.. SPDX-License-Identifier: GPL-2.0

===========================
Test Style and Nomenclature
===========================

To make finding, writing, and using KUnit tests as simple as possible, it is
strongly encouraged that they are named and written according to the guidelines
below. While it is 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 is recommended that you only deviate from these guidelines when:

1. Porting tests to KUnit which are already known with an existing name.
2. Writing tests which would cause serious problems if automatically run. For
   example, non-deterministically producing false positives or negatives, or
   taking a long time to run.

Subsystems, Suites, and Tests
=============================

To make tests easy to find, they are grouped into suites and subsystems. A test
suite is a group of tests which test a related area of the kernel. A subsystem
is a set of test suites which test different parts of a kernel subsystem
or a 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
test subsystem should match a single kernel module. If the code being tested
cannot 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 we are actually testing other tests or the kunit framework itself. For
example, subsystems could be called:

``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 as shown in examples below:

``linear-ranges``
  Names should use underscores, not dashes, to separate words. Prefer
  ``linear_ranges``.
``qos-kunit-test``
  This name should use underscores, and not have "kunit-test" as a
  suffix. ``qos`` is also ambiguous as a subsystem name, because several parts
  of the kernel have a ``qos`` subsystem. ``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 are
        a way of categorizing test suites and naming modules which provides a
        simple, consistent way for humans to find and run tests. This may change
        in the future.

Suites
------

KUnit tests are grouped into test suites, which cover a specific area of
functionality being tested. Test suites can have shared initialization and
shutdown code which is run for all tests in the suite. Not all subsystems need
to be split into multiple test suites (for example, 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 (for example, 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. For 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, for example:

``ext4_ext4_inode``
  There is 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 as ``kasan``. Do not redundantly add
  ``integration_test``. It should be a separate test suite. For example, if the
  unit tests are added, then that suite could be named as ``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, an individual test's
results will show up as subtests of the suite's results.

Tests should be named after what they are 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 is 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.
* depend on the feature being tested, rather than selecting it (so that
  enabling ``CONFIG_KUNIT_ALL_TESTS`` does not enable unrelated functionality).
* be ``tristate``, unless there is a specific reason that the test cannot be
  built as a module.

For example, a Kconfig entry might look like:

.. 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 are often compiled as a separate module. To avoid conflicting
with regular modules, KUnit modules should be named after the test suite,
followed by ``_kunit`` (e.g. if "foobar" is the core module, then
"foobar_kunit" is the KUnit test module).

Test source files, whether compiled as a separate module or an
``#include`` in another source file, are best kept in a ``tests/``
subdirectory to not conflict with other source files (e.g. for
tab-completion).

Note that the ``_test`` suffix has also been used in some existing
tests. The ``_kunit`` suffix is preferred, as it makes the distinction
between KUnit and non-KUnit tests clearer.

So for the common case, name the file containing the test suite
``tests/<suite>_kunit.c``. The ``tests`` directory should be placed at
the same level as the code under test. For example, tests for
``lib/string.c`` live in ``lib/tests/string_kunit.c``.

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/tests/firmware_kunit.c`` file.