3
Sooner or later, the time comes when your work is ready to be presented to
4
the community for review and, eventually, inclusion into the mainline
5
kernel. Unsurprisingly, the kernel development community has evolved a set
6
of conventions and procedures which are used in the posting of patches;
7
following them will make life much easier for everybody involved. This
8
document will attempt to cover these expectations in reasonable detail;
9
more information can also be found in the files SubmittingPatches,
10
SubmittingDrivers, and SubmitChecklist in the kernel documentation
16
There is a constant temptation to avoid posting patches before they are
17
completely "ready." For simple patches, that is not a problem. If the
18
work being done is complex, though, there is a lot to be gained by getting
19
feedback from the community before the work is complete. So you should
20
consider posting in-progress work, or even making a git tree available so
21
that interested developers can catch up with your work at any time.
23
When posting code which is not yet considered ready for inclusion, it is a
24
good idea to say so in the posting itself. Also mention any major work
25
which remains to be done and any known problems. Fewer people will look at
26
patches which are known to be half-baked, but those who do will come in
27
with the idea that they can help you drive the work in the right direction.
30
5.2: BEFORE CREATING PATCHES
32
There are a number of things which should be done before you consider
33
sending patches to the development community. These include:
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.
40
- Make sure your code is compliant with the kernel coding style
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.
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.
51
As a general rule, putting in some extra thought before posting code almost
52
always pays back the effort in short order.
55
5.3: PATCH PREPARATION
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The preparation of patches for posting can be a surprising amount of work,
58
but, once again, attempting to save time here is not generally advisable
59
even in the short term.
61
Patches must be prepared against a specific version of the kernel. As a
62
general rule, a patch should be based on the current mainline as found in
63
Linus's git tree. When basing on mainline, start with a well-known release
64
point - a stable or -rc release - rather than branching off the mainline at
67
It may become necessary to make versions against -mm, linux-next, or a
68
subsystem tree, though, to facilitate wider testing and review. Depending
69
on the area of your patch and what is going on elsewhere, basing a patch
70
against these other trees can require a significant amount of work
71
resolving conflicts and dealing with API changes.
73
Only the most simple changes should be formatted as a single patch;
74
everything else should be made as a logical series of changes. Splitting
75
up patches is a bit of an art; some developers spend a long time figuring
76
out how to do it in the way that the community expects. There are a few
77
rules of thumb, however, which can help considerably:
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
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.
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
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.
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*
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.
120
Working to create the perfect patch series can be a frustrating process
121
which takes quite a bit of time and thought after the "real work" has been
122
done. When done properly, though, it is time well spent.
125
5.4: PATCH FORMATTING AND CHANGELOGS
127
So now you have a perfect series of patches for posting, but the work is
128
not done quite yet. Each patch needs to be formatted into a message which
129
quickly and clearly communicates its purpose to the rest of the world. To
130
that end, each patch will be composed of the following:
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.
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
143
gpio: fix build on CONFIG_GPIO_SYSFS=n
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.
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.
152
The items above, together, form the changelog for the patch. Writing good
153
changelogs is a crucial but often-neglected art; it's worth spending
154
another moment discussing this issue. When writing a changelog, you should
155
bear in mind that a number of different people will be reading your words.
156
These include subsystem maintainers and reviewers who need to decide
157
whether the patch should be included, distributors and other maintainers
158
trying to decide whether a patch should be backported to other kernels, bug
159
hunters wondering whether the patch is responsible for a problem they are
160
chasing, users who want to know how the kernel has changed, and more. A
161
good changelog conveys the needed information to all of these people in the
162
most direct and concise way possible.
164
To that end, the summary line should describe the effects of and motivation
165
for the change as well as possible given the one-line constraint. The
166
detailed description can then amplify on those topics and provide any
167
needed additional information. If the patch fixes a bug, cite the commit
168
which introduced the bug if possible (and please provide both the commit ID
169
and the title when citing commits). If a problem is associated with
170
specific log or compiler output, include that output to help others
171
searching for a solution to the same problem. If the change is meant to
172
support other changes coming in later patch, say so. If internal APIs are
173
changed, detail those changes and how other developers should respond. In
174
general, the more you can put yourself into the shoes of everybody who will
175
be reading your changelog, the better that changelog (and the kernel as a
178
Needless to say, the changelog should be the text used when committing the
179
change to a revision control system. It will be followed by:
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.
185
You should avoid including changes to irrelevant files (those generated by
186
the build process, for example, or editor backup files) in the patch. The
187
file "dontdiff" in the Documentation directory can help in this regard;
188
pass it to diff with the "-X" option.
190
The tags mentioned above are used to describe how various developers have
191
been associated with the development of this patch. They are described in
192
detail in the SubmittingPatches document; what follows here is a brief
193
summary. Each of these lines has the format:
195
tag: Full Name <email address> optional-other-stuff
197
The tags in common use are:
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.
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.
209
- Tested-by: states that the named person has tested the patch and found
212
- Reviewed-by: the named developer has reviewed the patch for correctness;
213
see the reviewer's statement in Documentation/SubmittingPatches for more
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
221
- Cc: the named person received a copy of the patch and had the
222
opportunity to comment on it.
224
Be careful in the addition of tags to your patches: only Cc: is appropriate
225
for addition without the explicit permission of the person named.
228
5.5: SENDING THE PATCH
230
Before you mail your patches, there are a couple of other things you should
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.
239
Documentation/email-clients.txt has some helpful hints on making
240
specific mail clients work for sending patches.
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.
249
Patches should always be sent as plain text. Please do not send them as
250
attachments; that makes it much harder for reviewers to quote sections of
251
the patch in their replies. Instead, just put the patch directly into your
254
When mailing patches, it is important to send copies to anybody who might
255
be interested in it. Unlike some other projects, the kernel encourages
256
people to err on the side of sending too many copies; don't assume that the
257
relevant people will see your posting on the mailing lists. In particular,
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.
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.
267
- If you are responding to a bug report or a feature request, copy the
268
original poster as well.
270
- Send a copy to the relevant mailing list, or, if nothing else applies,
271
the linux-kernel list.
273
- If you are fixing a bug, think about whether the fix should go into the
274
next stable update. If so, stable@kernel.org should get a copy of the
275
patch. Also add a "Cc: stable@kernel.org" to the tags within the patch
276
itself; that will cause the stable team to get a notification when your
277
fix goes into the mainline.
279
When selecting recipients for a patch, it is good to have an idea of who
280
you think will eventually accept the patch and get it merged. While it
281
is possible to send patches directly to Linus Torvalds and have him merge
282
them, things are not normally done that way. Linus is busy, and there are
283
subsystem maintainers who watch over specific parts of the kernel. Usually
284
you will be wanting that maintainer to merge your patches. If there is no
285
obvious maintainer, Andrew Morton is often the patch target of last resort.
287
Patches need good subject lines. The canonical format for a patch line is
290
[PATCH nn/mm] subsys: one-line description of the patch
292
where "nn" is the ordinal number of the patch, "mm" is the total number of
293
patches in the series, and "subsys" is the name of the affected subsystem.
294
Clearly, nn/mm can be omitted for a single, standalone patch.
296
If you have a significant series of patches, it is customary to send an
297
introductory description as part zero. This convention is not universally
298
followed though; if you use it, remember that information in the
299
introduction does not make it into the kernel changelogs. So please ensure
300
that the patches, themselves, have complete changelog information.
302
In general, the second and following parts of a multi-part patch should be
303
sent as a reply to the first part so that they all thread together at the
304
receiving end. Tools like git and quilt have commands to mail out a set of
305
patches with the proper threading. If you have a long series, though, and
306
are using git, please stay away from the --chain-reply-to option to avoid
307
creating exceptionally deep nesting.