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Submitting Patches

How to Get Your Change Into the Linux Kernel


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. --------------------------------------------


1) "diff -up"

------------ Use "diff -up" or "diff -uprN" to create patches. 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 may want to look into splitting them into individual patches which modify things in logical stages. This will facilitate easier reviewing by other kernel developers, very important if you want your patch accepted. There are a number of scripts which can aid in this: Quilt: Andrew Morton's patch scripts: Instead of these scripts, quilt is the recommended patch management tool (see above).

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. If the patch fixes a logged bug entry, refer to that bug entry by number and URL. 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/ 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/ 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, 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 . 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 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: 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 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)

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 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 [ struct foo moved from foo.c to foo.h] Signed-off-by: Lucky K Maintainer 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

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:// 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.


----------------------------------- 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/ 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." ----------------------


---------------------- Andrew Morton, "The perfect patch" (tpp). Jeff Garzik, "Linux kernel patch submission format". Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer". NO!!!! No more huge patch bombs to people! Kernel Documentation/CodingStyle: Linus Torvalds's mail on the canonical patch format: Andi Kleen, "On submitting kernel patches" Some strategies to get difficult or controversial changes in. --