1Bisecting a bug 2+++++++++++++++ 3 4Last updated: 28 October 2016 5 6Introduction 7============ 8 9Always try the latest kernel from kernel.org and build from source. If you are 10not confident in doing that please report the bug to your distribution vendor 11instead of to a kernel developer. 12 13Finding bugs is not always easy. Have a go though. If you can't find it don't 14give up. Report as much as you have found to the relevant maintainer. See 15MAINTAINERS for who that is for the subsystem you have worked on. 16 17Before you submit a bug report read 18'Documentation/admin-guide/reporting-issues.rst'. 19 20Devices not appearing 21===================== 22 23Often this is caused by udev/systemd. Check that first before blaming it 24on the kernel. 25 26Finding patch that caused a bug 27=============================== 28 29Using the provided tools with ``git`` makes finding bugs easy provided the bug 30is reproducible. 31 32Steps to do it: 33 34- build the Kernel from its git source 35- start bisect with [#f1]_:: 36 37 $ git bisect start 38 39- mark the broken changeset with:: 40 41 $ git bisect bad [commit] 42 43- mark a changeset where the code is known to work with:: 44 45 $ git bisect good [commit] 46 47- rebuild the Kernel and test 48- interact with git bisect by using either:: 49 50 $ git bisect good 51 52 or:: 53 54 $ git bisect bad 55 56 depending if the bug happened on the changeset you're testing 57- After some interactions, git bisect will give you the changeset that 58 likely caused the bug. 59 60- For example, if you know that the current version is bad, and version 61 4.8 is good, you could do:: 62 63 $ git bisect start 64 $ git bisect bad # Current version is bad 65 $ git bisect good v4.8 66 67 68.. [#f1] You can, optionally, provide both good and bad arguments at git 69 start with ``git bisect start [BAD] [GOOD]`` 70 71For further references, please read: 72 73- The man page for ``git-bisect`` 74- `Fighting regressions with git bisect <https://www.kernel.org/pub/software/scm/git/docs/git-bisect-lk2009.html>`_ 75- `Fully automated bisecting with "git bisect run" <https://lwn.net/Articles/317154>`_ 76- `Using Git bisect to figure out when brokenness was introduced <http://webchick.net/node/99>`_ 77