mind that checkpatch.pl, while being the
283   embodiment of a fair amount of thought about what kernel patches should
284   look like, is not smarter than you.  If fixing a checkpatch.pl complaint
285   would make the code worse, don't do it.
286
287Patches should always be sent as plain text.  Please do not send them as
288attachments; that makes it much harder for reviewers to quote sections of
289the patch in their replies.  Instead, just put the patch directly into your
290message.
291
292When mailing patches, it is important to send copies to anybody who might
293be interested in it.  Unlike some other projects, the kernel encourages
294people to err on the side of sending too many copies; don't assume that the
295relevant people will see your posting on the mailing lists.  In particular,
296copies should go to:
297
298 - The maintainer(s) of the affected subsystem(s).  As described earlier,
299   the MAINTAINERS file is the first place to look for these people.
300
301 - Other developers who have been working in the same area - especially
302   those who might be working there now.  Using git to see who else has
303   modified the files you are working on can be helpful.
304
305 - If you are responding to a bug report or a feature request, copy the
306   original poster as well.
307
308 - Send a copy to the relevant mailing list, or, if nothing else applies,
309   the linux-kernel list.
310
311 - If you are fixing a bug, think about whether the fix should go into the
312   next stable update.  If so, stable@vger.kernel.org should get a copy of
313   the patch.  Also add a "Cc: stable@vger.kernel.org" to the tags within
314   the patch itself; that will cause the stable team to get a notification
315   when your fix goes into the mainline.
316
317When selecting recipients for a patch, it is good to have an idea of who
318you think will eventually accept the patch and get it merged.  While it
319is possible to send patches directly to Linus Torvalds and have him merge
320them, things are not normally done that way.  Linus is busy, and there are
321subsystem maintainers who watch over specific parts of the kernel.  Usually
322you will be wanting that maintainer to merge your patches.  If there is no
323obvious maintainer, Andrew Morton is often the patch target of last resort.
324
325Patches need good subject lines.  The canonical format for a patch line is
326something like:
327
328::
329
330	[PATCH nn/mm] subsys: one-line description of the patch
331
332where "nn" is the ordinal number of the patch, "mm" is the total number of
333patches in the series, and "subsys" is the name of the affected subsystem.
334Clearly, nn/mm can be omitted for a single, standalone patch.
335
336If you have a significant series of patches, it is customary to send an
337introductory description as part zero.  This convention is not universally
338followed though; if you use it, remember that information in the
339introduction does not make it into the kernel changelogs.  So please ensure
340that the patches, themselves, have complete changelog information.
341
342In general, the second and following parts of a multi-part patch should be
343sent as a reply to the first part so that they all thread together at the
344receiving end.  Tools like git and quilt have commands to mail out a set of
345patches with the proper threading.  If you have a long series, though, and
346are using git, please stay away from the --chain-reply-to option to avoid
347creating exceptionally deep nesting.
348