Documentation: kunit: Add naming guidelines

As discussed in [1], KUnit tests have hitherto not had a particularly
consistent naming scheme. This adds documentation outlining how tests
and test suites should be named, including how those names should be
used in Kconfig entries and filenames.

[1]:
https://lore.kernel.org/linux-kselftest/202006141005.BA19A9D3@keescook/t/#u

Signed-off-by: David Gow <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Brendan Higgins <[e-mail]>
Reviewed-by: Marco Elver <[e-mail]>
Reviewed-by: Tim Bird <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

Documentation/dev-tools/kunit/index.rst

···
 	usage
 	kunit-tool
 	api/index
+	style
 	faq
 
 What is KUnit?

Documentation/dev-tools/kunit/style.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================
+Test Style and Nomenclature
+===========================
+
+To make finding, writing, and using KUnit tests as simple as possible, it's
+strongly encouraged that they are named and written according to the guidelines
+below. While it's possible to write KUnit tests which do not follow these rules,
+they may break some tooling, may conflict with other tests, and may not be run
+automatically by testing systems.
+
+It's recommended that you only deviate from these guidelines when:
+
+1. Porting tests to KUnit which are already known with an existing name, or
+2. Writing tests which would cause serious problems if automatically run (e.g.,
+   non-deterministically producing false positives or negatives, or taking an
+   extremely long time to run).
+
+Subsystems, Suites, and Tests
+=============================
+
+In order to make tests as easy to find as possible, they're grouped into suites
+and subsystems. A test suite is a group of tests which test a related area of
+the kernel, and a subsystem is a set of test suites which test different parts
+of the same kernel subsystem or driver.
+
+Subsystems
+----------
+
+Every test suite must belong to a subsystem. A subsystem is a collection of one
+or more KUnit test suites which test the same driver or part of the kernel. A
+rule of thumb is that a test subsystem should match a single kernel module. If
+the code being tested can't be compiled as a module, in many cases the subsystem
+should correspond to a directory in the source tree or an entry in the
+MAINTAINERS file. If unsure, follow the conventions set by tests in similar
+areas.
+
+Test subsystems should be named after the code being tested, either after the
+module (wherever possible), or after the directory or files being tested. Test
+subsystems should be named to avoid ambiguity where necessary.
+
+If a test subsystem name has multiple components, they should be separated by
+underscores. *Do not* include "test" or "kunit" directly in the subsystem name
+unless you are actually testing other tests or the kunit framework itself.
+
+Example subsystems could be:
+
+``ext4``
+  Matches the module and filesystem name.
+``apparmor``
+  Matches the module name and LSM name.
+``kasan``
+  Common name for the tool, prominent part of the path ``mm/kasan``
+``snd_hda_codec_hdmi``
+  Has several components (``snd``, ``hda``, ``codec``, ``hdmi``) separated by
+  underscores. Matches the module name.
+
+Avoid names like these:
+
+``linear-ranges``
+  Names should use underscores, not dashes, to separate words. Prefer
+  ``linear_ranges``.
+``qos-kunit-test``
+  As well as using underscores, this name should not have "kunit-test" as a
+  suffix, and ``qos`` is ambiguous as a subsystem name. ``power_qos`` would be a
+  better name.
+``pc_parallel_port``
+  The corresponding module name is ``parport_pc``, so this subsystem should also
+  be named ``parport_pc``.
+
+.. note::
+        The KUnit API and tools do not explicitly know about subsystems. They're
+        simply a way of categorising test suites and naming modules which
+        provides a simple, consistent way for humans to find and run tests. This
+        may change in the future, though.
+
+Suites
+------
+
+KUnit tests are grouped into test suites, which cover a specific area of
+functionality being tested. Test suites can have shared initialisation and
+shutdown code which is run for all tests in the suite.
+Not all subsystems will need to be split into multiple test suites (e.g. simple drivers).
+
+Test suites are named after the subsystem they are part of. If a subsystem
+contains several suites, the specific area under test should be appended to the
+subsystem name, separated by an underscore.
+
+In the event that there are multiple types of test using KUnit within a
+subsystem (e.g., both unit tests and integration tests), they should be put into
+separate suites, with the type of test as the last element in the suite name.
+Unless these tests are actually present, avoid using ``_test``, ``_unittest`` or
+similar in the suite name.
+
+The full test suite name (including the subsystem name) should be specified as
+the ``.name`` member of the ``kunit_suite`` struct, and forms the base for the
+module name (see below).
+
+Example test suites could include:
+
+``ext4_inode``
+  Part of the ``ext4`` subsystem, testing the ``inode`` area.
+``kunit_try_catch``
+  Part of the ``kunit`` implementation itself, testing the ``try_catch`` area.
+``apparmor_property_entry``
+  Part of the ``apparmor`` subsystem, testing the ``property_entry`` area.
+``kasan``
+  The ``kasan`` subsystem has only one suite, so the suite name is the same as
+  the subsystem name.
+
+Avoid names like:
+
+``ext4_ext4_inode``
+  There's no reason to state the subsystem twice.
+``property_entry``
+  The suite name is ambiguous without the subsystem name.
+``kasan_integration_test``
+  Because there is only one suite in the ``kasan`` subsystem, the suite should
+  just be called ``kasan``. There's no need to redundantly add
+  ``integration_test``. Should a separate test suite with, for example, unit
+  tests be added, then that suite could be named ``kasan_unittest`` or similar.
+
+Test Cases
+----------
+
+Individual tests consist of a single function which tests a constrained
+codepath, property, or function. In the test output, individual tests' results
+will show up as subtests of the suite's results.
+
+Tests should be named after what they're testing. This is often the name of the
+function being tested, with a description of the input or codepath being tested.
+As tests are C functions, they should be named and written in accordance with
+the kernel coding style.
+
+.. note::
+        As tests are themselves functions, their names cannot conflict with
+        other C identifiers in the kernel. This may require some creative
+        naming. It's a good idea to make your test functions `static` to avoid
+        polluting the global namespace.
+
+Example test names include:
+
+``unpack_u32_with_null_name``
+  Tests the ``unpack_u32`` function when a NULL name is passed in.
+``test_list_splice``
+  Tests the ``list_splice`` macro. It has the prefix ``test_`` to avoid a
+  name conflict with the macro itself.
+
+
+Should it be necessary to refer to a test outside the context of its test suite,
+the *fully-qualified* name of a test should be the suite name followed by the
+test name, separated by a colon (i.e. ``suite:test``).
+
+Test Kconfig Entries
+====================
+
+Every test suite should be tied to a Kconfig entry.
+
+This Kconfig entry must:
+
+* be named ``CONFIG_<name>_KUNIT_TEST``: where <name> is the name of the test
+  suite.
+* be listed either alongside the config entries for the driver/subsystem being
+  tested, or be under [Kernel Hacking]→[Kernel Testing and Coverage]
+* depend on ``CONFIG_KUNIT``
+* be visible only if ``CONFIG_KUNIT_ALL_TESTS`` is not enabled.
+* have a default value of ``CONFIG_KUNIT_ALL_TESTS``.
+* have a brief description of KUnit in the help text
+
+Unless there's a specific reason not to (e.g. the test is unable to be built as
+a module), Kconfig entries for tests should be tristate.
+
+An example Kconfig entry:
+
+.. code-block:: none
+
+        config FOO_KUNIT_TEST
+                tristate "KUnit test for foo" if !KUNIT_ALL_TESTS
+                depends on KUNIT
+                default KUNIT_ALL_TESTS
+                help
+                    This builds unit tests for foo.
+
+                    For more information on KUnit and unit tests in general, please refer
+                    to the KUnit documentation in Documentation/dev-tools/kunit
+
+                    If unsure, say N
+
+
+Test File and Module Names
+==========================
+
+KUnit tests can often be compiled as a module. These modules should be named
+after the test suite, followed by ``_test``. If this is likely to conflict with
+non-KUnit tests, the suffix ``_kunit`` can also be used.
+
+The easiest way of achieving this is to name the file containing the test suite
+``<suite>_test.c`` (or, as above, ``<suite>_kunit.c``). This file should be
+placed next to the code under test.
+
+If the suite name contains some or all of the name of the test's parent
+directory, it may make sense to modify the source filename to reduce redundancy.
+For example, a ``foo_firmware`` suite could be in the ``foo/firmware_test.c``
+file.