Documentation/admin-guide/reporting-issues.rst

.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
..
   If you want to distribute this text under CC-BY-4.0 only, please use 'The
   Linux kernel developers' for author attribution and link this as source:
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   Note: Only the content of this RST file as found in the Linux kernel sources
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.. important::

   This document is being prepared to replace
   'Documentation/admin-guide/reporting-bugs.rst'. The main work is done and
   you are already free to follow its instructions when reporting issues to the
   Linux kernel developers. But keep in mind, below text still needs a few
   finishing touches and review. It was merged to the Linux kernel sources at
   this stage to make this process easier and increase the text's visibility.

   Any improvements for the text or other feedback is thus very much welcome.
   Please send it to 'Thorsten Leemhuis <linux@leemhuis.info>' and 'Jonathan
   Corbet <corbet@lwn.net>', ideally with 'Linux kernel mailing list (LKML)
   <linux-kernel@vger.kernel.org>' and the 'Linux Kernel Documentation List
   <linux-doc@vger.kernel.org>' in CC.

   Areas in the text that still need work or discussion contain a hint like this
   which point out the remaining issues; all of them start with the word "FIXME"
   to make them easy to find.


Reporting issues
++++++++++++++++


The short guide (aka TL;DR)
===========================

If you're facing multiple issues with the Linux kernel at once, report each
separately to its developers. Try your best guess which kernel part might be
causing the issue. Check the :ref:`MAINTAINERS <maintainers>` file for how its
developers expect to be told about issues. Note, it's rarely
`bugzilla.kernel.org <https://bugzilla.kernel.org/>`_, as in almost all cases
the report needs to be sent by email!

Check the destination thoroughly for existing reports; also search the LKML
archives and the web. Join existing discussion if you find matches. If you
don't find any, install `the latest Linux mainline kernel
<https://kernel.org/>`_. Make sure it's vanilla, thus is not patched or using
add-on kernel modules. Also ensure the kernel is running in a healthy
environment and is not already tainted before the issue occurs.

If you can reproduce your issue with the mainline kernel, send a report to the
destination you determined earlier. Make sure it includes all relevant
information, which in case of a regression should mention the change that's
causing it which can often can be found with a bisection. Also ensure the
report reaches all people that need to know about it, for example the security
team, the stable maintainers or the developers of the patch that causes a
regression. Once the report is out, answer any questions that might be raised
and help where you can. That includes keeping the ball rolling: every time a
new rc1 mainline kernel is released, check if the issue is still happening
there and attach a status update to your initial report.

If you can not reproduce the issue with the mainline kernel, consider sticking
with it; if you'd like to use an older version line and want to see it fixed
there, first make sure it's still supported. Install its latest release as
vanilla kernel. If you cannot reproduce the issue there, try to find the commit
that fixed it in mainline or any discussion preceding it: those will often
mention if backporting is planed or considered too complex. If backporting was
not discussed, ask if it's in the cards. In case you don't find any commits or
a preceding discussion, see the Linux-stable mailing list archives for existing
reports, as it might be a regression specific to the version line. If it is,
report it like you would report a problem in mainline (including the
bisection).

If you reached this point without a solution, ask for advice one the
subsystem's mailing list.


Step-by-step guide how to report issues to the kernel maintainers
=================================================================

The above TL;DR outlines roughly how to report issues to the Linux kernel
developers. It might be all that's needed for people already familiar with
reporting issues to Free/Libre & Open Source Software (FLOSS) projects. For
everyone else there is this section. It is more detailed and uses a
step-by-step approach. It still tries to be brief for readability and leaves
out a lot of details; those are described below the step-by-step guide in a
reference section, which explains each of the steps in more detail.

Note: this section covers a few more aspects than the TL;DR and does things in
a slightly different order. That's in your interest, to make sure you notice
early if an issue that looks like a Linux kernel problem is actually caused by
something else. These steps thus help to ensure the time you invest in this
process won't feel wasted in the end:

 * Stop reading this document and report the problem to your vendor instead,
   unless you are running the latest mainline kernel already or are willing to
   install it. This kernel must not be modified or enhanced in any way, and
   thus be considered 'vanilla'.

 * See if the issue you are dealing with qualifies as regression, security
   issue, or a really severe problem: those are 'issues of high priority' that
   need special handling in some steps that are about to follow.

 * Check if your kernel was 'tainted' when the issue occurred, as the event
   that made the kernel set this flag might be causing the issue you face.

 * Locate the driver or kernel subsystem that seems to be causing the issue.
   Find out how and where its developers expect reports. Note: most of the
   time this won't be bugzilla.kernel.org, as issues typically need to be sent
   by mail to a maintainer and a public mailing list.

 * Search the archives of the bug tracker or mailing list in question
   thoroughly for reports that might match your issue. Also check if you find
   something with your favorite internet search engine or in the Linux Kernel
   Mailing List (LKML) archives. If you find anything, join the discussion
   instead of sending a new report.

 * Create a fresh backup and put system repair and restore tools at hand.

 * Ensure your system does not enhance its kernels by building additional
   kernel modules on-the-fly, which solutions like DKMS might be doing locally
   without your knowledge.

 * Make sure it's not the kernel's surroundings that are causing the issue
   you face.

 * Write down coarsely how to reproduce the issue. If you deal with multiple
   issues at once, create separate notes for each of them and make sure they
   work independently on a freshly booted system. That's needed, as each issue
   needs to get reported to the kernel developers separately, unless they are
   strongly entangled.

After these preparations you'll now enter the main part:

 * Install the latest Linux mainline kernel: that's where all issues get
   fixed first, because it's the version line the kernel developers mainly
   care about. Testing and reporting with the latest Linux stable kernel can
   be an acceptable alternative in some situations, for example during the
   merge window; but during that period you might want to suspend your efforts
   till its end anyway.

 * Ensure the kernel you just installed does not 'taint' itself when
   running.

 * Reproduce the issue with the kernel you just installed. If it doesn't show
   up there, head over to the instructions for issues only happening with
   stable and longterm kernels.

 * Optimize your notes: try to find and write the most straightforward way to
   reproduce your issue. Make sure the end result has all the important
   details, and at the same time is easy to read and understand for others
   that hear about it for the first time. And if you learned something in this
   process, consider searching again for existing reports about the issue.

 * If your failure involves a 'panic', 'Oops', 'warning', or 'BUG', consider
   decoding the kernel log to find the line of code that triggered the error.

 * If your problem is a regression, try to narrow down when the issue was
   introduced as much as possible.

 * Start to compile the report by writing a detailed description about the
   issue. Always mention a few things: the latest kernel version you installed
   for reproducing, the Linux Distribution used, and your notes on how to
   reproduce the issue. Ideally, make the kernel's build configuration
   (.config) and the output from ``dmesg`` available somewhere on the net and
   link to it. Include or upload all other information that might be relevant,
   like the output/screenshot of an Oops or the output from ``lspci``. Once
   you wrote this main part, insert a normal length paragraph on top of it
   outlining the issue and the impact quickly. On top of this add one sentence
   that briefly describes the problem and gets people to read on. Now give the
   thing a descriptive title or subject that yet again is shorter. Then you're
   ready to send or file the report like the MAINTAINERS file told you, unless
   you are dealing with one of those 'issues of high priority': they need
   special care which is explained in 'Special handling for high priority
   issues' below.

 * Wait for reactions and keep the thing rolling until you can accept the
   outcome in one way or the other. Thus react publicly and in a timely manner
   to any inquiries. Test proposed fixes. Do proactive testing: retest with at
   least every first release candidate (RC) of a new mainline version and
   report your results. Send friendly reminders if things stall. And try to
   help yourself, if you don't get any help or if it's unsatisfying.


Reporting issues only occurring in older kernel version lines
-------------------------------------------------------------

This section is for you, if you tried the latest mainline kernel as outlined
above, but failed to reproduce your issue there; at the same time you want to
see the issue fixed in older version lines or a vendor kernel that's regularly
rebased on new stable or longterm releases. If that case follow these steps:

 * Prepare yourself for the possibility that going through the next few steps
   might not get the issue solved in older releases: the fix might be too big
   or risky to get backported there.

 * Check if the kernel developers still maintain the Linux kernel version
   line you care about: go to the front page of kernel.org and make sure it
   mentions the latest release of the particular version line without an
   '[EOL]' tag.

 * Check the archives of the Linux stable mailing list for existing reports.

 * Install the latest release from the particular version line as a vanilla
   kernel. Ensure this kernel is not tainted and still shows the problem, as
   the issue might have already been fixed there.

 * Search the Linux kernel version control system for the change that fixed
   the issue in mainline, as its commit message might tell you if the fix is
   scheduled for backporting already. If you don't find anything that way,
   search the appropriate mailing lists for posts that discuss such an issue
   or peer-review possible fixes; then check the discussions if the fix was
   deemed unsuitable for backporting. If backporting was not considered at
   all, join the newest discussion, asking if it's in the cards.

 * Check if you're dealing with a regression that was never present in
   mainline by installing the first release of the version line you care
   about. If the issue doesn't show up with it, you basically need to report
   the issue with this version like you would report a problem with mainline
   (see above). This ideally includes a bisection followed by a search for
   existing reports on the net; with the help of the subject and the two
   relevant commit-ids. If that doesn't turn up anything, write the report; CC
   or forward the report to the stable maintainers, the stable mailing list,
   and those who authored the change. Include the shortened commit-id if you
   found the change that causes it.

 * One of the former steps should lead to a solution. If that doesn't work
   out, ask the maintainers for the subsystem that seems to be causing the
   issue for advice; CC the mailing list for the particular subsystem as well
   as the stable mailing list.


Reference section: Reporting issues to the kernel maintainers
=============================================================

The detailed guides above outline all the major steps in brief fashion, which
should be enough for most people. But sometimes there are situations where even
experienced users might wonder how to actually do one of those steps. That's
what this section is for, as it will provide a lot more details on each of the
above steps. Consider this as reference documentation: it's possible to read it
from top to bottom. But it's mainly meant to skim over and a place to look up
details how to actually perform those steps.

A few words of general advice before digging into the details:

 * The Linux kernel developers are well aware this process is complicated and
   demands more than other FLOSS projects. We'd love to make it simpler. But
   that would require work in various places as well as some infrastructure,
   which would need constant maintenance; nobody has stepped up to do that
   work, so that's just how things are for now.

 * A warranty or support contract with some vendor doesn't entitle you to
   request fixes from developers in the upstream Linux kernel community: such
   contracts are completely outside the scope of the Linux kernel, its
   development community, and this document. That's why you can't demand
   anything such a contract guarantees in this context, not even if the
   developer handling the issue works for the vendor in question. If you want
   to claim your rights, use the vendor's support channel instead. When doing
   so, you might want to mention you'd like to see the issue fixed in the
   upstream Linux kernel; motivate them by saying it's the only way to ensure
   the fix in the end will get incorporated in all Linux distributions.

 * If you never reported an issue to a FLOSS project before you should consider
   reading `How to Report Bugs Effectively
   <https://www.chiark.greenend.org.uk/~sgtatham/bugs.html>`_, `How To Ask
   Questions The Smart Way
   <http://www.catb.org/esr/faqs/smart-questions.html>`_, and `How to ask good
   questions <https://jvns.ca/blog/good-questions/>`_.

With that off the table, find below the details on how to properly report
issues to the Linux kernel developers.


Make sure you're using the upstream Linux kernel
------------------------------------------------

   *Stop reading this document and report the problem to your vendor instead,
   unless you are running the latest mainline kernel already or are willing to
   install it. This kernel must not be modified or enhanced in any way, and
   thus be considered 'vanilla'.*

Like most programmers, Linux kernel developers don't like to spend time dealing
with reports for issues that don't even happen with the source code they
maintain: it's just a waste everybody's time, yours included. That's why you
later will have to test your issue with the latest 'vanilla' kernel: a kernel
that was build using the Linux sources taken straight from `kernel.org
<https://kernel.org/>`_ and not modified or enhanced in any way.

Almost all kernels used in devices (Computers, Laptops, Smartphones, Routers,
…) and most kernels shipped by Linux distributors are ancient from the point of
kernel development and heavily modified. They thus do not qualify for reporting
an issue to the Linux kernel developers: the issue you face with such a kernel
might be fixed already or caused by the changes or additions, even if they look
small or totally unrelated. That's why issues with such kernels need to be
reported to the vendor that distributed it. Its developers should look into the
report and, in case it turns out to be an upstream issue, fix it directly
upstream or report it there. In practice that sometimes does not work out. If
that the case, you might want to circumvent the vendor by installing the latest
mainline kernel yourself and reporting the issue as outlined in this document;
just make sure to use really fresh kernel (see below).


.. note::

   FIXME: Should we accept reports for issues with kernel images that are pretty
   close to vanilla? But when are they close enough and how to put that line in
   words? Maybe something like this?

    *Note: Some Linux kernel developers accept reports from vendor kernels that
    are known to be close to upstream. That for example is often the case for
    the kernels that Debian GNU/Linux Sid or Fedora Rawhide ship, which are
    normally following mainline closely and carry only a few patches. So a
    report with one of these might be accepted by the developers that need to
    handle it. But if they do, depends heavily on the individual developers and
    the issue at hand. That's why installing a mainline vanilla kernel is the
    safe bet.*

    *Arch Linux, other Fedora releases, and openSUSE Tumbleweed often use quite
    recent stable kernels that are pretty close to upstream, too. Some
    developers accept bugs from them as well. But note that you normally should
    avoid stable kernels for reporting issues and use a mainline kernel instead
    (see below).*

   Are there any other major Linux distributions that should be mentioned here?


Issue of high priority?
-----------------------

    *See if the issue you are dealing with qualifies as regression, security
    issue, or a really severe problem: those are 'issues of high priority' that
    need special handling in some steps that are about to follow.*

Linus Torvalds and the leading Linux kernel developers want to see some issues
fixed as soon as possible, hence there are 'issues of high priority' that get
handled slightly differently in the reporting process. Three type of cases
qualify: regressions, security issues, and really severe problems.

You deal with a 'regression' if something that worked with an older version of
the Linux kernel does not work with a newer one or somehow works worse with it.
It thus is a regression when a WiFi driver that did a fine job with Linux 5.7
somehow misbehaves with 5.8 or doesn't work at all. It's also a regression if
an application shows erratic behavior with a newer kernel, which might happen
due to incompatible changes in the interface between the kernel and the
userland (like procfs and sysfs). Significantly reduced performance or
increased power consumption also qualify as regression. But keep in mind: the
new kernel needs to be built with a configuration that is similar to the one
from the old kernel (see below how to achieve that). That's because the kernel
developers sometimes can not avoid incompatibilities when implementing new
features; but to avoid regressions such features have to be enabled explicitly
during build time configuration.

What qualifies as security issue is left to your judgment. Consider reading
'Documentation/admin-guide/security-bugs.rst' before proceeding, as it
provides additional details how to best handle security issues.

An issue is a 'really severe problem' when something totally unacceptably bad
happens. That's for example the case when a Linux kernel corrupts the data it's
handling or damages hardware it's running on. You're also dealing with a severe
issue when the kernel suddenly stops working with an error message ('kernel
panic') or without any farewell note at all. Note: do not confuse a 'panic' (a
fatal error where the kernel stop itself) with a 'Oops' (a recoverable error),
as the kernel remains running after the latter.


Check 'taint' flag
------------------

    *Check if your kernel was 'tainted' when the issue occurred, as the event
    that made the kernel set this flag might be causing the issue you face.*

The kernel marks itself with a 'taint' flag when something happens that might
lead to follow-up errors that look totally unrelated. The issue you face might
be such an error if your kernel is tainted. That's why it's in your interest to
rule this out early before investing more time into this process. This is the
only reason why this step is here, as this process later will tell you to
install the latest mainline kernel; you will need to check the taint flag again
then, as that's when it matters because it's the kernel the report will focus
on.

On a running system is easy to check if the kernel tainted itself: if ``cat
/proc/sys/kernel/tainted`` returns '0' then the kernel is not tainted and
everything is fine. Checking that file is impossible in some situations; that's
why the kernel also mentions the taint status when it reports an internal
problem (a 'kernel bug'), a recoverable error (a 'kernel Oops') or a
non-recoverable error before halting operation (a 'kernel panic'). Look near
the top of the error messages printed when one of these occurs and search for a
line starting with 'CPU:'. It should end with 'Not tainted' if the kernel was
not tainted when it noticed the problem; it was tainted if you see 'Tainted:'
followed by a few spaces and some letters.

If your kernel is tainted, study 'Documentation/admin-guide/tainted-kernels.rst'
to find out why. Try to eliminate the reason. Often it's caused by one these
three things:

 1. A recoverable error (a 'kernel Oops') occurred and the kernel tainted
    itself, as the kernel knows it might misbehave in strange ways after that
    point. In that case check your kernel or system log and look for a section
    that starts with this::

       Oops: 0000 [#1] SMP

    That's the first Oops since boot-up, as the '#1' between the brackets shows.
    Every Oops and any other problem that happens after that point might be a
    follow-up problem to that first Oops, even if both look totally unrelated.
    Rule this out by getting rid of the cause for the first Oops and reproducing
    the issue afterwards. Sometimes simply restarting will be enough, sometimes
    a change to the configuration followed by a reboot can eliminate the Oops.
    But don't invest too much time into this at this point of the process, as
    the cause for the Oops might already be fixed in the newer Linux kernel
    version you are going to install later in this process.

 2. Your system uses a software that installs its own kernel modules, for
    example Nvidia's proprietary graphics driver or VirtualBox. The kernel
    taints itself when it loads such module from external sources (even if
    they are Open Source): they sometimes cause errors in unrelated kernel
    areas and thus might be causing the issue you face. You therefore have to
    prevent those modules from loading when you want to report an issue to the
    Linux kernel developers. Most of the time the easiest way to do that is:
    temporarily uninstall such software including any modules they might have
    installed. Afterwards reboot.

 3. The kernel also taints itself when it's loading a module that resides in
    the staging tree of the Linux kernel source. That's a special area for
    code (mostly drivers) that does not yet fulfill the normal Linux kernel
    quality standards. When you report an issue with such a module it's
    obviously okay if the kernel is tainted; just make sure the module in
    question is the only reason for the taint. If the issue happens in an
    unrelated area reboot and temporarily block the module from being loaded
    by specifying ``foo.blacklist=1`` as kernel parameter (replace 'foo' with
    the name of the module in question).


Locate kernel area that causes the issue
----------------------------------------

    *Locate the driver or kernel subsystem that seems to be causing the issue.
    Find out how and where its developers expect reports. Note: most of the
    time this won't be bugzilla.kernel.org, as issues typically need to be sent
    by mail to a maintainer and a public mailing list.*

It's crucial to send your report to the right people, as the Linux kernel is a
big project and most of its developers are only familiar with a small subset of
it. Quite a few programmers for example only care for just one driver, for
example one for a WiFi chip; its developer likely will only have small or no
knowledge about the internals of remote or unrelated "subsystems", like the TCP
stack, the PCIe/PCI subsystem, memory management or file systems.

Problem is: the Linux kernel lacks a central bug tracker where you can simply
file your issue and make it reach the developers that need to know about it.
That's why you have to find the right place and way to report issues yourself.
You can do that with the help of a script (see below), but it mainly targets
kernel developers and experts. For everybody else the MAINTAINERS file is the
better place.

How to read the MAINTAINERS file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To illustrate how to use the :ref:`MAINTAINERS <maintainers>` file, lets assume
the WiFi in your Laptop suddenly misbehaves after updating the kernel. In that
case it's likely an issue in the WiFi driver. Obviously it could also be some
code it builds upon, but unless you suspect something like that stick to the
driver. If it's really something else, the driver's developers will get the
right people involved.

Sadly, there is no way to check which code is driving a particular hardware
component that is both universal and easy.

In case of a problem with the WiFi driver you for example might want to look at
the output of ``lspci -k``, as it lists devices on the PCI/PCIe bus and the
kernel module driving it::

       [user@something ~]$ lspci -k
       [...]
       3a:00.0 Network controller: Qualcomm Atheros QCA6174 802.11ac Wireless Network Adapter (rev 32)
         Subsystem: Bigfoot Networks, Inc. Device 1535
         Kernel driver in use: ath10k_pci
         Kernel modules: ath10k_pci
       [...]

But this approach won't work if your WiFi chip is connected over USB or some
other internal bus. In those cases you might want to check your WiFi manager or
the output of ``ip link``. Look for the name of the problematic network
interface, which might be something like 'wlp58s0'. This name can be used like
this to find the module driving it::

       [user@something ~]$ realpath --relative-to=/sys/module/ /sys/class/net/wlp58s0/device/driver/module
       ath10k_pci

In case tricks like these don't bring you any further, try to search the
internet on how to narrow down the driver or subsystem in question. And if you
are unsure which it is: just try your best guess, somebody will help you if you
guessed poorly.

Once you know the driver or subsystem, you want to search for it in the
MAINTAINERS file. In the case of 'ath10k_pci' you won't find anything, as the
name is too specific. Sometimes you will need to search on the net for help;
but before doing so, try a somewhat shorted or modified name when searching the
MAINTAINERS file, as then you might find something like this::

       QUALCOMM ATHEROS ATH10K WIRELESS DRIVER
       Mail:          A. Some Human <shuman@example.com>
       Mailing list:  ath10k@lists.infradead.org
       Status:        Supported
       Web-page:      https://wireless.wiki.kernel.org/en/users/Drivers/ath10k
       SCM:           git git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git
       Files:         drivers/net/wireless/ath/ath10k/

Note: the line description will be abbreviations, if you read the plain
MAINTAINERS file found in the root of the Linux source tree. 'Mail:' for
example will be 'M:', 'Mailing list:' will be 'L', and 'Status:' will be 'S:'.
A section near the top of the file explains these and other abbreviations.

First look at the line 'Status'. Ideally it should be 'Supported' or
'Maintained'. If it states 'Obsolete' then you are using some outdated approach
that was replaced by a newer solution you need to switch to. Sometimes the code
only has someone who provides 'Odd Fixes' when feeling motivated. And with
'Orphan' you are totally out of luck, as nobody takes care of the code anymore.
That only leaves these options: arrange yourself to live with the issue, fix it
yourself, or find a programmer somewhere willing to fix it.

After checking the status, look for a line starting with 'bugs:': it will tell
you where to find a subsystem specific bug tracker to file your issue. The
example above does not have such a line. That is the case for most sections, as
Linux kernel development is completely driven by mail. Very few subsystems use
a bug tracker, and only some of those rely on bugzilla.kernel.org.


.. note::

   FIXME: The old text took a totally different approach to bugzilla.kernel.org,
   as it mentions it as the place to file issue for people that don't known how
   to contact the appropriate people. The new one mentions it rarely; and when
   it does like here, it warns users that it's often the wrong place to go.

   This approach was chosen as the main author of this document noticed quite a
   few users (or even a lot?) get no reply to the bugs they file in bugzilla.
   That's kind of expected, as quite a few (many? most?) of the maintainers
   don't even get notified when reports for their subsystem get filed there. And
   not getting a single reply to report is something that is just annoying for
   users and might make them angry. Improving bugzilla.k.o would be an option,
   but on the kernel and maintainers summit 2017 it was agreed on to first go
   this route (sorry it took so long): it's easier to achieve and less
   controversial, as putting additional burden on already overworked maintainers
   is unlikely to get well received.


In this and many other cases you thus have to look for lines starting with
'Mail:' instead. Those mention the name and the email addresses for the
maintainers of the particular code. Also look for a line starting with 'Mailing
list:', which tells you the public mailing list where the code is developed.
Your report later needs to go by mail to those addresses. Additionally, for all
issue reports sent by email, make sure to add the Linux Kernel Mailing List
(LKML) <linux-kernel@vger.kernel.org> to CC. Don't omit either of the mailing
lists when sending your issue report by mail later! Maintainers are busy people
and might leave some work for other developers on the subsystem specific list;
and LKML is important to have one place where all issue reports can be found.


.. note::

   FIXME: Above section tells users to always CC LKML. These days it's a kind of
   "catch-all" list anyway, which nearly nobody seems to follow closely. So it
   seems appropriate to go "all in" and make people send their reports here,
   too, as everything (reports, fixes, ...) then can be found in one place (at
   least for all reports sent by mail and all subsystems that CC LKML).

   Related: Should we create mailing list like 'linux-issues@vger.kernel.org'
   and tell users above to always CC it when reporting issues? Then there would
   be one central place reporters could search for existing reports (at least
   for issues reported by mail) without getting regular LKML traffic mixed into
   the results.


Finding the maintainers with the help of a script
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For people that have the Linux sources at hand there is a second option to find
the proper place to report: the script 'scripts/get_maintainer.pl' which tries
to find all people to contact. It queries the MAINTAINERS file and needs to be
called with a path to the source code in question. For drivers compiled as
module if often can be found with a command like this::

       $ modinfo ath10k_pci | grep filename | sed 's!/lib/modules/.*/kernel/!!; s!filename:!!; s!\.ko\(\|\.xz\)!!'
       drivers/net/wireless/ath/ath10k/ath10k_pci.ko

Pass parts of this to the script::

       $ ./scripts/get_maintainer.pl -f drivers/net/wireless/ath/ath10k*
       Some Human <shuman@example.com> (supporter:QUALCOMM ATHEROS ATH10K WIRELESS DRIVER)
       Another S. Human <asomehuman@example.com> (maintainer:NETWORKING DRIVERS)
       ath10k@lists.infradead.org (open list:QUALCOMM ATHEROS ATH10K WIRELESS DRIVER)
       linux-wireless@vger.kernel.org (open list:NETWORKING DRIVERS (WIRELESS))
       netdev@vger.kernel.org (open list:NETWORKING DRIVERS)
       linux-kernel@vger.kernel.org (open list)

Don't sent your report to all of them. Send it to the maintainers, which the
script calls "supporter:"; additionally CC the most specific mailing list for
the code as well as the Linux Kernel Mailing List (LKML). In this case you thus
would need to send the report to 'Some Human <shuman@example.com>' with
'ath10k@lists.infradead.org' and 'linux-kernel@vger.kernel.org' in CC.

Note: in case you cloned the Linux sources with git you might want to call
``get_maintainer.pl`` a second time with ``--git``. The script then will look
at the commit history to find which people recently worked on the code in
question, as they might be able to help. But use these results with care, as it
can easily send you in a wrong direction. That for example happens quickly in
areas rarely changed (like old or unmaintained drivers): sometimes such code is
modified during tree-wide cleanups by developers that do not care about the
particular driver at all.


Search for existing reports
---------------------------

    *Search the archives of the bug tracker or mailing list in question
    thoroughly for reports that might match your issue. Also check if you find
    something with your favorite internet search engine or in the Linux Kernel
    Mailing List (LKML) archives. If you find anything, join the discussion
    instead of sending a new report.*

Reporting an issue that someone else already brought forward is often a waste
of time for everyone involved, especially you as the reporter. So it's in your
own interest to thoroughly check if somebody reported the issue already. Thus
do not hurry with this step of the reporting process. Spending 30 to 60 minutes
or even more time can save you and others quite a lot of time and trouble.

The best place to search is the bug tracker or the mailing list where your
report needs to be filed. You'll find quite a few of those lists on
`lore.kernel.org <https://lore.kernel.org/>`_, but some are hosted in
different places. That for example is the case for the ath10k WiFi driver used
as example in the previous step. But you'll often find the archives for these
lists easily on the net. Searching for 'archive ath10k@lists.infradead.org' for
example will quickly lead you to the `Info page for the ath10k mailing list
<https://lists.infradead.org/mailman/listinfo/ath10k>`_, which at the top links
to its `list archives <https://lists.infradead.org/pipermail/ath10k/>`_.

Sadly this and quite a few other lists miss a way to search the archives. In
those cases use a regular internet search engine and add something like
'site:lists.infradead.org/pipermail/ath10k/' to your search terms, which limits
the results to the archives at that URL.

Additionally, search the internet and the `Linux Kernel Mailing List (LKML)
archives <https://lore.kernel.org/lkml/>`_, as maybe the real culprit might be
in some other subsystem. Searching in `bugzilla.kernel.org
<https://bugzilla.kernel.org/>`_ might also be a good idea, but if you find
anything there keep in mind: most subsystems expect reports in different
places, hence those you find there might have not even reached the people
responsible for the subsystem in question. Nevertheless, the data there might
provide valuable insights.

If you get flooded with results consider telling your search engine to limit
search timeframe to the past month or year. And wherever you search, make sure
to use good search terms; vary them a few times, too. While doing so try to
look at the issue from the perspective of someone else: that will help you to
come up with other words to use as search terms. Also make sure not to use too
many search terms at once. Remember to search with and without information like
the name of the kernel driver or the name of the affected hardware component.
But its exact brand name (say 'ASUS Red Devil Radeon RX 5700 XT Gaming OC')
often is not much helpful, as it is too specific. Instead try search terms like
the model line (Radeon 5700 or Radeon 5000) and the code name of the main chip
('Navi' or 'Navi10') with and without its manufacturer ('AMD').

In case you find an existing report about your issue, join the discussion, as
you might be able to provide valuable additional information. That can be
important even when a fix is prepared or in its final stages already, as
developers might look for people that can provide additional information or
test a proposed fix. Jump to the section 'Duties after the report went out' for
details on how to get properly involved.


Prepare for emergencies
-----------------------

    *Create a fresh backup and put system repair and restore tools at hand.*

Reminder, you are dealing with computers, which sometimes do unexpected things,
especially if you fiddle with crucial parts like the kernel of its operating
system. That's what you are about to do in this process. Thus, make sure to
create a fresh backup; also ensure you have all tools at hand to repair or
reinstall the operating system as well as everything you need to restore the
backup.


Make sure your kernel doesn't get enhanced
------------------------------------------

    *Ensure your system does not enhance its kernels by building additional
    kernel modules on-the-fly, which solutions like DKMS might be doing locally
    without your knowledge.*

Your kernel must be 'vanilla' when reporting an issue, but stops being pure as
soon as it loads a kernel module not built from the sources used to compile the
kernel image itself. That's why you need to ensure your Linux kernel stays
vanilla by removing or disabling mechanisms like akmods and DKMS: those might
build additional kernel modules automatically, for example when your boot into
a newly installed Linux kernel the first time. Reboot after removing them and
any modules they installed.

Note, you might not be aware that your system is using one of these solutions:
they often get set up silently when you install Nvidia's proprietary graphics
driver, VirtualBox, or other software that requires a some support from a
module not part of the Linux kernel. That why your might need to uninstall the
packages with such software to get rid of any 3rd party kernel module.


Ensure a healthy environment
----------------------------

    *Make sure it's not the kernel's surroundings that are causing the issue
    you face.*

Problems that look a lot like a kernel issue are sometimes caused by build or
runtime environment. It's hard to rule out that problem completely, but you
should minimize it:

 * Use proven tools when building your kernel, as bugs in the compiler or the
   binutils can cause the resulting kernel to misbehave.

 * Ensure your computer components run within their design specifications;
   that's especially important for the main processor, the main memory, and the
   motherboard. Therefore, stop undervolting or overclocking when facing a
   potential kernel issue.

 * Try to make sure it's not faulty hardware that is causing your issue. Bad
   main memory for example can result in a multitude of issues that will
   manifest itself in problems looking like kernel issues.

 * If you're dealing with a filesystem issue, you might want to check the file
   system in question with ``fsck``, as it might be damaged in a way that leads
   to unexpected kernel behavior.

 * When dealing with a regression, make sure it's not something else that
   changed in parallel to updating the kernel. The problem for example might be
   caused by other software that was updated at the same time. It can also
   happen that a hardware component coincidentally just broke when you rebooted
   into a new kernel for the first time. Updating the systems BIOS or changing
   something in the BIOS Setup can also lead to problems that on look a lot
   like a kernel regression.


Document how to reproduce issue
-------------------------------

    *Write down coarsely how to reproduce the issue. If you deal with multiple
    issues at once, create separate notes for each of them and make sure they
    work independently on a freshly booted system. That's needed, as each issue
    needs to get reported to the kernel developers separately, unless they are
    strongly entangled.*

If you deal with multiple issues at once, you'll have to report each of them
separately, as they might be handled by different developers. Describing
various issues in one report also makes it quite difficult for others to tear
it apart. Hence, only combine issues in one report if they are very strongly
entangled.

Additionally, during the reporting process you will have to test if the issue
happens with other kernel versions. Therefore, it will make your work easier if
you know exactly how to reproduce an issue quickly on a freshly booted system.

Note: it's often fruitless to report issues that only happened once, as they
might be caused by a bit flip due to cosmic radiation. That's why you should
try to rule that out by reproducing the issue before going further. Feel free
to ignore this advice if you are experienced enough to tell a one-time error
due to faulty hardware apart from a kernel issue that rarely happens and thus
is hard to reproduce.


Install a fresh kernel for testing
----------------------------------

    *Install the latest Linux mainline kernel: that's where all issues get
    fixed first, because it's the version line the kernel developers mainly
    care about. Testing and reporting with the latest Linux stable kernel can
    be an acceptable alternative in some situations, for example during the
    merge window; but during that period you might want to suspend your efforts
    till its end anyway.*

Reporting an issue to the Linux kernel developers they fixed weeks or months
ago is annoying for them and wasting their and your time. That's why it's in
everybody's interest to check if the issue occurs with the latest codebase
before reporting it.

In the scope of the Linux kernel the term 'latest' means: a kernel version
recently created from the main line of development, as this 'mainline' tree is
where developers first apply fixes; only after that are they are allowed to get
backported to older, still supported version lines called 'stable' and
'longterm' kernels. That's why you should check a recent mainline kernel, even
if you deal with an issue you only want to see fixed in an older version line.
Another reason: some fixes are only applied to mainline or recent version
lines, as it's too hard or risky to backport them to older versions. If that
the case, reporting the issue again is unlikely to change anything.

Longterm kernels (sometimes called "LTS kernels") are therefore unsuitable for
testing; they simply are too distant from current development. Even the latest
Linux 'stable' kernel is a significant bit behind and thus better avoided. At
least most of the time, as sometimes a stable kernel can the best choice; but
in those situations you might want to wait a few days anyway:

Choosing between mainline, stable and waiting
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Head over to `kernel.org <https://kernel.org/>`_ to decide which version to
use. Ignore the big yellow button that says 'Latest release' and look a little
lower for a table. At its top you'll see a line starting with 'mainline', which
most of the time will point to a pre-release with a version number like
'5.8-rc2'. If that's the case, you'll want to use this mainline kernel for
testing. Do not let that 'rc' scare you, these 'development kernels' are pretty
reliable — and you made a backup, as you were instructed above, didn't you?

In about two out of every nine to ten weeks, 'mainline' might point you to a
proper release with a version number like '5.7'. If that happens, consider
suspending the reporting process until the first pre-release of the next
version (5.8-rc1) shows up on kernel.org. That's because the Linux development
cycle then is in its two-week long 'merge window'. The bulk of the changes and
all intrusive ones get merged for the next release during this time. It's a bit
more risky to use mainline during this period. Kernel developers are also often
quite busy then and might have no spare time to deal with issue reports. It's
also quite possible that one of the many changes applied during the merge
window fixes the issue you face; that's why you soon would have to retest with
a newer kernel version anyway, as outlined below in the section 'Duties after
the report went out'.

That's why it might make sense to wait till the merge window is over. But don't
to that if you're dealing with something that shouldn't wait. In that case
consider obtaining the latest mainline kernel via git (see below) or use the
latest stable version offered on kernel.org. Using that is also acceptable in
case mainline for some reason does currently not work for you. An in general:
using it for reproducing the issue is also better than not reporting it issue
at all.

How to obtain a fresh Linux kernel
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can use pre-built or self-compiled kernel for testing; if you choose the
latter approach, you can either obtain the source code using git or download it
as tar archive.

Using a pre-compiled kernel for testing is often the quickest, easiest, and
safest way – especially is you are unfamiliar with the Linux kernel. But it
needs to be a vanilla kernel, which can be hard to come buy. You are in luck if
you are using a popular Linux distribution: for quite a few of them you'll find
repositories on the net that contain packages with the latest mainline or
stable kernels in vanilla fashion. It's totally okay to use these, just make
sure from the repository's documentation they are really vanilla. And ensure
the packages contain the latest versions as offered on kernel.org; they are
likely unsuitable if the package is older than a week, as new mainline and
stable kernels typically get released at least once a week. And be aware that
you might need to get build your own kernel later anyway when it comes to
helping test fixes, as described later in this document.

Developers and experienced Linux users familiar with git are often best served
by obtaining the latest Linux kernel sources straight from the `official
development repository on kernel.org
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/>`_.
Those are likely a bit ahead of the latest mainline pre-release. Don't worry
about it: they are as reliable as a proper pre-release, unless the kernel's
development cycle is currently in the middle of a merge window. But even then
they are quite reliable.

People unfamiliar with git are often best served by downloading the sources as
tarball from `kernel.org <https://kernel.org/>`_.

How to actually build a kernel isnot described here, as many websites explain
the necessary steps already. If you are new to it, consider following one of
those how-to's that suggest to use ``make localmodconfig``, as that tries to
pick up the configuration of your current kernel and then tries to adjust it
somewhat for your system. That does not make the resulting kernel any better,
but quicker to compile.

Note: If you are dealing with a panic, Oops, warning, or BUG from the kernel,
please try to enable CONFIG_KALLSYMS when configuring your kernel.
Additionally, enable CONFIG_DEBUG_KERNEL and CONFIG_DEBUG_INFO, too; the
latter is the relevant one of those two, but can only be reached if you enable
the former. Be aware CONFIG_DEBUG_INFO increases the storage space required to
build a kernel by quite a bit. But that's worth it, as these options will allow
you later to pinpoint the exact line of code that triggers your issue. The
section 'Decode failure messages' below explains this in more detail.

But keep in mind: Always keep a record of the issue encountered in case it is
hard to reproduce. Sending an undecoded report is better than not reporting
the issue at all.


Check 'taint' flag
------------------

    *Ensure the kernel you just installed does not 'taint' itself when
    running.*

As outlined above in more detail already: the kernel sets a 'taint' flag when
something happens that can lead to follow-up errors that look totally
unrelated. That's why you need to check if the kernel you just installed does
not set this flag. And if it does, you in almost all the cases needs to
eliminate the reason for it before you reporting issues that occur with it. See
the section above for details how to do that.


Reproduce issue with the fresh kernel
-------------------------------------

    *Reproduce the issue with the kernel you just installed. If it doesn't show
    up there, head over to the instructions for issues only happening with
    stable and longterm kernels.*

Check if the issue occurs with the fresh Linux kernel version you just
installed. If it was fixed there already, consider sticking with this version
line and abandoning your plan to report the issue. But keep in mind that other
users might still be plagued by it, as long as it's not fixed in either stable
and longterm version from kernel.org (and thus vendor kernels derived from
those). If you prefer to use one of those or just want to help their users,
head over to the section "Details about reporting issues only occurring in
older kernel version lines" below.


Optimize description to reproduce issue
---------------------------------------

    *Optimize your notes: try to find and write the most straightforward way to
    reproduce your issue. Make sure the end result has all the important
    details, and at the same time is easy to read and understand for others
    that hear about it for the first time. And if you learned something in this
    process, consider searching again for existing reports about the issue.*

An unnecessarily complex report will make it hard for others to understand your
report. Thus try to find a reproducer that's straight forward to describe and
thus easy to understand in written form. Include all important details, but at
the same time try to keep it as short as possible.

In this in the previous steps you likely have learned a thing or two about the
issue you face. Use this knowledge and search again for existing reports
instead you can join.


Decode failure messages
-----------------------

    *If your failure involves a 'panic', 'Oops', 'warning', or 'BUG', consider
    decoding the kernel log to find the line of code that triggered the error.*

When the kernel detects an internal problem, it will log some information about
the executed code. This makes it possible to pinpoint the exact line in the
source code that triggered the issue and shows how it was called. But that only
works if you enabled CONFIG_DEBUG_INFO and CONFIG_KALLSYMS when configuring
your kernel. If you did so, consider to decode the information from the
kernel's log. That will make it a lot easier to understand what lead to the
'panic', 'Oops', 'warning', or 'BUG', which increases the chances that someone
can provide a fix.

Decoding can be done with a script you find in the Linux source tree. If you
are running a kernel you compiled yourself earlier, call it like this::

       [user@something ~]$ sudo dmesg | ./linux-5.10.5/scripts/decode_stacktrace.sh ./linux-5.10.5/vmlinux

If you are running a packaged vanilla kernel, you will likely have to install
the corresponding packages with debug symbols. Then call the script (which you
might need to get from the Linux sources if your distro does not package it)
like this::

       [user@something ~]$ sudo dmesg | ./linux-5.10.5/scripts/decode_stacktrace.sh \
        /usr/lib/debug/lib/modules/5.10.10-4.1.x86_64/vmlinux /usr/src/kernels/5.10.10-4.1.x86_64/

The script will work on log lines like the following, which show the address of
the code the kernel was executing when the error occurred::

       [   68.387301] RIP: 0010:test_module_init+0x5/0xffa [test_module]

Once decoded, these lines will look like this::

       [   68.387301] RIP: 0010:test_module_init (/home/username/linux-5.10.5/test-module/test-module.c:16) test_module

In this case the executed code was built from the file
'~/linux-5.10.5/test-module/test-module.c' and the error occurred by the
instructions found in line '16'.

The script will similarly decode the addresses mentioned in the section
starting with 'Call trace', which show the path to the function where the
problem occurred. Additionally, the script will show the assembler output for
the code section the kernel was executing.

Note, if you can't get this to work, simply skip this step and mention the
reason for it in the report. If you're lucky, it might not be needed. And if it
is, someone might help you to get things going. Also be aware this is just one
of several ways to decode kernel stack traces. Sometimes different steps will
be required to retrieve the relevant details. Don't worry about that, if that's
needed in your case, developers will tell you what to do.


Special care for regressions
----------------------------

    *If your problem is a regression, try to narrow down when the issue was
    introduced as much as possible.*

Linux lead developer Linus Torvalds insists that the Linux kernel never
worsens, that's why he deems regressions as unacceptable and wants to see them
fixed quickly. That's why changes that introduced a regression are often
promptly reverted if the issue they cause can't get solved quickly any other
way. Reporting a regression is thus a bit like playing a kind of trump card to
get something quickly fixed. But for that to happen the change that's causing
the regression needs to be known. Normally it's up to the reporter to track
down the culprit, as maintainers often won't have the time or setup at hand to
reproduce it themselves.

To find the change there is a process called 'bisection' which the document
'Documentation/admin-guide/bug-bisect.rst' describes in detail. That process
will often require you to build about ten to twenty kernel images, trying to
reproduce the issue with each of them before building the next. Yes, that takes
some time, but don't worry, it works a lot quicker than most people assume.
Thanks to a 'binary search' this will lead you to the one commit in the source
code management system that's causing the regression. Once you find it, search
the net for the subject of the change, its commit id and the shortened commit id
(the first 12 characters of the commit id). This will lead you to existing
reports about it, if there are any.

Note, a bisection needs a bit of know-how, which not everyone has, and quite a
bit of effort, which not everyone is willing to invest. Nevertheless, it's
highly recommended performing a bisection yourself. If you really can't or
don't want to go down that route at least find out which mainline kernel
introduced the regression. If something for example breaks when switching from
5.5.15 to 5.8.4, then try at least all the mainline releases in that area (5.6,
5.7 and 5.8) to check when it first showed up. Unless you're trying to find a
regression in a stable or longterm kernel, avoid testing versions which number
has three sections (5.6.12, 5.7.8), as that makes the outcome hard to
interpret, which might render your testing useless. Once you found the major
version which introduced the regression, feel free to move on in the reporting
process. But keep in mind: it depends on the issue at hand if the developers
will be able to help without knowing the culprit. Sometimes they might
recognize from the report want went wrong and can fix it; other times they will
be unable to help unless you perform a bisection.

When dealing with regressions make sure the issue you face is really caused by
the kernel and not by something else, as outlined above already.

In the whole process keep in mind: an issue only qualifies as regression if the
older and the newer kernel got built with a similar configuration. The best way
to archive this: copy the configuration file (``.config``) from the old working
kernel freshly to each newer kernel version you try. Afterwards run ``make
oldnoconfig`` to adjust it for the needs of the new version without enabling
any new feature, as those are allowed to cause regressions.


Write and send the report
-------------------------

    *Start to compile the report by writing a detailed description about the
    issue. Always mention a few things: the latest kernel version you installed
    for reproducing, the Linux Distribution used, and your notes on how to
    reproduce the issue. Ideally, make the kernel's build configuration
    (.config) and the output from ``dmesg`` available somewhere on the net and
    link to it. Include or upload all other information that might be relevant,
    like the output/screenshot of an Oops or the output from ``lspci``. Once
    you wrote this main part, insert a normal length paragraph on top of it
    outlining the issue and the impact quickly. On top of this add one sentence
    that briefly describes the problem and gets people to read on. Now give the
    thing a descriptive title or subject that yet again is shorter. Then you're
    ready to send or file the report like the MAINTAINERS file told you, unless
    you are dealing with one of those 'issues of high priority': they need
    special care which is explained in 'Special handling for high priority
    issues' below.*

Now that you have prepared everything it's time to write your report. How to do
that is partly explained by the three documents linked to in the preface above.
That's why this text will only mention a few of the essentials as well as
things specific to the Linux kernel.

There is one thing that fits both categories: the most crucial parts of your
report are the title/subject, the first sentence, and the first paragraph.
Developers often get quite a lot of mail. They thus often just take a few
seconds to skim a mail before deciding to move on or look closer. Thus: the
better the top section of your report, the higher are the chances that someone
will look into it and help you. And that is why you should ignore them for now
and write the detailed report first. ;-)

Things each report should mention
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Describe in detail how your issue happens with the fresh vanilla kernel you
installed. Try to include the step-by-step instructions you wrote and optimized
earlier that outline how you and ideally others can reproduce the issue; in
those rare cases where that's impossible try to describe what you did to
trigger it.

Also include all the relevant information others might need to understand the
issue and its environment. What's actually needed depends a lot on the issue,
but there are some things you should include always:

 * the output from ``cat /proc/version``, which contains the Linux kernel
   version number and the compiler it was built with.

 * the Linux distribution the machine is running (``hostnamectl | grep
   "Operating System"``)

 * the architecture of the CPU and the operating system (``uname -mi``)

 * if you are dealing with a regression and performed a bisection, mention the
   subject and the commit-id of the change that is causing it.

In a lot of cases it's also wise to make two more things available to those
that read your report:

 * the configuration used for building your Linux kernel (the '.config' file)

 * the kernel's messages that you get from ``dmesg`` written to a file. Make
   sure that it starts with a line like 'Linux version 5.8-1
   (foobar@example.com) (gcc (GCC) 10.2.1, GNU ld version 2.34) #1 SMP Mon Aug
   3 14:54:37 UTC 2020' If it's missing, then important messages from the first
   boot phase already got discarded. In this case instead consider using
   ``journalctl -b 0 -k``; alternatively you can also reboot, reproduce the
   issue and call ``dmesg`` right afterwards.

These two files are big, that's why it's a bad idea to put them directly into
your report. If you are filing the issue in a bug tracker then attach them to
the ticket. If you report the issue by mail do not attach them, as that makes
the mail too large; instead do one of these things:

 * Upload the files somewhere public (your website, a public file paste
   service, a ticket created just for this purpose on `bugzilla.kernel.org
   <https://bugzilla.kernel.org/>`_, ...) and include a link to them in your
   report. Ideally use something where the files stay available for years, as
   they could be useful to someone many years from now; this for example can
   happen if five or ten years from now a developer works on some code that was
   changed just to fix your issue.

 * Put the files aside and mention you will send them later in individual
   replies to your own mail. Just remember to actually do that once the report
   went out. ;-)

Things that might be wise to provide
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Depending on the issue you might need to add more background data. Here are a
few suggestions what often is good to provide:

 * If you are dealing with a 'warning', an 'OOPS' or a 'panic' from the kernel,
   include it. If you can't copy'n'paste it, try to capture a netconsole trace
   or at least take a picture of the screen.

 * If the issue might be related to your computer hardware, mention what kind
   of system you use. If you for example have problems with your graphics card,
   mention its manufacturer, the card's model, and what chip is uses. If it's a
   laptop mention its name, but try to make sure it's meaningful. 'Dell XPS 13'
   for example is not, because it might be the one from 2012; that one looks
   not that different from the one sold today, but apart from that the two have
   nothing in common. Hence, in such cases add the exact model number, which
   for example are '9380' or '7390' for XPS 13 models introduced during 2019.
   Names like 'Lenovo Thinkpad T590' are also somewhat ambiguous: there are
   variants of this laptop with and without a dedicated graphics chip, so try
   to find the exact model name or specify the main components.

 * Mention the relevant software in use. If you have problems with loading
   modules, you want to mention the versions of kmod, systemd, and udev in use.
   If one of the DRM drivers misbehaves, you want to state the versions of
   libdrm and Mesa; also specify your Wayland compositor or the X-Server and
   its driver. If you have a filesystem issue, mention the version of
   corresponding filesystem utilities (e2fsprogs, btrfs-progs, xfsprogs, ...).

 * Gather additional information from the kernel that might be of interest. The
   output from ``lspci -nn`` will for example help others to identify what
   hardware you use. If you have a problem with hardware you even might want to
   make the output from ``sudo lspci -vvv`` available, as that provides
   insights how the components were configured. For some issues it might be
   good to include the contents of files like ``/proc/cpuinfo``,
   ``/proc/ioports``, ``/proc/iomem``, ``/proc/modules``, or
   ``/proc/scsi/scsi``. Some subsystem also offer tools to collect relevant
   information. One such tool is ``alsa-info.sh`` `which the audio/sound
   subsystem developers provide <https://www.alsa-project.org/wiki/AlsaInfo>`_.

Those examples should give your some ideas of what data might be wise to
attach, but you have to think yourself what will be helpful for others to know.
Don't worry too much about forgetting something, as developers will ask for
additional details they need. But making everything important available from
the start increases the chance someone will take a closer look.


The important part: the head of your report
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now that you have the detailed part of the report prepared let's get to the
most important section: the first few sentences. Thus go to the top, add
something like 'The detailed description:' before the part you just wrote and
insert two newlines at the top. Now write one normal length paragraph that
describes the issue roughly. Leave out all boring details and focus on the
crucial parts readers need to know to understand what this is all about; if you
think this bug affects a lot of users, mention this to get people interested.

Once you did that insert two more lines at the top and write a one sentence
summary that explains quickly what the report is about. After that you have to
get even more abstract and write an even shorter subject/title for the report.

Now that you have written this part take some time to optimize it, as it is the
most important parts of your report: a lot of people will only read this before
they decide if reading the rest is time well spent.

Now send or file the report like the :ref:`MAINTAINERS <maintainers>` file told
you, unless it's one of those 'issues of high priority' outlined earlier: in
that case please read the next subsection first before sending the report on
its way.

Special handling for high priority issues
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reports for high priority issues need special handling.

**Severe bugs**: make sure the subject or ticket title as well as the first
paragraph makes the severeness obvious.

**Regressions**: If the issue is a regression add [REGRESSION] to the mail's
subject or the title in the bug-tracker. If you did not perform a bisection
mention at least the latest mainline version you tested that worked fine (say
5.7) and the oldest where the issue occurs (say 5.8). If you did a successful
bisection mention the commit id and subject of the change that causes the
regression. Also make sure to add the author of that change to your report; if
you need to file your bug in a bug-tracker forward the report to him in a
private mail and mention where your filed it.

**Security issues**: for these issues your will have to evaluate if a
short-term risk to other users would arise if details were publicly disclosed.
If that's not the case simply proceed with reporting the issue as described.
For issues that bear such a risk you will need to adjust the reporting process
slightly:

 * If the MAINTAINERS file instructed you to report the issue by mail, do not
   CC any public mailing lists.

 * If you were supposed to file the issue in a bug tracker make sure to mark
   the ticket as 'private' or 'security issue'. If the bug tracker does not
   offer a way to keep reports private, forget about it and send your report as
   a private mail to the maintainers instead.

In both cases make sure to also mail your report to the addresses the
MAINTAINERS file lists in the section 'security contact'. Ideally directly CC
them when sending the report by mail. If you filed it in a bug tracker, forward
the report's text to these addresses; but on top of it put a small note where
you mention that you filed it with a link to the ticket.

See 'Documentation/admin-guide/security-bugs.rst' for more information.


Duties after the report went out
--------------------------------

    *Wait for reactions and keep the thing rolling until you can accept the
    outcome in one way or the other. Thus react publicly and in a timely manner
    to any inquiries. Test proposed fixes. Do proactive testing: retest with at
    least every first release candidate (RC) of a new mainline version and
    report your results. Send friendly reminders if things stall. And try to
    help yourself, if you don't get any help or if it's unsatisfying.*

If your report was good and you are really lucky then one of the developers
might immediately spot what's causing the issue; they then might write a patch
to fix it, test it, and send it straight for integration in mainline while
tagging it for later backport to stable and longterm kernels that need it. Then
all you need to do is reply with a 'Thank you very much' and switch to a version
with the fix once it gets released.

But this ideal scenario rarely happens. That's why the job is only starting
once you got the report out. What you'll have to do depends on the situations,
but often it will be the things listed below. But before digging into the
details, here are a few important things you need to keep in mind for this part
of the process.


General advice for further interactions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**Always reply in public**: When you filed the issue in a bug tracker, always
reply there and do not contact any of the developers privately about it. For
mailed reports always use the 'Reply-all' function when replying to any mails
you receive. That includes mails with any additional data you might want to add
to your report: go to your mail applications 'Sent' folder and use 'reply-all'
on your mail with the report. This approach will make sure the public mailing
list(s) and everyone else that gets involved over time stays in the loop; it
also keeps the mail thread intact, which among others is really important for
mailing lists to group all related mails together.

There are just two situations where a comment in a bug tracker or a 'Reply-all'
is unsuitable:

 * Someone tells you to send something privately.

 * You were told to send something, but noticed it contains sensitive
   information that needs to be kept private. In that case it's okay to send it
   in private to the developer that asked for it. But note in the ticket or a
   mail that you did that, so everyone else knows you honored the request.

**Do research before asking for clarifications or help**: In this part of the
process someone might tell you to do something that requires a skill you might
not have mastered yet. For example, you might be asked to use some test tools
you never have heard of yet; or you might be asked to apply a patch to the
Linux kernel sources to test if it helps. In some cases it will be fine sending
a reply asking for instructions how to do that. But before going that route try
to find the answer own your own by searching the internet; alternatively
consider asking in other places for advice. For example ask a fried or post
about it to a chatroom or forum you normally hang out.

**Be patient**: If you are really lucky you might get a reply to your report
within a few hours. But most of the time it will take longer, as maintainers
are scattered around the globe and thus might be in a different time zone – one
where they already enjoy their night away from keyboard.

In general, kernel developers will take one to five business days to respond to
reports. Sometimes it will take longer, as they might be busy with the merge
windows, other work, visiting developer conferences, or simply enjoying a long
summer holiday.

The 'issues of high priority' (see above for an explanation) are an exception
here: maintainers should address them as soon as possible; that's why you
should wait a week at maximum (or just two days if it's something urgent)
before sending a friendly reminder.

Sometimes the maintainer might not be responding in a timely manner; other
times there might be disagreements, for example if an issue qualifies as
regression or not. In such cases raise your concerns on the mailing list and
ask others for public or private replies how to move on. If that fails, it
might be appropriate to get a higher authority involved. In case of a WiFi
driver that would be the wireless maintainers; if there are no higher level
maintainers or all else fails, it might be one of those rare situations where
it's okay to get Linus Torvalds involved.

**Proactive testing**: Every time the first pre-release (the 'rc1') of a new
mainline kernel version gets released, go and check if the issue is fixed there
or if anything of importance changed. Mention the outcome in the ticket or in a
mail you sent as reply to your report (make sure it has all those in the CC
that up to that point participated in the discussion). This will show your
commitment and that you are willing to help. It also tells developers if the
issue persists and makes sure they do not forget about it. A few other
occasional retests (for example with rc3, rc5 and the final) are also a good
idea, but only report your results if something relevant changed or if you are
writing something anyway.

With all these general things off the table let's get into the details of how
to help to get issues resolved once they were reported.

Inquires and testing request
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here are your duties in case you got replies to your report:

**Check who you deal with**: Most of the time it will be the maintainer or a
developer of the particular code area that will respond to your report. But as
issues are normally reported in public it could be anyone that's replying —
including people that want to help, but in the end might guide you totally off
track with their questions or requests. That rarely happens, but it's one of
many reasons why it's wise to quickly run an internet search to see who you're
interacting with. By doing this you also get aware if your report was heard by
the right people, as a reminder to the maintainer (see below) might be in order
later if discussion fades out without leading to a satisfying solution for the
issue.

**Inquiries for data**: Often you will be asked to test something or provide
additional details. Try to provide the requested information soon, as you have
the attention of someone that might help and risk losing it the longer you
wait; that outcome is even likely if you do not provide the information within
a few business days.

**Requests for testing**: When you are asked to test a diagnostic patch or a
possible fix, try to test it in timely manner, too. But do it properly and make
sure to not rush it: mixing things up can happen easily and can lead to a lot
of confusion for everyone involved. A common mistake for example is thinking a
proposed patch with a fix was applied, but in fact wasn't. Things like that
happen even to experienced testers occasionally, but they most of the time will
notice when the kernel with the fix behaves just as one without it.

What to do when nothing of substance happens
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some reports will not get any reaction from the responsible Linux kernel
developers; or a discussion around the issue evolved, but faded out with
nothing of substance coming out of it.

In these cases wait two (better: three) weeks before sending a friendly
reminder: maybe the maintainer was just away from keyboard for a while when
your report arrived or had something more important to take care of. When
writing the reminder, kindly ask if anything else from your side is needed to
get the ball running somehow. If the report got out by mail, do that in the
first lines of a mail that is a reply to your initial mail (see above) which
includes a full quote of the original report below: that's on of those few
situations where such a 'TOFU' (Text Over, Fullquote Under) is the right
approach, as then all the recipients will have the details at hand immediately
in the proper order.

After the reminder wait three more weeks for replies. If you still don't get a
proper reaction, you first should reconsider your approach. Did you maybe try
to reach out to the wrong people? Was the report maybe offensive or so
confusing that people decided to completely stay away from it? The best way to
rule out such factors: show the report to one or two people familiar with FLOSS
issue reporting and ask for their opinion. Also ask them for their advice how
to move forward. That might mean: prepare a better report and make those people
review it before you send it out. Such an approach is totally fine; just
mention that this is the second and improved report on the issue and include a
link to the first report.

If the report was proper you can send a second reminder; in it ask for advice
why the report did not get any replies. A good moment for this second reminder
mail is shortly after the first pre-release (the 'rc1') of a new Linux kernel
version got published, as you should retest and provide a status update at that
point anyway (see above).

If the second reminder again results in no reaction within a week, try to
contact a higher-level maintainer asking for advice: even busy maintainers by
then should at least have sent some kind of acknowledgment.

Remember to prepare yourself for a disappointment: maintainers ideally should
react somehow to every issue report, but they are only obliged to fix those
'issues of high priority' outlined earlier. So don't be too devastating if you
get a reply along the lines of 'thanks for the report, I have more important
issues to deal with currently and won't have time to look into this for the
foreseeable future'.

It's also possible that after some discussion in the bug tracker or on a list
nothing happens anymore and reminders don't help to motivate anyone to work out
a fix. Such situations can be devastating, but is within the cards when it
comes to Linux kernel development. This and several other reasons for not
getting help are explained in 'Why some issues won't get any reaction or remain
unfixed after being reported' near the end of this document.

Don't get devastated if you don't find any help or if the issue in the end does
not get solved: the Linux kernel is FLOSS and thus you can still help yourself.
You for example could try to find others that are affected and team up with
them to get the issue resolved. Such a team could prepare a fresh report
together that mentions how many you are and why this is something that in your
option should get fixed. Maybe together you can also narrow down the root cause
or the change that introduced a regression, which often makes developing a fix
easier. And with a bit of luck there might be someone in the team that knows a
bit about programming and might be able to write a fix.


Details about reporting issues only occurring in older kernel version lines
---------------------------------------------------------------------------

This subsection provides details for steps you need to take if you could not
reproduce your issue with a mainline kernel, but want to see it fixed in older
version lines (aka stable and longterm kernels).

Some fixes are too complex
~~~~~~~~~~~~~~~~~~~~~~~~~~

    *Prepare yourself for the possibility that going through the next few steps
    might not get the issue solved in older releases: the fix might be too big
    or risky to get backported there.*

Even small and seemingly obvious code-changes sometimes introduce new and
totally unexpected problems. The maintainers of the stable and longterm kernels
are very aware of that and thus only apply changes to these kernels that are
within rules outlined in 'Documentation/process/stable-kernel-rules.rst'.

Complex or risky changes for example do not qualify and thus only get applied
to mainline. Other fixes are easy to get backported to the newest stable and
longterm kernels, but too risky to integrate into older ones. So be aware the
fix you are hoping for might be one of those that won't be backported to the
version line your care about. In that case you'll have no other choice then to
live with the issue or switch to a newer Linux version, unless you want to
patch the fix into your kernels yourself.

Make sure the particular version line still gets support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    *Check if the kernel developers still maintain the Linux kernel version
    line you care about: go to the front page of kernel.org and make sure it
    mentions the latest release of the particular version line without an
    '[EOL]' tag.*

Most kernel version lines only get supported for about three months, as
maintaining them longer is quite a lot of work. Hence, only one per year is
chosen and gets supported for at least two years (often six). That's why you
need to check if the kernel developers still support the version line you care
for.

Note, if kernel.org lists two 'stable' version lines on the front page, you
should consider switching to the newer one and forget about the older one:
support for it is likely to be abandoned soon. Then it will get a "end-of-life"
(EOL) stamp. Version lines that reached that point still get mentioned on the
kernel.org front page for a week or two, but are unsuitable for testing and
reporting.

Search stable mailing list
~~~~~~~~~~~~~~~~~~~~~~~~~~

    *Check the archives of the Linux stable mailing list for existing reports.*

Maybe the issue you face is already known and was fixed or is about to. Hence,
`search the archives of the Linux stable mailing list
<https://lore.kernel.org/stable/>`_ for reports about an issue like yours. If
you find any matches, consider joining the discussion, unless the fix is
already finished and scheduled to get applied soon.

Reproduce issue with the newest release
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    *Install the latest release from the particular version line as a vanilla
    kernel. Ensure this kernel is not tainted and still shows the problem, as
    the issue might have already been fixed there.*

Before investing any more time in this process you want to check if the issue
was already fixed in the latest release of version line you're interested in.
This kernel needs to be vanilla and shouldn't be tainted before the issue
happens, as detailed outlined already above in the process of testing mainline.

Check code history and search for existing discussions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    *Search the Linux kernel version control system for the change that fixed
    the issue in mainline, as its commit message might tell you if the fix is
    scheduled for backporting already. If you don't find anything that way,
    search the appropriate mailing lists for posts that discuss such an issue
    or peer-review possible fixes; then check the discussions if the fix was
    deemed unsuitable for backporting. If backporting was not considered at
    all, join the newest discussion, asking if it's in the cards.*

In a lot of cases the issue you deal with will have happened with mainline, but
got fixed there. The commit that fixed it would need to get backported as well
to get the issue solved. That's why you want to search for it or any
discussions abound it.

 * First try to find the fix in the Git repository that holds the Linux kernel
   sources. You can do this with the web interfaces `on kernel.org
   <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/>`_
   or its mirror `on GitHub <https://github.com/torvalds/linux>`_; if you have
   a local clone you alternatively can search on the command line with ``git
   log --grep=<pattern>``.

   If you find the fix, look if the commit message near the end contains a
   'stable tag' that looks like this:

          Cc: <stable@vger.kernel.org> # 5.4+

   If that's case the developer marked the fix safe for backporting to version
   line 5.4 and later. Most of the time it's getting applied there within two
   weeks, but sometimes it takes a bit longer.

 * If the commit doesn't tell you anything or if you can't find the fix, look
   again for discussions about the issue. Search the net with your favorite
   internet search engine as well as the archives for the `Linux kernel
   developers mailing list <https://lore.kernel.org/lkml/>`_. Also read the
   section `Locate kernel area that causes the issue` above and follow the
   instructions to find the subsystem in question: its bug tracker or mailing
   list archive might have the answer you are looking for.

 * If you see a proposed fix, search for it in the version control system as
   outlined above, as the commit might tell you if a backport can be expected.

   * Check the discussions for any indicators the fix might be too risky to get
     backported to the version line you care about. If that's the case you have
     to live with the issue or switch to the kernel version line where the fix
     got applied.

   * If the fix doesn't contain a stable tag and backporting was not discussed,
     join the discussion: mention the version where you face the issue and that
     you would like to see it fixed, if suitable.

Check if it's a regression specific to stable or longterm kernels
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    *Check if you're dealing with a regression that was never present in
    mainline by installing the first release of the version line you care
    about. If the issue doesn't show up with it, you basically need to report
    the issue with this version like you would report a problem with mainline
    (see above). This ideally includes a bisection followed by a search for
    existing reports on the net; with the help of the subject and the two
    relevant commit-ids. If that doesn't turn up anything, write the report; CC
    or forward the report to the stable maintainers, the stable mailing list,
    and those who authored the change. Include the shortened commit-id if you
    found the change that causes it.*

Sometimes you won't find anything in the previous step: the issue you face
might have never occurred in mainline, as it is caused by some change that is
incomplete or not correctly applied. To check this, install the first release
from version line you care about, e.g., if you care about 5.4.x, install 5.4.

If the issue doesn't show itself there, it's a regression specific to the
particular version line. In that case you need to report it like an issue
happening in mainline, like the last few steps in the main section in the above
outline.

One of them suggests doing a bisection, which you are strongly advised to do in
this case. After finding the culprit, search the net for existing reports
again: not only search for the exact subject and the commit-id (proper and
shortened to twelve characters) of the change, but also for the commit-id
(proper and shortened) mentioned as 'Upstream commit' in the commit message.

Write the report; just keep a few specialties in mind: CC or forward the report
to the stable maintainers, the stable mailing list, which the :ref:`MAINTAINERS
<maintainers>` file mentions in the section "STABLE BRANCH". If you performed a
successful bisection, CC the author of the change and include its subject and
the shortened commit-id.

Ask for advice
~~~~~~~~~~~~~~

    *One of the former steps should lead to a solution. If that doesn't work
    out, ask the maintainers for the subsystem that seems to be causing the
    issue for advice; CC the mailing list for the particular subsystem as well
    as the stable mailing list.*

If the previous three steps didn't get you closer to a solution there is only
one option left: ask for advice. Do that in a mail you sent to the maintainers
for the subsystem where the issue seems to have its roots; CC the mailing list
for the subsystem as well as the stable mailing list the :ref:`MAINTAINERS
<maintainers>` file mention in the section "STABLE BRANCH".


Why some issues won't get any reaction or remain unfixed after being reported
=============================================================================

When reporting a problem to the Linux developers, be aware only 'issues of high
priority' (regressions, security issues, severe problems) are definitely going
to get resolved. The maintainers or if all else fails Linus Torvalds himself
will make sure of that. They and the other kernel developers will fix a lot of
other issues as well. But be aware that sometimes they can't or won't help; and
sometimes there isn't even anyone to send a report to.

This is best explained with kernel developers that contribute to the Linux
kernel in their spare time. Quite a few of the drivers in the kernel were
written by such programmers, often because they simply wanted to make their
hardware usable on their favorite operating system.

These programmers most of the time will happily fix problems other people
report. But nobody can force them to do, as they are contributing voluntarily.

Then there are situations where such developers really want to fix an issue,
but can't: sometimes they lack hardware programming documentation to do so.
This often happens when the publicly available docs are superficial or the
driver was written with the help of reverse engineering.

Sooner or later spare time developers will also stop caring for the driver.
Maybe their test hardware broke, got replaced by something more fancy, or is so
old that it's something you don't find much outside of computer museums
anymore. Sometimes developer stops caring for their code and Linux at all, as
something different in their life became way more important. In some cases
nobody is willing to take over the job as maintainer – and nobody can be forced
to, as contributing to the Linux kernel is done on a voluntary basis. Abandoned
drivers nevertheless remain in the kernel: they are still useful for people and
removing would be a regression.

The situation is not that different with developers that are paid for their
work on the Linux kernel. Those contribute most changes these days. But their
employers sooner or later also stop caring for their code or make its
programmer focus on other things. Hardware vendors for example earn their money
mainly by selling new hardware; quite a few of them hence are not investing
much time and energy in maintaining a Linux kernel driver for something they
stopped selling years ago. Enterprise Linux distributors often care for a
longer time period, but in new versions often leave support for old and rare
hardware aside to limit the scope. Often spare time contributors take over once
a company orphans some code, but as mentioned above: sooner or later they will
leave the code behind, too.

Priorities are another reason why some issues are not fixed, as maintainers
quite often are forced to set those, as time to work on Linux is limited.
That's true for spare time or the time employers grant their developers to
spend on maintenance work on the upstream kernel. Sometimes maintainers also
get overwhelmed with reports, even if a driver is working nearly perfectly. To
not get completely stuck, the programmer thus might have no other choice than
to prioritize issue reports and reject some of them.

But don't worry too much about all of this, a lot of drivers have active
maintainers who are quite interested in fixing as many issues as possible.


Closing words
=============

Compared with other Free/Libre & Open Source Software it's hard to report
issues to the Linux kernel developers: the length and complexity of this
document and the implications between the lines illustrate that. But that's how
it is for now. The main author of this text hopes documenting the state of the
art will lay some groundwork to improve the situation over time.