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   1HOWTO do Linux kernel development
   2---------------------------------
   3
   4This is the be-all, end-all document on this topic.  It contains
   5instructions on how to become a Linux kernel developer and how to learn
   6to work with the Linux kernel development community.  It tries to not
   7contain anything related to the technical aspects of kernel programming,
   8but will help point you in the right direction for that.
   9
  10If anything in this document becomes out of date, please send in patches
  11to the maintainer of this file, who is listed at the bottom of the
  12document.
  13
  14
  15Introduction
  16------------
  17
  18So, you want to learn how to become a Linux kernel developer?  Or you
  19have been told by your manager, "Go write a Linux driver for this
  20device."  This document's goal is to teach you everything you need to
  21know to achieve this by describing the process you need to go through,
  22and hints on how to work with the community.  It will also try to
  23explain some of the reasons why the community works like it does.
  24
  25The kernel is written mostly in C, with some architecture-dependent
  26parts written in assembly. A good understanding of C is required for
  27kernel development.  Assembly (any architecture) is not required unless
  28you plan to do low-level development for that architecture.  Though they
  29are not a good substitute for a solid C education and/or years of
  30experience, the following books are good for, if anything, reference:
  31 - "The C Programming Language" by Kernighan and Ritchie [Prentice Hall]
  32 - "Practical C Programming" by Steve Oualline [O'Reilly]
  33 - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall]
  34
  35The kernel is written using GNU C and the GNU toolchain.  While it
  36adheres to the ISO C89 standard, it uses a number of extensions that are
  37not featured in the standard.  The kernel is a freestanding C
  38environment, with no reliance on the standard C library, so some
  39portions of the C standard are not supported.  Arbitrary long long
  40divisions and floating point are not allowed.  It can sometimes be
  41difficult to understand the assumptions the kernel has on the toolchain
  42and the extensions that it uses, and unfortunately there is no
  43definitive reference for them.  Please check the gcc info pages (`info
  44gcc`) for some information on them.
  45
  46Please remember that you are trying to learn how to work with the
  47existing development community.  It is a diverse group of people, with
  48high standards for coding, style and procedure.  These standards have
  49been created over time based on what they have found to work best for
  50such a large and geographically dispersed team.  Try to learn as much as
  51possible about these standards ahead of time, as they are well
  52documented; do not expect people to adapt to you or your company's way
  53of doing things.
  54
  55
  56Legal Issues
  57------------
  58
  59The Linux kernel source code is released under the GPL.  Please see the
  60file, COPYING, in the main directory of the source tree, for details on
  61the license.  If you have further questions about the license, please
  62contact a lawyer, and do not ask on the Linux kernel mailing list.  The
  63people on the mailing lists are not lawyers, and you should not rely on
  64their statements on legal matters.
  65
  66For common questions and answers about the GPL, please see:
  67        http://www.gnu.org/licenses/gpl-faq.html
  68
  69
  70Documentation
  71------------
  72
  73The Linux kernel source tree has a large range of documents that are
  74invaluable for learning how to interact with the kernel community.  When
  75new features are added to the kernel, it is recommended that new
  76documentation files are also added which explain how to use the feature.
  77When a kernel change causes the interface that the kernel exposes to
  78userspace to change, it is recommended that you send the information or
  79a patch to the manual pages explaining the change to the manual pages
  80maintainer at mtk.manpages@gmail.com, and CC the list
  81linux-api@vger.kernel.org.
  82
  83Here is a list of files that are in the kernel source tree that are
  84required reading:
  85  README
  86    This file gives a short background on the Linux kernel and describes
  87    what is necessary to do to configure and build the kernel.  People
  88    who are new to the kernel should start here.
  89
  90  Documentation/Changes
  91    This file gives a list of the minimum levels of various software
  92    packages that are necessary to build and run the kernel
  93    successfully.
  94
  95  Documentation/CodingStyle
  96    This describes the Linux kernel coding style, and some of the
  97    rationale behind it. All new code is expected to follow the
  98    guidelines in this document. Most maintainers will only accept
  99    patches if these rules are followed, and many people will only
 100    review code if it is in the proper style.
 101
 102  Documentation/SubmittingPatches
 103  Documentation/SubmittingDrivers
 104    These files describe in explicit detail how to successfully create
 105    and send a patch, including (but not limited to):
 106       - Email contents
 107       - Email format
 108       - Who to send it to
 109    Following these rules will not guarantee success (as all patches are
 110    subject to scrutiny for content and style), but not following them
 111    will almost always prevent it.
 112
 113    Other excellent descriptions of how to create patches properly are:
 114        "The Perfect Patch"
 115                http://userweb.kernel.org/~akpm/stuff/tpp.txt
 116        "Linux kernel patch submission format"
 117                http://linux.yyz.us/patch-format.html
 118
 119  Documentation/stable_api_nonsense.txt
 120    This file describes the rationale behind the conscious decision to
 121    not have a stable API within the kernel, including things like:
 122      - Subsystem shim-layers (for compatibility?)
 123      - Driver portability between Operating Systems.
 124      - Mitigating rapid change within the kernel source tree (or
 125        preventing rapid change)
 126    This document is crucial for understanding the Linux development
 127    philosophy and is very important for people moving to Linux from
 128    development on other Operating Systems.
 129
 130  Documentation/SecurityBugs
 131    If you feel you have found a security problem in the Linux kernel,
 132    please follow the steps in this document to help notify the kernel
 133    developers, and help solve the issue.
 134
 135  Documentation/ManagementStyle
 136    This document describes how Linux kernel maintainers operate and the
 137    shared ethos behind their methodologies.  This is important reading
 138    for anyone new to kernel development (or anyone simply curious about
 139    it), as it resolves a lot of common misconceptions and confusion
 140    about the unique behavior of kernel maintainers.
 141
 142  Documentation/stable_kernel_rules.txt
 143    This file describes the rules on how the stable kernel releases
 144    happen, and what to do if you want to get a change into one of these
 145    releases.
 146
 147  Documentation/kernel-docs.txt
 148    A list of external documentation that pertains to kernel
 149    development.  Please consult this list if you do not find what you
 150    are looking for within the in-kernel documentation.
 151
 152  Documentation/applying-patches.txt
 153    A good introduction describing exactly what a patch is and how to
 154    apply it to the different development branches of the kernel.
 155
 156The kernel also has a large number of documents that can be
 157automatically generated from the source code itself.  This includes a
 158full description of the in-kernel API, and rules on how to handle
 159locking properly.  The documents will be created in the
 160Documentation/DocBook/ directory and can be generated as PDF,
 161Postscript, HTML, and man pages by running:
 162        make pdfdocs
 163        make psdocs
 164        make htmldocs
 165        make mandocs
 166respectively from the main kernel source directory.
 167
 168
 169Becoming A Kernel Developer
 170---------------------------
 171
 172If you do not know anything about Linux kernel development, you should
 173look at the Linux KernelNewbies project:
 174        http://kernelnewbies.org
 175It consists of a helpful mailing list where you can ask almost any type
 176of basic kernel development question (make sure to search the archives
 177first, before asking something that has already been answered in the
 178past.)  It also has an IRC channel that you can use to ask questions in
 179real-time, and a lot of helpful documentation that is useful for
 180learning about Linux kernel development.
 181
 182The website has basic information about code organization, subsystems,
 183and current projects (both in-tree and out-of-tree). It also describes
 184some basic logistical information, like how to compile a kernel and
 185apply a patch.
 186
 187If you do not know where you want to start, but you want to look for
 188some task to start doing to join into the kernel development community,
 189go to the Linux Kernel Janitor's project:
 190        http://kernelnewbies.org/KernelJanitors 
 191It is a great place to start.  It describes a list of relatively simple
 192problems that need to be cleaned up and fixed within the Linux kernel
 193source tree.  Working with the developers in charge of this project, you
 194will learn the basics of getting your patch into the Linux kernel tree,
 195and possibly be pointed in the direction of what to go work on next, if
 196you do not already have an idea.
 197
 198If you already have a chunk of code that you want to put into the kernel
 199tree, but need some help getting it in the proper form, the
 200kernel-mentors project was created to help you out with this.  It is a
 201mailing list, and can be found at:
 202        http://selenic.com/mailman/listinfo/kernel-mentors
 203
 204Before making any actual modifications to the Linux kernel code, it is
 205imperative to understand how the code in question works.  For this
 206purpose, nothing is better than reading through it directly (most tricky
 207bits are commented well), perhaps even with the help of specialized
 208tools.  One such tool that is particularly recommended is the Linux
 209Cross-Reference project, which is able to present source code in a
 210self-referential, indexed webpage format. An excellent up-to-date
 211repository of the kernel code may be found at:
 212        http://lxr.linux.no/+trees
 213
 214
 215The development process
 216-----------------------
 217
 218Linux kernel development process currently consists of a few different
 219main kernel "branches" and lots of different subsystem-specific kernel
 220branches.  These different branches are:
 221  - main 3.x kernel tree
 222  - 3.x.y -stable kernel tree
 223  - 3.x -git kernel patches
 224  - subsystem specific kernel trees and patches
 225  - the 3.x -next kernel tree for integration tests
 226
 2273.x kernel tree
 228-----------------
 2293.x kernels are maintained by Linus Torvalds, and can be found on
 230kernel.org in the pub/linux/kernel/v3.x/ directory.  Its development
 231process is as follows:
 232  - As soon as a new kernel is released a two weeks window is open,
 233    during this period of time maintainers can submit big diffs to
 234    Linus, usually the patches that have already been included in the
 235    -next kernel for a few weeks.  The preferred way to submit big changes
 236    is using git (the kernel's source management tool, more information
 237    can be found at http://git-scm.com/) but plain patches are also just
 238    fine.
 239  - After two weeks a -rc1 kernel is released it is now possible to push
 240    only patches that do not include new features that could affect the
 241    stability of the whole kernel.  Please note that a whole new driver
 242    (or filesystem) might be accepted after -rc1 because there is no
 243    risk of causing regressions with such a change as long as the change
 244    is self-contained and does not affect areas outside of the code that
 245    is being added.  git can be used to send patches to Linus after -rc1
 246    is released, but the patches need to also be sent to a public
 247    mailing list for review.
 248  - A new -rc is released whenever Linus deems the current git tree to
 249    be in a reasonably sane state adequate for testing.  The goal is to
 250    release a new -rc kernel every week.
 251  - Process continues until the kernel is considered "ready", the
 252    process should last around 6 weeks.
 253  - Known regressions in each release are periodically posted to the 
 254    linux-kernel mailing list.  The goal is to reduce the length of 
 255    that list to zero before declaring the kernel to be "ready," but, in
 256    the real world, a small number of regressions often remain at 
 257    release time.
 258
 259It is worth mentioning what Andrew Morton wrote on the linux-kernel
 260mailing list about kernel releases:
 261        "Nobody knows when a kernel will be released, because it's
 262        released according to perceived bug status, not according to a
 263        preconceived timeline."
 264
 2653.x.y -stable kernel tree
 266---------------------------
 267Kernels with 3-part versions are -stable kernels. They contain
 268relatively small and critical fixes for security problems or significant
 269regressions discovered in a given 3.x kernel.
 270
 271This is the recommended branch for users who want the most recent stable
 272kernel and are not interested in helping test development/experimental
 273versions.
 274
 275If no 3.x.y kernel is available, then the highest numbered 3.x
 276kernel is the current stable kernel.
 277
 2783.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
 279are released as needs dictate.  The normal release period is approximately
 280two weeks, but it can be longer if there are no pressing problems.  A
 281security-related problem, instead, can cause a release to happen almost
 282instantly.
 283
 284The file Documentation/stable_kernel_rules.txt in the kernel tree
 285documents what kinds of changes are acceptable for the -stable tree, and
 286how the release process works.
 287
 2883.x -git patches
 289------------------
 290These are daily snapshots of Linus' kernel tree which are managed in a
 291git repository (hence the name.) These patches are usually released
 292daily and represent the current state of Linus' tree.  They are more
 293experimental than -rc kernels since they are generated automatically
 294without even a cursory glance to see if they are sane.
 295
 296Subsystem Specific kernel trees and patches
 297-------------------------------------------
 298The maintainers of the various kernel subsystems --- and also many
 299kernel subsystem developers --- expose their current state of
 300development in source repositories.  That way, others can see what is
 301happening in the different areas of the kernel.  In areas where
 302development is rapid, a developer may be asked to base his submissions
 303onto such a subsystem kernel tree so that conflicts between the
 304submission and other already ongoing work are avoided.
 305
 306Most of these repositories are git trees, but there are also other SCMs
 307in use, or patch queues being published as quilt series.  Addresses of
 308these subsystem repositories are listed in the MAINTAINERS file.  Many
 309of them can be browsed at http://git.kernel.org/.
 310
 311Before a proposed patch is committed to such a subsystem tree, it is
 312subject to review which primarily happens on mailing lists (see the
 313respective section below).  For several kernel subsystems, this review
 314process is tracked with the tool patchwork.  Patchwork offers a web
 315interface which shows patch postings, any comments on a patch or
 316revisions to it, and maintainers can mark patches as under review,
 317accepted, or rejected.  Most of these patchwork sites are listed at
 318http://patchwork.kernel.org/.
 319
 3203.x -next kernel tree for integration tests
 321---------------------------------------------
 322Before updates from subsystem trees are merged into the mainline 3.x
 323tree, they need to be integration-tested.  For this purpose, a special
 324testing repository exists into which virtually all subsystem trees are
 325pulled on an almost daily basis:
 326        http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
 327        http://linux.f-seidel.de/linux-next/pmwiki/
 328
 329This way, the -next kernel gives a summary outlook onto what will be
 330expected to go into the mainline kernel at the next merge period.
 331Adventurous testers are very welcome to runtime-test the -next kernel.
 332
 333
 334Bug Reporting
 335-------------
 336
 337bugzilla.kernel.org is where the Linux kernel developers track kernel
 338bugs.  Users are encouraged to report all bugs that they find in this
 339tool.  For details on how to use the kernel bugzilla, please see:
 340        http://bugzilla.kernel.org/page.cgi?id=faq.html
 341
 342The file REPORTING-BUGS in the main kernel source directory has a good
 343template for how to report a possible kernel bug, and details what kind
 344of information is needed by the kernel developers to help track down the
 345problem.
 346
 347
 348Managing bug reports
 349--------------------
 350
 351One of the best ways to put into practice your hacking skills is by fixing
 352bugs reported by other people. Not only you will help to make the kernel
 353more stable, you'll learn to fix real world problems and you will improve
 354your skills, and other developers will be aware of your presence. Fixing
 355bugs is one of the best ways to get merits among other developers, because
 356not many people like wasting time fixing other people's bugs.
 357
 358To work in the already reported bug reports, go to http://bugzilla.kernel.org.
 359If you want to be advised of the future bug reports, you can subscribe to the
 360bugme-new mailing list (only new bug reports are mailed here) or to the
 361bugme-janitor mailing list (every change in the bugzilla is mailed here)
 362
 363        http://lists.linux-foundation.org/mailman/listinfo/bugme-new
 364        http://lists.linux-foundation.org/mailman/listinfo/bugme-janitors
 365
 366
 367
 368Mailing lists
 369-------------
 370
 371As some of the above documents describe, the majority of the core kernel
 372developers participate on the Linux Kernel Mailing list.  Details on how
 373to subscribe and unsubscribe from the list can be found at:
 374        http://vger.kernel.org/vger-lists.html#linux-kernel
 375There are archives of the mailing list on the web in many different
 376places.  Use a search engine to find these archives.  For example:
 377        http://dir.gmane.org/gmane.linux.kernel
 378It is highly recommended that you search the archives about the topic
 379you want to bring up, before you post it to the list. A lot of things
 380already discussed in detail are only recorded at the mailing list
 381archives.
 382
 383Most of the individual kernel subsystems also have their own separate
 384mailing list where they do their development efforts.  See the
 385MAINTAINERS file for a list of what these lists are for the different
 386groups.
 387
 388Many of the lists are hosted on kernel.org. Information on them can be
 389found at:
 390        http://vger.kernel.org/vger-lists.html
 391
 392Please remember to follow good behavioral habits when using the lists.
 393Though a bit cheesy, the following URL has some simple guidelines for
 394interacting with the list (or any list):
 395        http://www.albion.com/netiquette/
 396
 397If multiple people respond to your mail, the CC: list of recipients may
 398get pretty large. Don't remove anybody from the CC: list without a good
 399reason, or don't reply only to the list address. Get used to receiving the
 400mail twice, one from the sender and the one from the list, and don't try
 401to tune that by adding fancy mail-headers, people will not like it.
 402
 403Remember to keep the context and the attribution of your replies intact,
 404keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
 405add your statements between the individual quoted sections instead of
 406writing at the top of the mail.
 407
 408If you add patches to your mail, make sure they are plain readable text
 409as stated in Documentation/SubmittingPatches. Kernel developers don't
 410want to deal with attachments or compressed patches; they may want
 411to comment on individual lines of your patch, which works only that way.
 412Make sure you use a mail program that does not mangle spaces and tab
 413characters. A good first test is to send the mail to yourself and try
 414to apply your own patch by yourself. If that doesn't work, get your
 415mail program fixed or change it until it works.
 416
 417Above all, please remember to show respect to other subscribers.
 418
 419
 420Working with the community
 421--------------------------
 422
 423The goal of the kernel community is to provide the best possible kernel
 424there is.  When you submit a patch for acceptance, it will be reviewed
 425on its technical merits and those alone.  So, what should you be
 426expecting?
 427  - criticism
 428  - comments
 429  - requests for change
 430  - requests for justification
 431  - silence
 432
 433Remember, this is part of getting your patch into the kernel.  You have
 434to be able to take criticism and comments about your patches, evaluate
 435them at a technical level and either rework your patches or provide
 436clear and concise reasoning as to why those changes should not be made.
 437If there are no responses to your posting, wait a few days and try
 438again, sometimes things get lost in the huge volume.
 439
 440What should you not do?
 441  - expect your patch to be accepted without question
 442  - become defensive
 443  - ignore comments
 444  - resubmit the patch without making any of the requested changes
 445
 446In a community that is looking for the best technical solution possible,
 447there will always be differing opinions on how beneficial a patch is.
 448You have to be cooperative, and willing to adapt your idea to fit within
 449the kernel.  Or at least be willing to prove your idea is worth it.
 450Remember, being wrong is acceptable as long as you are willing to work
 451toward a solution that is right.
 452
 453It is normal that the answers to your first patch might simply be a list
 454of a dozen things you should correct.  This does _not_ imply that your
 455patch will not be accepted, and it is _not_ meant against you
 456personally.  Simply correct all issues raised against your patch and
 457resend it.
 458
 459
 460Differences between the kernel community and corporate structures
 461-----------------------------------------------------------------
 462
 463The kernel community works differently than most traditional corporate
 464development environments.  Here are a list of things that you can try to
 465do to try to avoid problems:
 466  Good things to say regarding your proposed changes:
 467    - "This solves multiple problems."
 468    - "This deletes 2000 lines of code."
 469    - "Here is a patch that explains what I am trying to describe."
 470    - "I tested it on 5 different architectures..."
 471    - "Here is a series of small patches that..."
 472    - "This increases performance on typical machines..."
 473
 474  Bad things you should avoid saying:
 475    - "We did it this way in AIX/ptx/Solaris, so therefore it must be
 476      good..."
 477    - "I've being doing this for 20 years, so..."
 478    - "This is required for my company to make money"
 479    - "This is for our Enterprise product line."
 480    - "Here is my 1000 page design document that describes my idea"
 481    - "I've been working on this for 6 months..."
 482    - "Here's a 5000 line patch that..."
 483    - "I rewrote all of the current mess, and here it is..."
 484    - "I have a deadline, and this patch needs to be applied now."
 485
 486Another way the kernel community is different than most traditional
 487software engineering work environments is the faceless nature of
 488interaction.  One benefit of using email and irc as the primary forms of
 489communication is the lack of discrimination based on gender or race.
 490The Linux kernel work environment is accepting of women and minorities
 491because all you are is an email address.  The international aspect also
 492helps to level the playing field because you can't guess gender based on
 493a person's name. A man may be named Andrea and a woman may be named Pat.
 494Most women who have worked in the Linux kernel and have expressed an
 495opinion have had positive experiences.
 496
 497The language barrier can cause problems for some people who are not
 498comfortable with English.  A good grasp of the language can be needed in
 499order to get ideas across properly on mailing lists, so it is
 500recommended that you check your emails to make sure they make sense in
 501English before sending them.
 502
 503
 504Break up your changes
 505---------------------
 506
 507The Linux kernel community does not gladly accept large chunks of code
 508dropped on it all at once.  The changes need to be properly introduced,
 509discussed, and broken up into tiny, individual portions.  This is almost
 510the exact opposite of what companies are used to doing.  Your proposal
 511should also be introduced very early in the development process, so that
 512you can receive feedback on what you are doing.  It also lets the
 513community feel that you are working with them, and not simply using them
 514as a dumping ground for your feature.  However, don't send 50 emails at
 515one time to a mailing list, your patch series should be smaller than
 516that almost all of the time.
 517
 518The reasons for breaking things up are the following:
 519
 5201) Small patches increase the likelihood that your patches will be
 521   applied, since they don't take much time or effort to verify for
 522   correctness.  A 5 line patch can be applied by a maintainer with
 523   barely a second glance. However, a 500 line patch may take hours to
 524   review for correctness (the time it takes is exponentially
 525   proportional to the size of the patch, or something).
 526
 527   Small patches also make it very easy to debug when something goes
 528   wrong.  It's much easier to back out patches one by one than it is
 529   to dissect a very large patch after it's been applied (and broken
 530   something).
 531
 5322) It's important not only to send small patches, but also to rewrite
 533   and simplify (or simply re-order) patches before submitting them.
 534
 535Here is an analogy from kernel developer Al Viro:
 536        "Think of a teacher grading homework from a math student.  The
 537        teacher does not want to see the student's trials and errors
 538        before they came up with the solution. They want to see the
 539        cleanest, most elegant answer.  A good student knows this, and
 540        would never submit her intermediate work before the final
 541        solution."
 542
 543        The same is true of kernel development. The maintainers and
 544        reviewers do not want to see the thought process behind the
 545        solution to the problem one is solving. They want to see a
 546        simple and elegant solution."
 547
 548It may be challenging to keep the balance between presenting an elegant
 549solution and working together with the community and discussing your
 550unfinished work. Therefore it is good to get early in the process to
 551get feedback to improve your work, but also keep your changes in small
 552chunks that they may get already accepted, even when your whole task is
 553not ready for inclusion now.
 554
 555Also realize that it is not acceptable to send patches for inclusion
 556that are unfinished and will be "fixed up later."
 557
 558
 559Justify your change
 560-------------------
 561
 562Along with breaking up your patches, it is very important for you to let
 563the Linux community know why they should add this change.  New features
 564must be justified as being needed and useful.
 565
 566
 567Document your change
 568--------------------
 569
 570When sending in your patches, pay special attention to what you say in
 571the text in your email.  This information will become the ChangeLog
 572information for the patch, and will be preserved for everyone to see for
 573all time.  It should describe the patch completely, containing:
 574  - why the change is necessary
 575  - the overall design approach in the patch
 576  - implementation details
 577  - testing results
 578
 579For more details on what this should all look like, please see the
 580ChangeLog section of the document:
 581  "The Perfect Patch"
 582      http://userweb.kernel.org/~akpm/stuff/tpp.txt
 583
 584
 585
 586
 587All of these things are sometimes very hard to do. It can take years to
 588perfect these practices (if at all). It's a continuous process of
 589improvement that requires a lot of patience and determination. But
 590don't give up, it's possible. Many have done it before, and each had to
 591start exactly where you are now.
 592
 593
 594
 595
 596----------
 597Thanks to Paolo Ciarrocchi who allowed the "Development Process"
 598(http://lwn.net/Articles/94386/) section
 599to be based on text he had written, and to Randy Dunlap and Gerrit
 600Huizenga for some of the list of things you should and should not say.
 601Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
 602Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
 603Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
 604David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
 605their review, comments, and contributions.  Without their help, this
 606document would not have been possible.
 607
 608
 609
 610Maintainer: Greg Kroah-Hartman <greg@kroah.com>
 611
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