[RFC PATCH v1 23/26] docs: reporting-bugs: details for issues specific to stable and longterm

[Thorsten Leemhuis <linux@leemhuis.info>]

Describe what users have to do if they can't reproduce a problem with
mainline they want to see fixed in stable and longterm kernels. This is
separated from the main flow, as integrating it there would make it
harder to follow.

Note users will only enter this section in two cases: (1) the issue was
fixed in mainline (on purpose or accidentally) (2) it's a regression
that never was present in mainline (for example due to a broken
backport).

Help users to differentiate between the two, as they ideally are handled
differently.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
---
 Documentation/admin-guide/reporting-bugs.rst | 191 +++++++++++++++++--
 1 file changed, 179 insertions(+), 12 deletions(-)

diff --git a/Documentation/admin-guide/reporting-bugs.rst b/Documentation/admin-guide/reporting-bugs.rst
index b8bc6c4e2340..340fa44b352c 100644
--- a/Documentation/admin-guide/reporting-bugs.rst
+++ b/Documentation/admin-guide/reporting-bugs.rst
@@ -1223,24 +1223,191 @@ 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
+---------------------------------------------------------------------------
+
+Find below details about the steps in the subsection for reporting issues that
+only happen in 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
+:ref:`within rules outlined in Documentation/process/stable-kernel-rules.rst
+<stable_kernel_rules>`.
+
+Complex or risky changes for example do not qualify and thus only get applied to
+mainline; sometimes 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 and that you have no other choice
+then to live with the issue or switch to a newer version, unless you want to
+patch the fix into your own kernels yourselfs.
+
+
+Make sure the particular version line is still supported
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    *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 developer still support the version line you care
+for.
+
+Note: if kernel.org lists two 'stable' version lines, you likely want to focus
+on the newer of the two, as support for the older is likely to be abandoned
+soon and thus "end-of-life" (EOL). Version lines that reached that point still
+get mentioned on the kernel.org-frontpage for a week or two, but then they 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 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. That might lead you to the commit with the fix
+    or tell you if it's unsuitable for backporting. If backporting was not
+    considered at all, join the newest discussion, asking if its 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 why you want to search for it or any
+discussions abound it.
+
+ * First try to find the fix on 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 that 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 its 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 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 `MAINTAINERS file
+<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/MAINTAINERS>`_
+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 `MAINTAINERS file
+<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/MAINTAINERS>`_
+mention in the section "STABLE BRANCH".
+
+
 .. ############################################################################
 .. Temporary marker added while this document is rewritten. Sections above
 .. are new and dual-licensed under GPLv2+ and CC-BY 4.0, those below are old.
 .. Both sections are incomplete as of now and thus sometimes inconsistent.
 .. ###########################################################################
 
-Background
-==========
-
-The upstream Linux kernel maintainers only fix bugs for specific kernel
-versions.  Those versions include the current "release candidate" (or -rc)
-kernel, any "stable" kernel versions, and any "long term" kernels.
-
-Please see https://www.kernel.org/ for a list of supported kernels.  Any
-kernel marked with [EOL] is "end of life" and will not have any fixes
-backported to it.
-
-
 How to report Linux kernel bugs
 ===============================
 
-- 
2.26.2