Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+Frequently Asked Questions
+==========================
+
+How is this different from Autotest, kselftest, and so on?
+==========================================================
+KUnit is a unit testing framework. Autotest, kselftest (and some others) are
+not.
+
+A `unit test <https://martinfowler.com/bliki/UnitTest.html>`_ is supposed to
+test a single unit of code in isolation and hence the name *unit test*. A unit
+test should be the finest granularity of testing and should allow all possible
+code paths to be tested in the code under test. This is only possible if the
+code under test is small and does not have any external dependencies outside of
+the test's control like hardware.
+
+There are no testing frameworks currently available for the kernel that do not
+require installing the kernel on a test machine or in a virtual machine. All
+testing frameworks require tests to be written in userspace and run on the
+kernel under test. This is true for Autotest, kselftest, and some others,
+disqualifying any of them from being considered unit testing frameworks.
+
+Does KUnit support running on architectures other than UML?
+===========================================================
+
+Yes, mostly.
+
+For the most part, the KUnit core framework (what we use to write the tests)
+can compile to any architecture. It compiles like just another part of the
+kernel and runs when the kernel boots, or when built as a module, when the
+module is loaded.  However, there is infrastructure, like the KUnit Wrapper
+(``tools/testing/kunit/kunit.py``) that might not support some architectures
+(see :ref:`kunit-on-qemu`).
+
+In short, yes, you can run KUnit on other architectures, but it might require
+more work than using KUnit on UML.
+
+For more information, see :ref:`kunit-on-non-uml`.
+
+.. _kinds-of-tests:
+
+What is the difference between a unit test and other kinds of tests?
+====================================================================
+Most existing tests for the Linux kernel would be categorized as an integration
+test, or an end-to-end test.
+
+- A unit test is supposed to test a single unit of code in isolation. A unit
+  test should be the finest granularity of testing and, as such, allows all
+  possible code paths to be tested in the code under test. This is only possible
+  if the code under test is small and does not have any external dependencies
+  outside of the test's control like hardware.
+- An integration test tests the interaction between a minimal set of components,
+  usually just two or three. For example, someone might write an integration
+  test to test the interaction between a driver and a piece of hardware, or to
+  test the interaction between the userspace libraries the kernel provides and
+  the kernel itself. However, one of these tests would probably not test the
+  entire kernel along with hardware interactions and interactions with the
+  userspace.
+- An end-to-end test usually tests the entire system from the perspective of the
+  code under test. For example, someone might write an end-to-end test for the
+  kernel by installing a production configuration of the kernel on production
+  hardware with a production userspace and then trying to exercise some behavior
+  that depends on interactions between the hardware, the kernel, and userspace.
+
+KUnit is not working, what should I do?
+=======================================
+
+Unfortunately, there are a number of things which can break, but here are some
+things to try.
+
+1. Run ``./tools/testing/kunit/kunit.py run`` with the ``--raw_output``
+   parameter. This might show details or error messages hidden by the kunit_tool
+   parser.
+2. Instead of running ``kunit.py run``, try running ``kunit.py config``,
+   ``kunit.py build``, and ``kunit.py exec`` independently. This can help track
+   down where an issue is occurring. (If you think the parser is at fault, you
+   can run it manually against ``stdin`` or a file with ``kunit.py parse``.)
+3. Running the UML kernel directly can often reveal issues or error messages,
+   ``kunit_tool`` ignores. This should be as simple as running ``./vmlinux``
+   after building the UML kernel (for example, by using ``kunit.py build``).
+   Note that UML has some unusual requirements (such as the host having a tmpfs
+   filesystem mounted), and has had issues in the past when built statically and
+   the host has KASLR enabled. (On older host kernels, you may need to run
+   ``setarch `uname -m` -R ./vmlinux`` to disable KASLR.)
+4. Make sure the kernel .config has ``CONFIG_KUNIT=y`` and at least one test
+   (e.g. ``CONFIG_KUNIT_EXAMPLE_TEST=y``). kunit_tool will keep its .config
+   around, so you can see what config was used after running ``kunit.py run``.
+   It also preserves any config changes you might make, so you can
+   enable/disable things with ``make ARCH=um menuconfig`` or similar, and then
+   re-run kunit_tool.
+5. Try to run ``make ARCH=um defconfig`` before running ``kunit.py run``. This
+   may help clean up any residual config items which could be causing problems.
+6. Finally, try running KUnit outside UML. KUnit and KUnit tests can be
+   built into any kernel, or can be built as a module and loaded at runtime.
+   Doing so should allow you to determine if UML is causing the issue you're
+   seeing. When tests are built-in, they will execute when the kernel boots, and
+   modules will automatically execute associated tests when loaded. Test results
+   can be collected from ``/sys/kernel/debug/kunit/<test suite>/results``, and
+   can be parsed with ``kunit.py parse``. For more details, see :ref:`kunit-on-qemu`.
+
+If none of the above tricks help, you are always welcome to email any issues to
+kunit-dev@googlegroups.com.