doc: Add doc how to add unit tests

Change-Id: I73f6add194a531c4f88b822d92c31ec67c23d2dc
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/80340
Reviewed-by: Peter Marheine <pmarheine@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

doc/contrib_howtos/how_to_add_unit_test.rst

@@ -0,0 +1,101 @@
+======================
+How to add a unit test
+======================
+
+Unit tests help to maintain higher bar for code quality. A new unit test which adds coverage to the code is always useful,
+no matter how small or large it is! Unit test is a valuable contribution, moreover it makes a good starter project, since
+you don't need specific hardware (apart from you development host machine).
+
+For more details on how to run unit tests and measure coverage, check the dev guide: :ref:`unit tests`.
+
+To see the examples of existing unit tests, check the ``/tests`` directory in the source tree. If it helps, you can also look
+at git history for examples of previous commits that add new tests.
+
+When is a good time to add a unit test? Any time is a good time. Test can go in its own separate patch, and also it can go
+together in a patch which introduces a new code.
+
+Unit tests are using `cmocka <https://cmocka.org/>`_ as a mocking framework, but we also have flashrom mock framework
+on the top of that.
+
+Mocking
+=========
+
+Unit tests mock all interactions with hardware, interactions with filesystem, syscalls, 3rd party libraries calls
+(e.g. libusb, libpci) etc. You can think of a flashrom unit test as a mini-emulator. The goal is to cover as much as possible
+flashrom code, but you don't need to go outside of that.
+
+See the list of all current mocks (which are called wraps in cmocka terminology) in ``/tests/wraps.h``. These might be enough for
+your new test, or you might need to add more wraps as a part of new test.
+
+New wrap needs to be added to ``/tests/wraps.h``, ``/tests/tests.c``, ``/tests/meson.build``. If it's fine for new wrap to
+do nothing, log invokation and return success, all good.
+
+If a wrap need to behave in a specific way for a test, and the behaviour can be different from one test to another, you need to
+extend the wrap into ``/tests/io_mock.h`` framework.
+
+Add corresponding member (a function pointer) to ``struct io_mock``
+and redirect calls from a wrap function in ``/tests/tests.c`` into a member of ``io_mock``. The exact implementation
+of the member function needs to be defined in your new test. At the beginning of a test scenario, define function pointers that your
+test needs in your own ``struct io_mock`` and then register by calling ``io_mock_register``. At the end of a test, clean up
+by calling ``io_mock_register(NULL)``.
+
+Note that ``io_mock`` can support state (if needed). State is a piece of custom data which is carried around for the duration
+of the test scenario and is available in all functions in ``io_mock``.
+
+Adding a new test to a framework
+================================
+
+To add new test you will either add a new test function in existing .c file, or add new .c file and new function(s) there.
+
+If you add new file, you need to add it into the list of test source files in ``/tests/meson.build``.
+
+Each new test function needs to be added into ``/tests/tests.h`` and ``/tests/tests.c``. Follow existing entries as examples
+how to do it.
+
+Types of tests
+==============
+
+Programmers tests
+-----------------
+
+The concept of a unit test for flashrom programmer is based on a programmer lifecycle. The options supported by the flashrom
+test framework are the following (but you are very welcome to try implement new ideas).
+
+The smallest possible lifecycle is called basic and it does initialisation -> shutdown of a programmer (nothing else).
+Another option is probing lifecycle, which does initialisation -> probing a chip -> shutdown.
+These two expect successful scenarious, the whole scenario is expected to run until the end with no errors and
+success return codes.
+
+One more option is to test expected failure for programmer initialisation. This is useful to test known invalid
+(and potentially dangerous) combination of programmer parameters, when such combination should be detected at init time.
+If invalid combination of parameters is detected, initialisation expected to fail early and programmer must not continue,
+and not send any opcodes to the chip.
+
+For more details, source code is in ``/tests/lifecycle.h`` and ``/tests/lifecycle.c``.
+
+If you want to add new test(s) for a programmer, first you look whether that programmer has any tests already, or none at all.
+Test source file has the same name as a programmer itself, for example programmer ``dummyflasher.c`` has its dedicated tests in
+``/tests/dummyflasher.c`` file. Either add your tests to an existing file, or create new file for a programmer that had no tests
+so far. The latter is more challenging, but it is very useful and highly appreciated.
+
+For programmers tests, the test scenario most likely won't be long: most likely it is one of the options to run lifecycle with
+given combination of programmer params as an input. Most time and effort is typically spent on mocking (see above), and this
+type of tests will indeed look like a mini-emulator.
+
+Chip operations tests
+---------------------
+
+These tests are based on dummyflasher programmer and they are running operations of a chip: read, write, erase, verify.
+The test defines mock chip(s) with given properties, and all the operations of the chip are redirected to mock functions.
+Mock chip has its own mock memory (an array of bytes) and all operations are performed on this array of bytes.
+As all the others, these tests are completely independent of hardware, and are focused on testing core flashrom logic
+for read, write, erase, verify.
+
+Examples of chip operation tests are: ``tests/chip.c``, ``/tests/chip_wp.c`` (focused on write-protection logic),
+``/tests/erase_func_algo.c`` (focused on erasing and writing logic, especially the choice of erase blocks for given
+layout regions).
+
+Misc
+----
+
+All other tests. You choose a function that you want to test, call it with given arguments, assert the results are as expected.

doc/contrib_howtos/index.rst

@@ -6,3 +6,4 @@ Contributors howtos
 
     how_to_add_new_chip
     how_to_mark_chip_tested
+    how_to_add_unit_test

doc/dev_guide/building_from_source.rst

@@ -224,6 +224,7 @@ If you want to change your initial configuration for some reason
 
     meson configure [updated builtin options] [updated flashrom options] <builddir>
 
+.. _unit tests:
 
 Unit Tests
 ----------