Documentation: KUnit: Rework writing page to focus on writing tests

We now have dedicated pages on running tests. Therefore refocus the
usage page on writing tests and add content from tips page and
information on other architectures.

Signed-off-by: Harinder Singh <sharinder@google.com>
Reviewed-by: Tim Bird <tim.bird@sony.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Link: https://lore.kernel.org/r/20211217044911.798817-6-sharinder@google.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

3 files changed, 255 insertions, 327 deletions

diff --git a/Documentation/dev-tools/kunit/index.rst b/Documentation/dev-tools/kunit/index.rst
index f9f37997b58c..595205348d2d 100644
--- a/Documentation/dev-tools/kunit/index.rst
+++ b/Documentation/dev-tools/kunit/index.rst
@@ -102,7 +102,7 @@ How do I use it?
 *   Documentation/dev-tools/kunit/architecture.rst - KUnit architecture.
 *   Documentation/dev-tools/kunit/run_wrapper.rst - run kunit_tool.
 *   Documentation/dev-tools/kunit/run_manual.rst - run tests without kunit_tool.
-*   Documentation/dev-tools/kunit/usage.rst - KUnit features.
+*   Documentation/dev-tools/kunit/usage.rst - write tests.
 *   Documentation/dev-tools/kunit/tips.rst - best practices with
     examples.
 *   Documentation/dev-tools/kunit/api/index.rst - KUnit APIs
diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
index af13f443c976..a858ab009944 100644
--- a/Documentation/dev-tools/kunit/start.rst
+++ b/Documentation/dev-tools/kunit/start.rst
@@ -243,7 +243,7 @@ Next Steps
 *   Documentation/dev-tools/kunit/architecture.rst - KUnit architecture.
 *   Documentation/dev-tools/kunit/run_wrapper.rst - run kunit_tool.
 *   Documentation/dev-tools/kunit/run_manual.rst - run tests without kunit_tool.
-*   Documentation/dev-tools/kunit/usage.rst - KUnit features.
+*   Documentation/dev-tools/kunit/usage.rst - write tests.
 *   Documentation/dev-tools/kunit/tips.rst - best practices with
     examples.
 *   Documentation/dev-tools/kunit/api/index.rst - KUnit APIs
diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index 63f1bb89ebf5..76af931a332c 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -1,57 +1,13 @@
 .. SPDX-License-Identifier: GPL-2.0
 
-===========
-Using KUnit
-===========
-
-The purpose of this document is to describe what KUnit is, how it works, how it
-is intended to be used, and all the concepts and terminology that are needed to
-understand it. This guide assumes a working knowledge of the Linux kernel and
-some basic knowledge of testing.
-
-For a high level introduction to KUnit, including setting up KUnit for your
-project, see Documentation/dev-tools/kunit/start.rst.
-
-Organization of this document
-=============================
-
-This document is organized into two main sections: Testing and Common Patterns.
-The first covers what unit tests are and how to use KUnit to write them. The
-second covers common testing patterns, e.g. how to isolate code and make it
-possible to unit test code that was otherwise un-unit-testable.
-
-Testing
-=======
-
-What is KUnit?
---------------
-
-"K" is short for "kernel" so "KUnit" is the "(Linux) Kernel Unit Testing
-Framework." KUnit is intended first and foremost for writing unit tests; it is
-general enough that it can be used to write integration tests; however, this is
-a secondary goal. KUnit has no ambition of being the only testing framework for
-the kernel; for example, it does not intend to be an end-to-end testing
-framework.
-
-What is Unit Testing?
----------------------
-
-A `unit test <https://martinfowler.com/bliki/UnitTest.html>`_ is a test that
-tests code at the smallest possible scope, a *unit* of code. In the C
-programming language that's a function.
-
-Unit tests should be written for all the publicly exposed functions in a
-compilation unit; so that is all the functions that are exported in either a
-*class* (defined below) or all functions which are **not** static.
-
 Writing Tests
--------------
+=============
 
 Test Cases
-~~~~~~~~~~
+----------
 
 The fundamental unit in KUnit is the test case. A test case is a function with
-the signature ``void (*)(struct kunit *test)``. It calls a function to be tested
+the signature ``void (*)(struct kunit *test)``. It calls the function under test
 and then sets *expectations* for what should happen. For example:
 
 .. code-block:: c
@@ -65,18 +21,19 @@ and then sets *expectations* for what should happen. For example:
 		KUNIT_FAIL(test, "This test never passes.");
 	}
 
-In the above example ``example_test_success`` always passes because it does
-nothing; no expectations are set, so all expectations pass. On the other hand
-``example_test_failure`` always fails because it calls ``KUNIT_FAIL``, which is
-a special expectation that logs a message and causes the test case to fail.
+In the above example, ``example_test_success`` always passes because it does
+nothing; no expectations are set, and therefore all expectations pass. On the
+other hand ``example_test_failure`` always fails because it calls ``KUNIT_FAIL``,
+which is a special expectation that logs a message and causes the test case to
+fail.
 
 Expectations
 ~~~~~~~~~~~~
-An *expectation* is a way to specify that you expect a piece of code to do
-something in a test. An expectation is called like a function. A test is made
-by setting expectations about the behavior of a piece of code under test; when
-one or more of the expectations fail, the test case fails and information about
-the failure is logged. For example:
+An *expectation* specifies that we expect a piece of code to do something in a
+test. An expectation is called like a function. A test is made by setting
+expectations about the behavior of a piece of code under test. When one or more
+expectations fail, the test case fails and information about the failure is
+logged. For example:
 
 .. code-block:: c
 
@@ -86,29 +43,28 @@ the failure is logged. For example:
 		KUNIT_EXPECT_EQ(test, 2, add(1, 1));
 	}
 
-In the above example ``add_test_basic`` makes a number of assertions about the
-behavior of a function called ``add``; the first parameter is always of type
-``struct kunit *``, which contains information about the current test context;
-the second parameter, in this case, is what the value is expected to be; the
+In the above example, ``add_test_basic`` makes a number of assertions about the
+behavior of a function called ``add``. The first parameter is always of type
+``struct kunit *``, which contains information about the current test context.
+The second parameter, in this case, is what the value is expected to be. The
 last value is what the value actually is. If ``add`` passes all of these
 expectations, the test case, ``add_test_basic`` will pass; if any one of these
 expectations fails, the test case will fail.
 
-It is important to understand that a test case *fails* when any expectation is
-violated; however, the test will continue running, potentially trying other
-expectations until the test case ends or is otherwise terminated. This is as
-opposed to *assertions* which are discussed later.
+A test case *fails* when any expectation is violated; however, the test will
+continue to run, and try other expectations until the test case ends or is
+otherwise terminated. This is as opposed to *assertions* which are discussed
+later.
 
-To learn about more expectations supported by KUnit, see
-Documentation/dev-tools/kunit/api/test.rst.
+To learn about more KUnit expectations, see Documentation/dev-tools/kunit/api/test.rst.
 
 .. note::
-   A single test case should be pretty short, pretty easy to understand,
-   focused on a single behavior.
+   A single test case should be short, easy to understand, and focused on a
+   single behavior.
 
-For example, if we wanted to properly test the add function above, we would
-create additional tests cases which would each test a different property that an
-add function should have like this:
+For example, if we want to rigorously test the ``add`` function above, create
+additional tests cases which would test each property that an ``add`` function
+should have as shown below:
 
 .. code-block:: c
 
@@ -134,56 +90,43 @@ add function should have like this:
 		KUNIT_EXPECT_EQ(test, INT_MIN, add(INT_MAX, 1));
 	}
 
-Notice how it is immediately obvious what all the properties that we are testing
-for are.
-
 Assertions
 ~~~~~~~~~~
 
-KUnit also has the concept of an *assertion*. An assertion is just like an
-expectation except the assertion immediately terminates the test case if it is
-not satisfied.
-
-For example:
+An assertion is like an expectation, except that the assertion immediately
+terminates the test case if the condition is not satisfied. For example:
 
 .. code-block:: c
 
-	static void mock_test_do_expect_default_return(struct kunit *test)
+	static void test_sort(struct kunit *test)
 	{
-		struct mock_test_context *ctx = test->priv;
-		struct mock *mock = ctx->mock;
-		int param0 = 5, param1 = -5;
-		const char *two_param_types[] = {"int", "int"};
-		const void *two_params[] = {&param0, &param1};
-		const void *ret;
-
-		ret = mock->do_expect(mock,
-				      "test_printk", test_printk,
-				      two_param_types, two_params,
-				      ARRAY_SIZE(two_params));
-		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ret);
-		KUNIT_EXPECT_EQ(test, -4, *((int *) ret));
+		int *a, i, r = 1;
+		a = kunit_kmalloc_array(test, TEST_LEN, sizeof(*a), GFP_KERNEL);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, a);
+		for (i = 0; i < TEST_LEN; i++) {
+			r = (r * 725861) % 6599;
+			a[i] = r;
+		}
+		sort(a, TEST_LEN, sizeof(*a), cmpint, NULL);
+		for (i = 0; i < TEST_LEN-1; i++)
+			KUNIT_EXPECT_LE(test, a[i], a[i + 1]);
 	}
 
-In this example, the method under test should return a pointer to a value, so
-if the pointer returned by the method is null or an errno, we don't want to
-bother continuing the test since the following expectation could crash the test
-case. `ASSERT_NOT_ERR_OR_NULL(...)` allows us to bail out of the test case if
-the appropriate conditions have not been satisfied to complete the test.
+In this example, the method under test should return pointer to a value. If the
+pointer returns null or an errno, we want to stop the test since the following
+expectation could crash the test case. `ASSERT_NOT_ERR_OR_NULL(...)` allows us
+to bail out of the test case if the appropriate conditions are not satisfied to
+complete the test.
 
 Test Suites
 ~~~~~~~~~~~
 
-Now obviously one unit test isn't very helpful; the power comes from having
-many test cases covering all of a unit's behaviors. Consequently it is common
-to have many *similar* tests; in order to reduce duplication in these closely
-related tests most unit testing frameworks - including KUnit - provide the
-concept of a *test suite*. A *test suite* is just a collection of test cases
-for a unit of code with a set up function that gets invoked before every test
-case and then a tear down function that gets invoked after every test case
-completes.
-
-Example:
+We need many test cases covering all the unit's behaviors. It is common to have
+many similar tests. In order to reduce duplication in these closely related
+tests, most unit testing frameworks (including KUnit) provide the concept of a
+*test suite*. A test suite is a collection of test cases for a unit of code
+with a setup function that gets invoked before every test case and then a tear
+down function that gets invoked after every test case completes. For example:
 
 .. code-block:: c
 
@@ -202,23 +145,48 @@ Example:
 	};
 	kunit_test_suite(example_test_suite);
 
-In the above example the test suite, ``example_test_suite``, would run the test
-cases ``example_test_foo``, ``example_test_bar``, and ``example_test_baz``;
-each would have ``example_test_init`` called immediately before it and would
-have ``example_test_exit`` called immediately after it.
+In the above example, the test suite ``example_test_suite`` would run the test
+cases ``example_test_foo``, ``example_test_bar``, and ``example_test_baz``. Each
+would have ``example_test_init`` called immediately before it and
+``example_test_exit`` called immediately after it.
 ``kunit_test_suite(example_test_suite)`` registers the test suite with the
 KUnit test framework.
 
 .. note::
-   A test case will only be run if it is associated with a test suite.
+   A test case will only run if it is associated with a test suite.
+
+``kunit_test_suite(...)`` is a macro which tells the linker to put the
+specified test suite in a special linker section so that it can be run by KUnit
+either after ``late_init``, or when the test module is loaded (if the test was
+built as a module).
+
+For more information, see Documentation/dev-tools/kunit/api/test.rst.
+
+Writing Tests For Other Architectures
+-------------------------------------
+
+It is better to write tests that run on UML to tests that only run under a
+particular architecture. It is better to write tests that run under QEMU or
+another easy to obtain (and monetarily free) software environment to a specific
+piece of hardware.
 
-``kunit_test_suite(...)`` is a macro which tells the linker to put the specified
-test suite in a special linker section so that it can be run by KUnit either
-after late_init, or when the test module is loaded (depending on whether the
-test was built in or not).
+Nevertheless, there are still valid reasons to write a test that is architecture
+or hardware specific. For example, we might want to test code that really
+belongs in ``arch/some-arch/*``. Even so, try to write the test so that it does
+not depend on physical hardware. Some of our test cases may not need hardware,
+only few tests actually require the hardware to test it. When hardware is not
+available, instead of disabling tests, we can skip them.
 
-For more information on these types of things see the
-Documentation/dev-tools/kunit/api/test.rst.
+Now that we have narrowed down exactly what bits are hardware specific, the
+actual procedure for writing and running the tests is same as writing normal
+KUnit tests.
+
+.. important::
+   We may have to reset hardware state. If this is not possible, we may only
+   be able to run one test case per invocation.
+
+.. TODO(brendanhiggins@google.com): Add an actual example of an architecture-
+   dependent KUnit test.
 
 Common Patterns
 ===============
@@ -226,43 +194,39 @@ Common Patterns
 Isolating Behavior
 ------------------
 
-The most important aspect of unit testing that other forms of testing do not
-provide is the ability to limit the amount of code under test to a single unit.
-In practice, this is only possible by being able to control what code gets run
-when the unit under test calls a function and this is usually accomplished
-through some sort of indirection where a function is exposed as part of an API
-such that the definition of that function can be changed without affecting the
-rest of the code base. In the kernel this primarily comes from two constructs,
-classes, structs that contain function pointers that are provided by the
-implementer, and architecture-specific functions which have definitions selected
-at compile time.
+Unit testing limits the amount of code under test to a single unit. It controls
+what code gets run when the unit under test calls a function. Where a function
+is exposed as part of an API such that the definition of that function can be
+changed without affecting the rest of the code base. In the kernel, this comes
+from two constructs: classes, which are structs that contain function pointers
+provided by the implementer, and architecture-specific functions, which have
+definitions selected at compile time.
 
 Classes
 ~~~~~~~
 
 Classes are not a construct that is built into the C programming language;
-however, it is an easily derived concept. Accordingly, pretty much every project
-that does not use a standardized object oriented library (like GNOME's GObject)
-has their own slightly different way of doing object oriented programming; the
-Linux kernel is no exception.
+however, it is an easily derived concept. Accordingly, in most cases, every
+project that does not use a standardized object oriented library (like GNOME's
+GObject) has their own slightly different way of doing object oriented
+programming; the Linux kernel is no exception.
 
 The central concept in kernel object oriented programming is the class. In the
 kernel, a *class* is a struct that contains function pointers. This creates a
 contract between *implementers* and *users* since it forces them to use the
-same function signature without having to call the function directly. In order
-for it to truly be a class, the function pointers must specify that a pointer
-to the class, known as a *class handle*, be one of the parameters; this makes
-it possible for the member functions (also known as *methods*) to have access
-to member variables (more commonly known as *fields*) allowing the same
-implementation to have multiple *instances*.
-
-Typically a class can be *overridden* by *child classes* by embedding the
-*parent class* in the child class. Then when a method provided by the child
-class is called, the child implementation knows that the pointer passed to it is
-of a parent contained within the child; because of this, the child can compute
-the pointer to itself because the pointer to the parent is always a fixed offset
-from the pointer to the child; this offset is the offset of the parent contained
-in the child struct. For example:
+same function signature without having to call the function directly. To be a
+class, the function pointers must specify that a pointer to the class, known as
+a *class handle*, be one of the parameters. Thus the member functions (also
+known as *methods*) have access to member variables (also known as *fields*)
+allowing the same implementation to have multiple *instances*.
+
+A class can be *overridden* by *child classes* by embedding the *parent class*
+in the child class. Then when the child class *method* is called, the child
+implementation knows that the pointer passed to it is of a parent contained
+within the child. Thus, the child can compute the pointer to itself because the
+pointer to the parent is always a fixed offset from the pointer to the child.
+This offset is the offset of the parent contained in the child struct. For
+example:
 
 .. code-block:: c
 
@@ -290,8 +254,8 @@ in the child struct. For example:
 		self->width = width;
 	}
 
-In this example (as in most kernel code) the operation of computing the pointer
-to the child from the pointer to the parent is done by ``container_of``.
+In this example, computing the pointer to the child from the pointer to the
+parent is done by ``container_of``.
 
 Faking Classes
 ~~~~~~~~~~~~~~
@@ -300,14 +264,11 @@ In order to unit test a piece of code that calls a method in a class, the
 behavior of the method must be controllable, otherwise the test ceases to be a
 unit test and becomes an integration test.
 
-A fake just provides an implementation of a piece of code that is different than
-what runs in a production instance, but behaves identically from the standpoint
-of the callers; this is usually done to replace a dependency that is hard to
-deal with, or is slow.
-
-A good example for this might be implementing a fake EEPROM that just stores the
-"contents" in an internal buffer. For example, let's assume we have a class that
-represents an EEPROM:
+A fake class implements a piece of code that is different than what runs in a
+production instance, but behaves identical from the standpoint of the callers.
+This is done to replace a dependency that is hard to deal with, or is slow. For
+example, implementing a fake EEPROM that stores the "contents" in an
+internal buffer. Assume we have a class that represents an EEPROM:
 
 .. code-block:: c
 
@@ -316,7 +277,7 @@ represents an EEPROM:
 		ssize_t (*write)(struct eeprom *this, size_t offset, const char *buffer, size_t count);
 	};
 
-And we want to test some code that buffers writes to the EEPROM:
+And we want to test code that buffers writes to the EEPROM:
 
 .. code-block:: c
 
@@ -329,7 +290,7 @@ And we want to test some code that buffers writes to the EEPROM:
 	struct eeprom_buffer *new_eeprom_buffer(struct eeprom *eeprom);
 	void destroy_eeprom_buffer(struct eeprom *eeprom);
 
-We can easily test this code by *faking out* the underlying EEPROM:
+We can test this code by *faking out* the underlying EEPROM:
 
 .. code-block:: c
 
@@ -456,14 +417,14 @@ We can now use it to test ``struct eeprom_buffer``:
 		destroy_eeprom_buffer(ctx->eeprom_buffer);
 	}
 
-Testing against multiple inputs
+Testing Against Multiple Inputs
 -------------------------------
 
-Testing just a few inputs might not be enough to have confidence that the code
-works correctly, e.g. for a hash function.
+Testing just a few inputs is not enough to ensure that the code works correctly,
+for example: testing a hash function.
 
-In such cases, it can be helpful to have a helper macro or function, e.g. this
-fictitious example for ``sha1sum(1)``
+We can write a helper macro or function. The function is called for each input.
+For example, to test ``sha1sum(1)``, we can write:
 
 .. code-block:: c
 
@@ -475,16 +436,15 @@ fictitious example for ``sha1sum(1)``
 	TEST_SHA1("hello world",  "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed");
 	TEST_SHA1("hello world!", "430ce34d020724ed75a196dfc2ad67c77772d169");
 
+Note the use of the ``_MSG`` version of ``KUNIT_EXPECT_STREQ`` to print a more
+detailed error and make the assertions clearer within the helper macros.
 
-Note the use of ``KUNIT_EXPECT_STREQ_MSG`` to give more context when it fails
-and make it easier to track down. (Yes, in this example, ``want`` is likely
-going to be unique enough on its own).
+The ``_MSG`` variants are useful when the same expectation is called multiple
+times (in a loop or helper function) and thus the line number is not enough to
+identify what failed, as shown below.
 
-The ``_MSG`` variants are even more useful when the same expectation is called
-multiple times (in a loop or helper function) and thus the line number isn't
-enough to identify what failed, like below.
-
-In some cases, it can be helpful to write a *table-driven test* instead, e.g.
+In complicated cases, we recommend using a *table-driven test* compared to the
+helper macro variation, for example:
 
 .. code-block:: c
 
@@ -513,17 +473,18 @@ In some cases, it can be helpful to write a *table-driven test* instead, e.g.
 	}
 
 
-There's more boilerplate involved, but it can:
+There is more boilerplate code involved, but it can:
+
+* be more readable when there are multiple inputs/outputs (due to field names).
 
-* be more readable when there are multiple inputs/outputs thanks to field names,
+  * For example, see ``fs/ext4/inode-test.c``.
 
-  * E.g. see ``fs/ext4/inode-test.c`` for an example of both.
-* reduce duplication if test cases can be shared across multiple tests.
+* reduce duplication if test cases are shared across multiple tests.
 
-  * E.g. if we wanted to also test ``sha256sum``, we could add a ``sha256``
+  * For example: if we want to test ``sha256sum``, we could add a ``sha256``
     field and reuse ``cases``.
 
-* be converted to a "parameterized test", see below.
+* be converted to a "parameterized test".
 
 Parameterized Testing
 ~~~~~~~~~~~~~~~~~~~~~
@@ -531,7 +492,7 @@ Parameterized Testing
 The table-driven testing pattern is common enough that KUnit has special
 support for it.
 
-Reusing the same ``cases`` array from above, we can write the test as a
+By reusing the same ``cases`` array from above, we can write the test as a
 "parameterized test" with the following.
 
 .. code-block:: c
@@ -582,193 +543,160 @@ Reusing the same ``cases`` array from above, we can write the test as a
 
 .. _kunit-on-non-uml:
 
-KUnit on non-UML architectures
-==============================
-
-By default KUnit uses UML as a way to provide dependencies for code under test.
-Under most circumstances KUnit's usage of UML should be treated as an
-implementation detail of how KUnit works under the hood. Nevertheless, there
-are instances where being able to run architecture-specific code or test
-against real hardware is desirable. For these reasons KUnit supports running on
-other architectures.
-
-Running existing KUnit tests on non-UML architectures
------------------------------------------------------
+Exiting Early on Failed Expectations
+------------------------------------
 
-There are some special considerations when running existing KUnit tests on
-non-UML architectures:
+We can use ``KUNIT_EXPECT_EQ`` to mark the test as failed and continue
+execution.  In some cases, it is unsafe to continue. We can use the
+``KUNIT_ASSERT`` variant to exit on failure.
 
-*   Hardware may not be deterministic, so a test that always passes or fails
-    when run under UML may not always do so on real hardware.
-*   Hardware and VM environments may not be hermetic. KUnit tries its best to
-    provide a hermetic environment to run tests; however, it cannot manage state
-    that it doesn't know about outside of the kernel. Consequently, tests that
-    may be hermetic on UML may not be hermetic on other architectures.
-*   Some features and tooling may not be supported outside of UML.
-*   Hardware and VMs are slower than UML.
+.. code-block:: c
 
-None of these are reasons not to run your KUnit tests on real hardware; they are
-only things to be aware of when doing so.
+	void example_test_user_alloc_function(struct kunit *test)
+	{
+		void *object = alloc_some_object_for_me();
 
-Currently, the KUnit Wrapper (``tools/testing/kunit/kunit.py``) (aka
-kunit_tool) only fully supports running tests inside of UML and QEMU; however,
-this is only due to our own time limitations as humans working on KUnit. It is
-entirely possible to support other emulators and even actual hardware, but for
-now QEMU and UML is what is fully supported within the KUnit Wrapper. Again, to
-be clear, this is just the Wrapper. The actualy KUnit tests and the KUnit
-library they are written in is fully architecture agnostic and can be used in
-virtually any setup, you just won't have the benefit of typing a single command
-out of the box and having everything magically work perfectly.
+		/* Make sure we got a valid pointer back. */
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, object);
+		do_something_with_object(object);
+	}
 
-Again, all core KUnit framework features are fully supported on all
-architectures, and using them is straightforward: Most popular architectures
-are supported directly in the KUnit Wrapper via QEMU. Currently, supported
-architectures on QEMU include:
+Allocating Memory
+-----------------
 
-*   i386
-*   x86_64
-*   arm
-*   arm64
-*   alpha
-*   powerpc
-*   riscv
-*   s390
-*   sparc
+Where you might use ``kzalloc``, you can instead use ``kunit_kzalloc`` as KUnit
+will then ensure that the memory is freed once the test completes.
 
-In order to run KUnit tests on one of these architectures via QEMU with the
-KUnit wrapper, all you need to do is specify the flags ``--arch`` and
-``--cross_compile`` when invoking the KUnit Wrapper. For example, we could run
-the default KUnit tests on ARM in the following manner (assuming we have an ARM
-toolchain installed):
+This is useful because it lets us use the ``KUNIT_ASSERT_EQ`` macros to exit
+early from a test without having to worry about remembering to call ``kfree``.
+For example:
 
-.. code-block:: bash
+.. code-block:: c
 
-	tools/testing/kunit/kunit.py run --timeout=60 --jobs=12 --arch=arm --cross_compile=arm-linux-gnueabihf-
+	void example_test_allocation(struct kunit *test)
+	{
+		char *buffer = kunit_kzalloc(test, 16, GFP_KERNEL);
+		/* Ensure allocation succeeded. */
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buffer);
 
-Alternatively, if you want to run your tests on real hardware or in some other
-emulation environment, all you need to do is to take your kunitconfig, your
-Kconfig options for the tests you would like to run, and merge them into
-whatever config your are using for your platform. That's it!
+		KUNIT_ASSERT_STREQ(test, buffer, "");
+	}
 
-For example, let's say you have the following kunitconfig:
 
-.. code-block:: none
+Testing Static Functions
+------------------------
 
-	CONFIG_KUNIT=y
-	CONFIG_KUNIT_EXAMPLE_TEST=y
+If we do not want to expose functions or variables for testing, one option is to
+conditionally ``#include`` the test file at the end of your .c file. For
+example:
 
-If you wanted to run this test on an x86 VM, you might add the following config
-options to your ``.config``:
+.. code-block:: c
 
-.. code-block:: none
+	/* In my_file.c */
 
-	CONFIG_KUNIT=y
-	CONFIG_KUNIT_EXAMPLE_TEST=y
-	CONFIG_SERIAL_8250=y
-	CONFIG_SERIAL_8250_CONSOLE=y
+	static int do_interesting_thing();
 
-All these new options do is enable support for a common serial console needed
-for logging.
+	#ifdef CONFIG_MY_KUNIT_TEST
+	#include "my_kunit_test.c"
+	#endif
 
-Next, you could build a kernel with these tests as follows:
+Injecting Test-Only Code
+------------------------
 
+Similar to as shown above, we can add test-specific logic. For example:
 
-.. code-block:: bash
+.. code-block:: c
 
-	make ARCH=x86 olddefconfig
-	make ARCH=x86
+	/* In my_file.h */
 
-Once you have built a kernel, you could run it on QEMU as follows:
+	#ifdef CONFIG_MY_KUNIT_TEST
+	/* Defined in my_kunit_test.c */
+	void test_only_hook(void);
+	#else
+	void test_only_hook(void) { }
+	#endif
 
-.. code-block:: bash
+This test-only code can be made more useful by accessing the current ``kunit_test``
+as shown in next section: *Accessing The Current Test*.
 
-	qemu-system-x86_64 -enable-kvm \
-			   -m 1024 \
-			   -kernel arch/x86_64/boot/bzImage \
-			   -append 'console=ttyS0' \
-			   --nographic
+Accessing The Current Test
+--------------------------
 
-Interspersed in the kernel logs you might see the following:
+In some cases, we need to call test-only code from outside the test file.
+For example, see example in section *Injecting Test-Only Code* or if
+we are providing a fake implementation of an ops struct. Using
+``kunit_test`` field in ``task_struct``, we can access it via
+``current->kunit_test``.
 
-.. code-block:: none
+The example below includes how to implement "mocking":
 
-	TAP version 14
-		# Subtest: example
-		1..1
-		# example_simple_test: initializing
-		ok 1 - example_simple_test
-	ok 1 - example
+.. code-block:: c
 
-Congratulations, you just ran a KUnit test on the x86 architecture!
+	#include <linux/sched.h> /* for current */
 
-In a similar manner, kunit and kunit tests can also be built as modules,
-so if you wanted to run tests in this way you might add the following config
-options to your ``.config``:
+	struct test_data {
+		int foo_result;
+		int want_foo_called_with;
+	};
 
-.. code-block:: none
+	static int fake_foo(int arg)
+	{
+		struct kunit *test = current->kunit_test;
+		struct test_data *test_data = test->priv;
 
-	CONFIG_KUNIT=m
-	CONFIG_KUNIT_EXAMPLE_TEST=m
+		KUNIT_EXPECT_EQ(test, test_data->want_foo_called_with, arg);
+		return test_data->foo_result;
+	}
 
-Once the kernel is built and installed, a simple
+	static void example_simple_test(struct kunit *test)
+	{
+		/* Assume priv (private, a member used to pass test data from
+		 * the init function) is allocated in the suite's .init */
+		struct test_data *test_data = test->priv;
 
-.. code-block:: bash
+		test_data->foo_result = 42;
+		test_data->want_foo_called_with = 1;
 
-	modprobe example-test
+		/* In a real test, we'd probably pass a pointer to fake_foo somewhere
+		 * like an ops struct, etc. instead of calling it directly. */
+		KUNIT_EXPECT_EQ(test, fake_foo(1), 42);
+	}
 
-...will run the tests.
+In this example, we are using the ``priv`` member of ``struct kunit`` as a way
+of passing data to the test from the init function. In general ``priv`` is
+pointer that can be used for any user data. This is preferred over static
+variables, as it avoids concurrency issues.
 
-.. note::
-   Note that you should make sure your test depends on ``KUNIT=y`` in Kconfig
-   if the test does not support module build.  Otherwise, it will trigger
-   compile errors if ``CONFIG_KUNIT`` is ``m``.
+Had we wanted something more flexible, we could have used a named ``kunit_resource``.
+Each test can have multiple resources which have string names providing the same
+flexibility as a ``priv`` member, but also, for example, allowing helper
+functions to create resources without conflicting with each other. It is also
+possible to define a clean up function for each resource, making it easy to
+avoid resource leaks. For more information, see Documentation/dev-tools/kunit/api/test.rst.
 
-Writing new tests for other architectures
------------------------------------------
+Failing The Current Test
+------------------------
 
-The first thing you must do is ask yourself whether it is necessary to write a
-KUnit test for a specific architecture, and then whether it is necessary to
-write that test for a particular piece of hardware. In general, writing a test
-that depends on having access to a particular piece of hardware or software (not
-included in the Linux source repo) should be avoided at all costs.
+If we want to fail the current test, we can use ``kunit_fail_current_test(fmt, args...)``
+which is defined in ``<kunit/test-bug.h>`` and does not require pulling in ``<kunit/test.h>``.
+For example, we have an option to enable some extra debug checks on some data
+structures as shown below:
 
-Even if you only ever plan on running your KUnit test on your hardware
-configuration, other people may want to run your tests and may not have access
-to your hardware. If you write your test to run on UML, then anyone can run your
-tests without knowing anything about your particular setup, and you can still
-run your tests on your hardware setup just by compiling for your architecture.
+.. code-block:: c
 
-.. important::
-   Always prefer tests that run on UML to tests that only run under a particular
-   architecture, and always prefer tests that run under QEMU or another easy
-   (and monetarily free) to obtain software environment to a specific piece of
-   hardware.
-
-Nevertheless, there are still valid reasons to write an architecture or hardware
-specific test: for example, you might want to test some code that really belongs
-in ``arch/some-arch/*``. Even so, try your best to write the test so that it
-does not depend on physical hardware: if some of your test cases don't need the
-hardware, only require the hardware for tests that actually need it.
-
-Now that you have narrowed down exactly what bits are hardware specific, the
-actual procedure for writing and running the tests is pretty much the same as
-writing normal KUnit tests. One special caveat is that you have to reset
-hardware state in between test cases; if this is not possible, you may only be
-able to run one test case per invocation.
+	#include <kunit/test-bug.h>
 
-.. TODO(brendanhiggins@google.com): Add an actual example of an architecture-
-   dependent KUnit test.
+	#ifdef CONFIG_EXTRA_DEBUG_CHECKS
+	static void validate_my_data(struct data *data)
+	{
+		if (is_valid(data))
+			return;
 
-KUnit debugfs representation
-============================
-When kunit test suites are initialized, they create an associated directory
-in ``/sys/kernel/debug/kunit/<test-suite>``.  The directory contains one file
+		kunit_fail_current_test("data %p is invalid", data);
 
-- results: "cat results" displays results of each test case and the results
-  of the entire suite for the last test run.
+		/* Normal, non-KUnit, error reporting code here. */
+	}
+	#else
+	static void my_debug_function(void) { }
+	#endif
 
-The debugfs representation is primarily of use when kunit test suites are
-run in a native environment, either as modules or builtin.  Having a way
-to display results like this is valuable as otherwise results can be
-intermixed with other events in dmesg output.  The maximum size of each
-results file is KUNIT_LOG_SIZE bytes (defined in ``include/kunit/test.h``).