[v18,15/19] Documentation: kunit: add documentation for KUnit
diff mbox series

Message ID 20190923090249.127984-16-brendanhiggins@google.com
State Mainlined
Commit c23a283b532c833de486e0fb8d756c7e7581803b
Headers show
Series
  • kunit: introduce KUnit, the Linux kernel unit testing framework
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Commit Message

Brendan Higgins Sept. 23, 2019, 9:02 a.m. UTC
Add documentation for KUnit, the Linux kernel unit testing framework.
- Add intro and usage guide for KUnit
- Add API reference

Signed-off-by: Felix Guo <felixguoxiuping@gmail.com>
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
---
 Documentation/dev-tools/index.rst           |   1 +
 Documentation/dev-tools/kunit/api/index.rst |  16 +
 Documentation/dev-tools/kunit/api/test.rst  |  11 +
 Documentation/dev-tools/kunit/faq.rst       |  62 +++
 Documentation/dev-tools/kunit/index.rst     |  79 +++
 Documentation/dev-tools/kunit/start.rst     | 180 ++++++
 Documentation/dev-tools/kunit/usage.rst     | 576 ++++++++++++++++++++
 7 files changed, 925 insertions(+)
 create mode 100644 Documentation/dev-tools/kunit/api/index.rst
 create mode 100644 Documentation/dev-tools/kunit/api/test.rst
 create mode 100644 Documentation/dev-tools/kunit/faq.rst
 create mode 100644 Documentation/dev-tools/kunit/index.rst
 create mode 100644 Documentation/dev-tools/kunit/start.rst
 create mode 100644 Documentation/dev-tools/kunit/usage.rst

Comments

Randy Dunlap Sept. 23, 2019, 3:47 p.m. UTC | #1
On 9/23/19 2:02 AM, Brendan Higgins wrote:
> Add documentation for KUnit, the Linux kernel unit testing framework.
> - Add intro and usage guide for KUnit
> - Add API reference
> 
> Signed-off-by: Felix Guo <felixguoxiuping@gmail.com>
> Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
> Reviewed-by: Stephen Boyd <sboyd@kernel.org>
> ---
>  Documentation/dev-tools/index.rst           |   1 +
>  Documentation/dev-tools/kunit/api/index.rst |  16 +
>  Documentation/dev-tools/kunit/api/test.rst  |  11 +
>  Documentation/dev-tools/kunit/faq.rst       |  62 +++
>  Documentation/dev-tools/kunit/index.rst     |  79 +++
>  Documentation/dev-tools/kunit/start.rst     | 180 ++++++
>  Documentation/dev-tools/kunit/usage.rst     | 576 ++++++++++++++++++++
>  7 files changed, 925 insertions(+)
>  create mode 100644 Documentation/dev-tools/kunit/api/index.rst
>  create mode 100644 Documentation/dev-tools/kunit/api/test.rst
>  create mode 100644 Documentation/dev-tools/kunit/faq.rst
>  create mode 100644 Documentation/dev-tools/kunit/index.rst
>  create mode 100644 Documentation/dev-tools/kunit/start.rst
>  create mode 100644 Documentation/dev-tools/kunit/usage.rst


> diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
> new file mode 100644
> index 000000000000..6dc229e46bb3
> --- /dev/null
> +++ b/Documentation/dev-tools/kunit/start.rst
> @@ -0,0 +1,180 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +===============
> +Getting Started
> +===============
> +
> +Installing dependencies
> +=======================
> +KUnit has the same dependencies as the Linux kernel. As long as you can build
> +the kernel, you can run KUnit.
> +
> +KUnit Wrapper
> +=============
> +Included with KUnit is a simple Python wrapper that helps format the output to
> +easily use and read KUnit output. It handles building and running the kernel, as
> +well as formatting the output.
> +
> +The wrapper can be run with:
> +
> +.. code-block:: bash
> +
> +   ./tools/testing/kunit/kunit.py run
> +
> +Creating a kunitconfig
> +======================
> +The Python script is a thin wrapper around Kbuild as such, it needs to be

                                       around Kbuild. As such,

> +configured with a ``kunitconfig`` file. This file essentially contains the
> +regular Kernel config, with the specific test targets as well.
> +
> +.. code-block:: bash
> +
> +	git clone -b master https://kunit.googlesource.com/kunitconfig $PATH_TO_KUNITCONFIG_REPO
> +	cd $PATH_TO_LINUX_REPO
> +	ln -s $PATH_TO_KUNIT_CONFIG_REPO/kunitconfig kunitconfig
> +
> +You may want to add kunitconfig to your local gitignore.
> +
> +Verifying KUnit Works
> +---------------------
> +
> +To make sure that everything is set up correctly, simply invoke the Python
> +wrapper from your kernel repo:
> +
> +.. code-block:: bash
> +
> +	./tools/testing/kunit/kunit.py
> +
> +.. note::
> +   You may want to run ``make mrproper`` first.

I normally use O=builddir when building kernels.
Does this support using O=builddir ?

> +
> +If everything worked correctly, you should see the following:
> +
> +.. code-block:: bash
> +
> +	Generating .config ...
> +	Building KUnit Kernel ...
> +	Starting KUnit Kernel ...
> +
> +followed by a list of tests that are run. All of them should be passing.
> +
> +.. note::
> +   Because it is building a lot of sources for the first time, the ``Building
> +   kunit kernel`` step may take a while.
> +
> +Writing your first test
> +=======================

[snip]

> diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
> new file mode 100644
> index 000000000000..c6e69634e274
> --- /dev/null
> +++ b/Documentation/dev-tools/kunit/usage.rst

TBD...
Brendan Higgins Sept. 23, 2019, 6:06 p.m. UTC | #2
On Mon, Sep 23, 2019 at 8:48 AM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> On 9/23/19 2:02 AM, Brendan Higgins wrote:
> > Add documentation for KUnit, the Linux kernel unit testing framework.
> > - Add intro and usage guide for KUnit
> > - Add API reference
> >
> > Signed-off-by: Felix Guo <felixguoxiuping@gmail.com>
> > Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
> > Reviewed-by: Stephen Boyd <sboyd@kernel.org>
> > ---
> >  Documentation/dev-tools/index.rst           |   1 +
> >  Documentation/dev-tools/kunit/api/index.rst |  16 +
> >  Documentation/dev-tools/kunit/api/test.rst  |  11 +
> >  Documentation/dev-tools/kunit/faq.rst       |  62 +++
> >  Documentation/dev-tools/kunit/index.rst     |  79 +++
> >  Documentation/dev-tools/kunit/start.rst     | 180 ++++++
> >  Documentation/dev-tools/kunit/usage.rst     | 576 ++++++++++++++++++++
> >  7 files changed, 925 insertions(+)
> >  create mode 100644 Documentation/dev-tools/kunit/api/index.rst
> >  create mode 100644 Documentation/dev-tools/kunit/api/test.rst
> >  create mode 100644 Documentation/dev-tools/kunit/faq.rst
> >  create mode 100644 Documentation/dev-tools/kunit/index.rst
> >  create mode 100644 Documentation/dev-tools/kunit/start.rst
> >  create mode 100644 Documentation/dev-tools/kunit/usage.rst
>
>
> > diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
> > new file mode 100644
> > index 000000000000..6dc229e46bb3
> > --- /dev/null
> > +++ b/Documentation/dev-tools/kunit/start.rst
> > @@ -0,0 +1,180 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +===============
> > +Getting Started
> > +===============
> > +
> > +Installing dependencies
> > +=======================
> > +KUnit has the same dependencies as the Linux kernel. As long as you can build
> > +the kernel, you can run KUnit.
> > +
> > +KUnit Wrapper
> > +=============
> > +Included with KUnit is a simple Python wrapper that helps format the output to
> > +easily use and read KUnit output. It handles building and running the kernel, as
> > +well as formatting the output.
> > +
> > +The wrapper can be run with:
> > +
> > +.. code-block:: bash
> > +
> > +   ./tools/testing/kunit/kunit.py run
> > +
> > +Creating a kunitconfig
> > +======================
> > +The Python script is a thin wrapper around Kbuild as such, it needs to be
>
>                                        around Kbuild. As such,

Thanks for pointing this out.

>
> > +configured with a ``kunitconfig`` file. This file essentially contains the
> > +regular Kernel config, with the specific test targets as well.
> > +
> > +.. code-block:: bash
> > +
> > +     git clone -b master https://kunit.googlesource.com/kunitconfig $PATH_TO_KUNITCONFIG_REPO
> > +     cd $PATH_TO_LINUX_REPO
> > +     ln -s $PATH_TO_KUNIT_CONFIG_REPO/kunitconfig kunitconfig
> > +
> > +You may want to add kunitconfig to your local gitignore.
> > +
> > +Verifying KUnit Works
> > +---------------------
> > +
> > +To make sure that everything is set up correctly, simply invoke the Python
> > +wrapper from your kernel repo:
> > +
> > +.. code-block:: bash
> > +
> > +     ./tools/testing/kunit/kunit.py
> > +
> > +.. note::
> > +   You may want to run ``make mrproper`` first.
>
> I normally use O=builddir when building kernels.
> Does this support using O=builddir ?

Yep, it supports specifying a separate build directory.

> > +
> > +If everything worked correctly, you should see the following:
> > +
> > +.. code-block:: bash
> > +
> > +     Generating .config ...
> > +     Building KUnit Kernel ...
> > +     Starting KUnit Kernel ...
> > +
> > +followed by a list of tests that are run. All of them should be passing.
> > +
> > +.. note::
> > +   Because it is building a lot of sources for the first time, the ``Building
> > +   kunit kernel`` step may take a while.
> > +
> > +Writing your first test
> > +=======================
>
> [snip]
>
> > diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
> > new file mode 100644
> > index 000000000000..c6e69634e274
> > --- /dev/null
> > +++ b/Documentation/dev-tools/kunit/usage.rst
>
> TBD...

What did you mean by this comment?

Cheers
Randy Dunlap Sept. 23, 2019, 7:49 p.m. UTC | #3
On 9/23/19 11:06 AM, Brendan Higgins wrote:
> On Mon, Sep 23, 2019 at 8:48 AM Randy Dunlap <rdunlap@infradead.org> wrote:
>>
>> On 9/23/19 2:02 AM, Brendan Higgins wrote:
>>> Add documentation for KUnit, the Linux kernel unit testing framework.
>>> - Add intro and usage guide for KUnit
>>> - Add API reference
>>>
>>> Signed-off-by: Felix Guo <felixguoxiuping@gmail.com>
>>> Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
>>> Cc: Jonathan Corbet <corbet@lwn.net>
>>> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>> Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
>>> Reviewed-by: Stephen Boyd <sboyd@kernel.org>
>>> ---
>>>  Documentation/dev-tools/index.rst           |   1 +
>>>  Documentation/dev-tools/kunit/api/index.rst |  16 +
>>>  Documentation/dev-tools/kunit/api/test.rst  |  11 +
>>>  Documentation/dev-tools/kunit/faq.rst       |  62 +++
>>>  Documentation/dev-tools/kunit/index.rst     |  79 +++
>>>  Documentation/dev-tools/kunit/start.rst     | 180 ++++++
>>>  Documentation/dev-tools/kunit/usage.rst     | 576 ++++++++++++++++++++
>>>  7 files changed, 925 insertions(+)
>>>  create mode 100644 Documentation/dev-tools/kunit/api/index.rst
>>>  create mode 100644 Documentation/dev-tools/kunit/api/test.rst
>>>  create mode 100644 Documentation/dev-tools/kunit/faq.rst
>>>  create mode 100644 Documentation/dev-tools/kunit/index.rst
>>>  create mode 100644 Documentation/dev-tools/kunit/start.rst
>>>  create mode 100644 Documentation/dev-tools/kunit/usage.rst
>>
>>
>>> diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
>>> new file mode 100644
>>> index 000000000000..6dc229e46bb3
>>> --- /dev/null
>>> +++ b/Documentation/dev-tools/kunit/start.rst
>>> @@ -0,0 +1,180 @@
>>> +.. SPDX-License-Identifier: GPL-2.0
>>> +
>>> +===============
>>> +Getting Started
>>> +===============
>>> +
>>> +Installing dependencies
>>> +=======================
>>> +KUnit has the same dependencies as the Linux kernel. As long as you can build
>>> +the kernel, you can run KUnit.
>>> +
>>> +KUnit Wrapper
>>> +=============
>>> +Included with KUnit is a simple Python wrapper that helps format the output to
>>> +easily use and read KUnit output. It handles building and running the kernel, as
>>> +well as formatting the output.
>>> +
>>> +The wrapper can be run with:
>>> +
>>> +.. code-block:: bash
>>> +
>>> +   ./tools/testing/kunit/kunit.py run
>>> +
>>> +Creating a kunitconfig
>>> +======================
>>> +The Python script is a thin wrapper around Kbuild as such, it needs to be
>>
>>                                        around Kbuild. As such,
> 
> Thanks for pointing this out.
> 
>>
>>> +configured with a ``kunitconfig`` file. This file essentially contains the
>>> +regular Kernel config, with the specific test targets as well.
>>> +
>>> +.. code-block:: bash
>>> +
>>> +     git clone -b master https://kunit.googlesource.com/kunitconfig $PATH_TO_KUNITCONFIG_REPO
>>> +     cd $PATH_TO_LINUX_REPO
>>> +     ln -s $PATH_TO_KUNIT_CONFIG_REPO/kunitconfig kunitconfig
>>> +
>>> +You may want to add kunitconfig to your local gitignore.
>>> +
>>> +Verifying KUnit Works
>>> +---------------------
>>> +
>>> +To make sure that everything is set up correctly, simply invoke the Python
>>> +wrapper from your kernel repo:
>>> +
>>> +.. code-block:: bash
>>> +
>>> +     ./tools/testing/kunit/kunit.py
>>> +
>>> +.. note::
>>> +   You may want to run ``make mrproper`` first.
>>
>> I normally use O=builddir when building kernels.
>> Does this support using O=builddir ?
> 
> Yep, it supports specifying a separate build directory.
> 
>>> +
>>> +If everything worked correctly, you should see the following:
>>> +
>>> +.. code-block:: bash
>>> +
>>> +     Generating .config ...
>>> +     Building KUnit Kernel ...
>>> +     Starting KUnit Kernel ...
>>> +
>>> +followed by a list of tests that are run. All of them should be passing.
>>> +
>>> +.. note::
>>> +   Because it is building a lot of sources for the first time, the ``Building
>>> +   kunit kernel`` step may take a while.
>>> +
>>> +Writing your first test
>>> +=======================
>>
>> [snip]
>>
>>> diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
>>> new file mode 100644
>>> index 000000000000..c6e69634e274
>>> --- /dev/null
>>> +++ b/Documentation/dev-tools/kunit/usage.rst
>>
>> TBD...
> 
> What did you mean by this comment?

I plan to review usage.rst soon... (To Be Done :)
shuah Sept. 23, 2019, 9:18 p.m. UTC | #4
On 9/23/19 1:49 PM, Randy Dunlap wrote:
> On 9/23/19 11:06 AM, Brendan Higgins wrote:
>> On Mon, Sep 23, 2019 at 8:48 AM Randy Dunlap <rdunlap@infradead.org> wrote:
>>>
>>> On 9/23/19 2:02 AM, Brendan Higgins wrote:
>>>> Add documentation for KUnit, the Linux kernel unit testing framework.
>>>> - Add intro and usage guide for KUnit
>>>> - Add API reference
>>>>
>>>> Signed-off-by: Felix Guo <felixguoxiuping@gmail.com>
>>>> Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
>>>> Cc: Jonathan Corbet <corbet@lwn.net>
>>>> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>> Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
>>>> Reviewed-by: Stephen Boyd <sboyd@kernel.org>
>>>> ---
>>>>   Documentation/dev-tools/index.rst           |   1 +
>>>>   Documentation/dev-tools/kunit/api/index.rst |  16 +
>>>>   Documentation/dev-tools/kunit/api/test.rst  |  11 +
>>>>   Documentation/dev-tools/kunit/faq.rst       |  62 +++
>>>>   Documentation/dev-tools/kunit/index.rst     |  79 +++
>>>>   Documentation/dev-tools/kunit/start.rst     | 180 ++++++
>>>>   Documentation/dev-tools/kunit/usage.rst     | 576 ++++++++++++++++++++
>>>>   7 files changed, 925 insertions(+)
>>>>   create mode 100644 Documentation/dev-tools/kunit/api/index.rst
>>>>   create mode 100644 Documentation/dev-tools/kunit/api/test.rst
>>>>   create mode 100644 Documentation/dev-tools/kunit/faq.rst
>>>>   create mode 100644 Documentation/dev-tools/kunit/index.rst
>>>>   create mode 100644 Documentation/dev-tools/kunit/start.rst
>>>>   create mode 100644 Documentation/dev-tools/kunit/usage.rst
>>>
>>>
>>>> diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
>>>> new file mode 100644
>>>> index 000000000000..6dc229e46bb3
>>>> --- /dev/null
>>>> +++ b/Documentation/dev-tools/kunit/start.rst
>>>> @@ -0,0 +1,180 @@
>>>> +.. SPDX-License-Identifier: GPL-2.0
>>>> +
>>>> +===============
>>>> +Getting Started
>>>> +===============
>>>> +
>>>> +Installing dependencies
>>>> +=======================
>>>> +KUnit has the same dependencies as the Linux kernel. As long as you can build
>>>> +the kernel, you can run KUnit.
>>>> +
>>>> +KUnit Wrapper
>>>> +=============
>>>> +Included with KUnit is a simple Python wrapper that helps format the output to
>>>> +easily use and read KUnit output. It handles building and running the kernel, as
>>>> +well as formatting the output.
>>>> +
>>>> +The wrapper can be run with:
>>>> +
>>>> +.. code-block:: bash
>>>> +
>>>> +   ./tools/testing/kunit/kunit.py run
>>>> +
>>>> +Creating a kunitconfig
>>>> +======================
>>>> +The Python script is a thin wrapper around Kbuild as such, it needs to be
>>>
>>>                                         around Kbuild. As such,
>>
>> Thanks for pointing this out.
>>
>>>
>>>> +configured with a ``kunitconfig`` file. This file essentially contains the
>>>> +regular Kernel config, with the specific test targets as well.
>>>> +
>>>> +.. code-block:: bash
>>>> +
>>>> +     git clone -b master https://kunit.googlesource.com/kunitconfig $PATH_TO_KUNITCONFIG_REPO
>>>> +     cd $PATH_TO_LINUX_REPO
>>>> +     ln -s $PATH_TO_KUNIT_CONFIG_REPO/kunitconfig kunitconfig
>>>> +
>>>> +You may want to add kunitconfig to your local gitignore.
>>>> +
>>>> +Verifying KUnit Works
>>>> +---------------------
>>>> +
>>>> +To make sure that everything is set up correctly, simply invoke the Python
>>>> +wrapper from your kernel repo:
>>>> +
>>>> +.. code-block:: bash
>>>> +
>>>> +     ./tools/testing/kunit/kunit.py
>>>> +
>>>> +.. note::
>>>> +   You may want to run ``make mrproper`` first.
>>>
>>> I normally use O=builddir when building kernels.
>>> Does this support using O=builddir ?
>>
>> Yep, it supports specifying a separate build directory.
>>
>>>> +
>>>> +If everything worked correctly, you should see the following:
>>>> +
>>>> +.. code-block:: bash
>>>> +
>>>> +     Generating .config ...
>>>> +     Building KUnit Kernel ...
>>>> +     Starting KUnit Kernel ...
>>>> +
>>>> +followed by a list of tests that are run. All of them should be passing.
>>>> +
>>>> +.. note::
>>>> +   Because it is building a lot of sources for the first time, the ``Building
>>>> +   kunit kernel`` step may take a while.
>>>> +
>>>> +Writing your first test
>>>> +=======================
>>>
>>> [snip]
>>>
>>>> diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
>>>> new file mode 100644
>>>> index 000000000000..c6e69634e274
>>>> --- /dev/null
>>>> +++ b/Documentation/dev-tools/kunit/usage.rst
>>>
>>> TBD...
>>
>> What did you mean by this comment?
> 
> I plan to review usage.rst soon... (To Be Done :)
> 

I would like to apply the series very soon so it gets some soak time
after this move in linux-next and it can still make the rc1.

Since there changes can be addressed after rc1, I would like to not
require Brendan to do another version before I apply.

Hope you are okay with that Randy!

thanks,
-- Shuah
Randy Dunlap Sept. 23, 2019, 9:30 p.m. UTC | #5
On 9/23/19 2:18 PM, shuah wrote:

> I would like to apply the series very soon so it gets some soak time
> after this move in linux-next and it can still make the rc1.
> 
> Since there changes can be addressed after rc1, I would like to not
> require Brendan to do another version before I apply.
> 
> Hope you are okay with that Randy!
> 
> thanks,
> -- Shuah

Sure, no problem, go for it .
Randy Dunlap Sept. 24, 2019, 12:51 a.m. UTC | #6
On 9/23/19 2:02 AM, Brendan Higgins wrote:

> diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
> new file mode 100644
> index 000000000000..c6e69634e274
> --- /dev/null
> +++ b/Documentation/dev-tools/kunit/usage.rst
> @@ -0,0 +1,576 @@
> +.. 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 :doc:`start`.
> +
> +Organization of this document
> +=============================
> +
> +This document is organized into two main sections: Testing and Isolating
> +Behavior. The first covers what a unit test is and how to use KUnit to write

                              what unit tests are
would agree with the following "them."

> +them. The second covers how to use KUnit to isolate code and make it possible
> +to unit test code that was otherwise un-unit-testable.
> +
> +Testing
> +=======
> +

[snip]


> +
> +Test Suites
> +~~~~~~~~~~~
> +
> +Now obviously one unit test isn't very helpful; the power comes from having
> +many test cases covering all of your behaviors. Consequently it is common to

                   covering all of a unit's behaviors.

> +have many *similar* tests; in order to reduce duplication in these closely
> +related tests most unit testing frameworks provide the concept of a *test
> +suite*, in KUnit we call it a *test suite*; all it is is just a collection of

                                             . This is just a collection of

> +test cases for a unit of code with a set up function that gets invoked before
> +every test cases and then a tear down function that gets invoked after every

   every test case

> +test case completes.
> +
> +Example:
> +
> +.. code-block:: c
> +
> +	static struct kunit_case example_test_cases[] = {
> +		KUNIT_CASE(example_test_foo),
> +		KUNIT_CASE(example_test_bar),
> +		KUNIT_CASE(example_test_baz),
> +		{}
> +	};
> +
> +	static struct kunit_suite example_test_suite = {
> +		.name = "example",
> +		.init = example_test_init,
> +		.exit = example_test_exit,
> +		.test_cases = example_test_cases,
> +	};
> +	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.
> +``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.
> +
> +For a more information on these types of things see the :doc:`api/test`.

   For more

> +
> +Isolating Behavior
> +==================
> +

[snip]

> +
> +.. _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

                           I would drop the comma above.

> +against real hardware is desirable. For these reasons KUnit supports running on
> +other architectures.
> +
> +Running existing KUnit tests on non-UML architectures
> +-----------------------------------------------------
> +

[snip]

> +Writing new tests for other architectures
> +-----------------------------------------
> +
> +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.
> +
> +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.
> +
> +.. 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 monitarily free) to obtain software environment to a specific piece of

           monetarily

> +   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.
> +
> +.. TODO(brendanhiggins@google.com): Add an actual example of an architecture
> +   dependent KUnit test.

Patch
diff mbox series

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index b0522a4dd107..09dee10d2592 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -24,6 +24,7 @@  whole; patches welcome!
    gdb-kernel-debugging
    kgdb
    kselftest
+   kunit/index
 
 
 .. only::  subproject and html
diff --git a/Documentation/dev-tools/kunit/api/index.rst b/Documentation/dev-tools/kunit/api/index.rst
new file mode 100644
index 000000000000..9b9bffe5d41a
--- /dev/null
+++ b/Documentation/dev-tools/kunit/api/index.rst
@@ -0,0 +1,16 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+API Reference
+=============
+.. toctree::
+
+	test
+
+This section documents the KUnit kernel testing API. It is divided into the
+following sections:
+
+================================= ==============================================
+:doc:`test`                       documents all of the standard testing API
+                                  excluding mocking or mocking related features.
+================================= ==============================================
diff --git a/Documentation/dev-tools/kunit/api/test.rst b/Documentation/dev-tools/kunit/api/test.rst
new file mode 100644
index 000000000000..aaa97f17e5b3
--- /dev/null
+++ b/Documentation/dev-tools/kunit/api/test.rst
@@ -0,0 +1,11 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+========
+Test API
+========
+
+This file documents all of the standard testing API excluding mocking or mocking
+related features.
+
+.. kernel-doc:: include/kunit/test.h
+   :internal:
diff --git a/Documentation/dev-tools/kunit/faq.rst b/Documentation/dev-tools/kunit/faq.rst
new file mode 100644
index 000000000000..bf2095112d89
--- /dev/null
+++ b/Documentation/dev-tools/kunit/faq.rst
@@ -0,0 +1,62 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+Frequently Asked Questions
+==========================
+
+How is this different from Autotest, kselftest, etc?
+====================================================
+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, hence the name. A unit test should be
+the finest granularity of testing and as such should allow all possible code
+paths to be tested in the code under test; this is only possible if the code
+under test is very 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 VM and all 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, well, mostly.
+
+For the most part, the KUnit core framework (what you 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. However, there is some infrastructure,
+like the KUnit Wrapper (``tools/testing/kunit/kunit.py``) that does not support
+other architectures.
+
+In short, this means that, 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`.
+
+What is the difference between a unit test and these 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, hence the
+  name. A unit test should be the finest granularity of testing and as such
+  should allow all possible code paths to be tested in the code under test; this
+  is only possible if the code under test is very 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.
diff --git a/Documentation/dev-tools/kunit/index.rst b/Documentation/dev-tools/kunit/index.rst
new file mode 100644
index 000000000000..26ffb46bdf99
--- /dev/null
+++ b/Documentation/dev-tools/kunit/index.rst
@@ -0,0 +1,79 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================================
+KUnit - Unit Testing for the Linux Kernel
+=========================================
+
+.. toctree::
+	:maxdepth: 2
+
+	start
+	usage
+	api/index
+	faq
+
+What is KUnit?
+==============
+
+KUnit is a lightweight unit testing and mocking framework for the Linux kernel.
+These tests are able to be run locally on a developer's workstation without a VM
+or special hardware.
+
+KUnit is heavily inspired by JUnit, Python's unittest.mock, and
+Googletest/Googlemock for C++. KUnit provides facilities for defining unit test
+cases, grouping related test cases into test suites, providing common
+infrastructure for running tests, and much more.
+
+Get started now: :doc:`start`
+
+Why KUnit?
+==========
+
+A unit test is supposed to test a single unit of code in isolation, hence the
+name. A unit test should be the finest granularity of testing and as such should
+allow all possible code paths to be tested in the code under test; this is only
+possible if the code under test is very small and does not have any external
+dependencies outside of the test's control like hardware.
+
+Outside of KUnit, there are no testing frameworks currently
+available for the kernel that do not require installing the kernel on a test
+machine or in a VM and all require tests to be written in userspace running on
+the kernel; this is true for Autotest, and kselftest, disqualifying
+any of them from being considered unit testing frameworks.
+
+KUnit addresses the problem of being able to run tests without needing a virtual
+machine or actual hardware with User Mode Linux. User Mode Linux is a Linux
+architecture, like ARM or x86; however, unlike other architectures it compiles
+to a standalone program that can be run like any other program directly inside
+of a host operating system; to be clear, it does not require any virtualization
+support; it is just a regular program.
+
+KUnit is fast. Excluding build time, from invocation to completion KUnit can run
+several dozen tests in only 10 to 20 seconds; this might not sound like a big
+deal to some people, but having such fast and easy to run tests fundamentally
+changes the way you go about testing and even writing code in the first place.
+Linus himself said in his `git talk at Google
+<https://gist.github.com/lorn/1272686/revisions#diff-53c65572127855f1b003db4064a94573R874>`_:
+
+	"... a lot of people seem to think that performance is about doing the
+	same thing, just doing it faster, and that is not true. That is not what
+	performance is all about. If you can do something really fast, really
+	well, people will start using it differently."
+
+In this context Linus was talking about branching and merging,
+but this point also applies to testing. If your tests are slow, unreliable, are
+difficult to write, and require a special setup or special hardware to run,
+then you wait a lot longer to write tests, and you wait a lot longer to run
+tests; this means that tests are likely to break, unlikely to test a lot of
+things, and are unlikely to be rerun once they pass. If your tests are really
+fast, you run them all the time, every time you make a change, and every time
+someone sends you some code. Why trust that someone ran all their tests
+correctly on every change when you can just run them yourself in less time than
+it takes to read their test log?
+
+How do I use it?
+================
+
+*   :doc:`start` - for new users of KUnit
+*   :doc:`usage` - for a more detailed explanation of KUnit features
+*   :doc:`api/index` - for the list of KUnit APIs used for testing
diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
new file mode 100644
index 000000000000..6dc229e46bb3
--- /dev/null
+++ b/Documentation/dev-tools/kunit/start.rst
@@ -0,0 +1,180 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+Getting Started
+===============
+
+Installing dependencies
+=======================
+KUnit has the same dependencies as the Linux kernel. As long as you can build
+the kernel, you can run KUnit.
+
+KUnit Wrapper
+=============
+Included with KUnit is a simple Python wrapper that helps format the output to
+easily use and read KUnit output. It handles building and running the kernel, as
+well as formatting the output.
+
+The wrapper can be run with:
+
+.. code-block:: bash
+
+   ./tools/testing/kunit/kunit.py run
+
+Creating a kunitconfig
+======================
+The Python script is a thin wrapper around Kbuild as such, it needs to be
+configured with a ``kunitconfig`` file. This file essentially contains the
+regular Kernel config, with the specific test targets as well.
+
+.. code-block:: bash
+
+	git clone -b master https://kunit.googlesource.com/kunitconfig $PATH_TO_KUNITCONFIG_REPO
+	cd $PATH_TO_LINUX_REPO
+	ln -s $PATH_TO_KUNIT_CONFIG_REPO/kunitconfig kunitconfig
+
+You may want to add kunitconfig to your local gitignore.
+
+Verifying KUnit Works
+---------------------
+
+To make sure that everything is set up correctly, simply invoke the Python
+wrapper from your kernel repo:
+
+.. code-block:: bash
+
+	./tools/testing/kunit/kunit.py
+
+.. note::
+   You may want to run ``make mrproper`` first.
+
+If everything worked correctly, you should see the following:
+
+.. code-block:: bash
+
+	Generating .config ...
+	Building KUnit Kernel ...
+	Starting KUnit Kernel ...
+
+followed by a list of tests that are run. All of them should be passing.
+
+.. note::
+   Because it is building a lot of sources for the first time, the ``Building
+   kunit kernel`` step may take a while.
+
+Writing your first test
+=======================
+
+In your kernel repo let's add some code that we can test. Create a file
+``drivers/misc/example.h`` with the contents:
+
+.. code-block:: c
+
+	int misc_example_add(int left, int right);
+
+create a file ``drivers/misc/example.c``:
+
+.. code-block:: c
+
+	#include <linux/errno.h>
+
+	#include "example.h"
+
+	int misc_example_add(int left, int right)
+	{
+		return left + right;
+	}
+
+Now add the following lines to ``drivers/misc/Kconfig``:
+
+.. code-block:: kconfig
+
+	config MISC_EXAMPLE
+		bool "My example"
+
+and the following lines to ``drivers/misc/Makefile``:
+
+.. code-block:: make
+
+	obj-$(CONFIG_MISC_EXAMPLE) += example.o
+
+Now we are ready to write the test. The test will be in
+``drivers/misc/example-test.c``:
+
+.. code-block:: c
+
+	#include <kunit/test.h>
+	#include "example.h"
+
+	/* Define the test cases. */
+
+	static void misc_example_add_test_basic(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, 1, misc_example_add(1, 0));
+		KUNIT_EXPECT_EQ(test, 2, misc_example_add(1, 1));
+		KUNIT_EXPECT_EQ(test, 0, misc_example_add(-1, 1));
+		KUNIT_EXPECT_EQ(test, INT_MAX, misc_example_add(0, INT_MAX));
+		KUNIT_EXPECT_EQ(test, -1, misc_example_add(INT_MAX, INT_MIN));
+	}
+
+	static void misc_example_test_failure(struct kunit *test)
+	{
+		KUNIT_FAIL(test, "This test never passes.");
+	}
+
+	static struct kunit_case misc_example_test_cases[] = {
+		KUNIT_CASE(misc_example_add_test_basic),
+		KUNIT_CASE(misc_example_test_failure),
+		{}
+	};
+
+	static struct kunit_suite misc_example_test_suite = {
+		.name = "misc-example",
+		.test_cases = misc_example_test_cases,
+	};
+	kunit_test_suite(misc_example_test_suite);
+
+Now add the following to ``drivers/misc/Kconfig``:
+
+.. code-block:: kconfig
+
+	config MISC_EXAMPLE_TEST
+		bool "Test for my example"
+		depends on MISC_EXAMPLE && KUNIT
+
+and the following to ``drivers/misc/Makefile``:
+
+.. code-block:: make
+
+	obj-$(CONFIG_MISC_EXAMPLE_TEST) += example-test.o
+
+Now add it to your ``kunitconfig``:
+
+.. code-block:: none
+
+	CONFIG_MISC_EXAMPLE=y
+	CONFIG_MISC_EXAMPLE_TEST=y
+
+Now you can run the test:
+
+.. code-block:: bash
+
+	./tools/testing/kunit/kunit.py
+
+You should see the following failure:
+
+.. code-block:: none
+
+	...
+	[16:08:57] [PASSED] misc-example:misc_example_add_test_basic
+	[16:08:57] [FAILED] misc-example:misc_example_test_failure
+	[16:08:57] EXPECTATION FAILED at drivers/misc/example-test.c:17
+	[16:08:57] 	This test never passes.
+	...
+
+Congrats! You just wrote your first KUnit test!
+
+Next Steps
+==========
+*   Check out the :doc:`usage` page for a more
+    in-depth explanation of KUnit.
diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
new file mode 100644
index 000000000000..c6e69634e274
--- /dev/null
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -0,0 +1,576 @@ 
+.. 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 :doc:`start`.
+
+Organization of this document
+=============================
+
+This document is organized into two main sections: Testing and Isolating
+Behavior. The first covers what a unit test is and how to use KUnit to write
+them. The second covers how to use KUnit 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
+and then sets *expectations* for what should happen. For example:
+
+.. code-block:: c
+
+	void example_test_success(struct kunit *test)
+	{
+	}
+
+	void example_test_failure(struct kunit *test)
+	{
+		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.
+
+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:
+
+.. code-block:: c
+
+	void add_test_basic(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, 1, add(1, 0));
+		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
+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 fail, 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.
+
+To learn about more expectations supported by KUnit, see :doc:`api/test`.
+
+.. note::
+   A single test case should be pretty short, pretty easy to understand,
+   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:
+
+.. code-block:: c
+
+	void add_test_basic(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, 1, add(1, 0));
+		KUNIT_EXPECT_EQ(test, 2, add(1, 1));
+	}
+
+	void add_test_negative(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, 0, add(-1, 1));
+	}
+
+	void add_test_max(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, INT_MAX, add(0, INT_MAX));
+		KUNIT_EXPECT_EQ(test, -1, add(INT_MAX, INT_MIN));
+	}
+
+	void add_test_overflow(struct kunit *test)
+	{
+		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:
+
+.. code-block:: c
+
+	static void mock_test_do_expect_default_return(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));
+	}
+
+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.
+
+Test Suites
+~~~~~~~~~~~
+
+Now obviously one unit test isn't very helpful; the power comes from having
+many test cases covering all of your behaviors. Consequently it is common to
+have many *similar* tests; in order to reduce duplication in these closely
+related tests most unit testing frameworks provide the concept of a *test
+suite*, in KUnit we call it a *test suite*; all it is is just a collection of
+test cases for a unit of code with a set up function that gets invoked before
+every test cases and then a tear down function that gets invoked after every
+test case completes.
+
+Example:
+
+.. code-block:: c
+
+	static struct kunit_case example_test_cases[] = {
+		KUNIT_CASE(example_test_foo),
+		KUNIT_CASE(example_test_bar),
+		KUNIT_CASE(example_test_baz),
+		{}
+	};
+
+	static struct kunit_suite example_test_suite = {
+		.name = "example",
+		.init = example_test_init,
+		.exit = example_test_exit,
+		.test_cases = example_test_cases,
+	};
+	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.
+``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.
+
+For a more information on these types of things see the :doc:`api/test`.
+
+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.
+
+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.
+
+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:
+
+.. code-block:: c
+
+	struct shape {
+		int (*area)(struct shape *this);
+	};
+
+	struct rectangle {
+		struct shape parent;
+		int length;
+		int width;
+	};
+
+	int rectangle_area(struct shape *this)
+	{
+		struct rectangle *self = container_of(this, struct shape, parent);
+
+		return self->length * self->width;
+	};
+
+	void rectangle_new(struct rectangle *self, int length, int width)
+	{
+		self->parent.area = rectangle_area;
+		self->length = length;
+		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``.
+
+Faking Classes
+~~~~~~~~~~~~~~
+
+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:
+
+.. code-block:: c
+
+	struct eeprom {
+		ssize_t (*read)(struct eeprom *this, size_t offset, char *buffer, size_t count);
+		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:
+
+.. code-block:: c
+
+	struct eeprom_buffer {
+		ssize_t (*write)(struct eeprom_buffer *this, const char *buffer, size_t count);
+		int flush(struct eeprom_buffer *this);
+		size_t flush_count; /* Flushes when buffer exceeds flush_count. */
+	};
+
+	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:
+
+.. code-block:: c
+
+	struct fake_eeprom {
+		struct eeprom parent;
+		char contents[FAKE_EEPROM_CONTENTS_SIZE];
+	};
+
+	ssize_t fake_eeprom_read(struct eeprom *parent, size_t offset, char *buffer, size_t count)
+	{
+		struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);
+
+		count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
+		memcpy(buffer, this->contents + offset, count);
+
+		return count;
+	}
+
+	ssize_t fake_eeprom_write(struct eeprom *this, size_t offset, const char *buffer, size_t count)
+	{
+		struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);
+
+		count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
+		memcpy(this->contents + offset, buffer, count);
+
+		return count;
+	}
+
+	void fake_eeprom_init(struct fake_eeprom *this)
+	{
+		this->parent.read = fake_eeprom_read;
+		this->parent.write = fake_eeprom_write;
+		memset(this->contents, 0, FAKE_EEPROM_CONTENTS_SIZE);
+	}
+
+We can now use it to test ``struct eeprom_buffer``:
+
+.. code-block:: c
+
+	struct eeprom_buffer_test {
+		struct fake_eeprom *fake_eeprom;
+		struct eeprom_buffer *eeprom_buffer;
+	};
+
+	static void eeprom_buffer_test_does_not_write_until_flush(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+		struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+		struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+		char buffer[] = {0xff};
+
+		eeprom_buffer->flush_count = SIZE_MAX;
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0);
+
+		eeprom_buffer->flush(eeprom_buffer);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+	}
+
+	static void eeprom_buffer_test_flushes_after_flush_count_met(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+		struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+		struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+		char buffer[] = {0xff};
+
+		eeprom_buffer->flush_count = 2;
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+	}
+
+	static void eeprom_buffer_test_flushes_increments_of_flush_count(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+		struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+		struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+		char buffer[] = {0xff, 0xff};
+
+		eeprom_buffer->flush_count = 2;
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 2);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+		/* Should have only flushed the first two bytes. */
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[2], 0);
+	}
+
+	static int eeprom_buffer_test_init(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx;
+
+		ctx = kunit_kzalloc(test, sizeof(*ctx), GFP_KERNEL);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx);
+
+		ctx->fake_eeprom = kunit_kzalloc(test, sizeof(*ctx->fake_eeprom), GFP_KERNEL);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->fake_eeprom);
+		fake_eeprom_init(ctx->fake_eeprom);
+
+		ctx->eeprom_buffer = new_eeprom_buffer(&ctx->fake_eeprom->parent);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->eeprom_buffer);
+
+		test->priv = ctx;
+
+		return 0;
+	}
+
+	static void eeprom_buffer_test_exit(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+
+		destroy_eeprom_buffer(ctx->eeprom_buffer);
+	}
+
+.. _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
+-----------------------------------------------------
+
+There are some special considerations when running existing KUnit tests on
+non-UML architectures:
+
+*   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.
+
+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.
+
+The biggest impediment will likely be that certain KUnit features and
+infrastructure may not support your target environment. For example, at this
+time the KUnit Wrapper (``tools/testing/kunit/kunit.py``) does not work outside
+of UML. Unfortunately, there is no way around this. Using UML (or even just a
+particular architecture) allows us to make a lot of assumptions that make it
+possible to do things which might otherwise be impossible.
+
+Nevertheless, all core KUnit framework features are fully supported on all
+architectures, and using them is straightforward: 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!
+
+For example, let's say you have the following kunitconfig:
+
+.. code-block:: none
+
+	CONFIG_KUNIT=y
+	CONFIG_KUNIT_EXAMPLE_TEST=y
+
+If you wanted to run this test on an x86 VM, you might add the following config
+options to your ``.config``:
+
+.. code-block:: none
+
+	CONFIG_KUNIT=y
+	CONFIG_KUNIT_EXAMPLE_TEST=y
+	CONFIG_SERIAL_8250=y
+	CONFIG_SERIAL_8250_CONSOLE=y
+
+All these new options do is enable support for a common serial console needed
+for logging.
+
+Next, you could build a kernel with these tests as follows:
+
+
+.. code-block:: bash
+
+	make ARCH=x86 olddefconfig
+	make ARCH=x86
+
+Once you have built a kernel, you could run it on QEMU as follows:
+
+.. code-block:: bash
+
+	qemu-system-x86_64 -enable-kvm \
+			   -m 1024 \
+			   -kernel arch/x86_64/boot/bzImage \
+			   -append 'console=ttyS0' \
+			   --nographic
+
+Interspersed in the kernel logs you might see the following:
+
+.. code-block:: none
+
+	TAP version 14
+		# Subtest: example
+		1..1
+		# example_simple_test: initializing
+		ok 1 - example_simple_test
+	ok 1 - example
+
+Congratulations, you just ran a KUnit test on the x86 architecture!
+
+Writing new tests for other architectures
+-----------------------------------------
+
+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.
+
+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.
+
+.. 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 monitarily 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.
+
+.. TODO(brendanhiggins@google.com): Add an actual example of an architecture
+   dependent KUnit test.