[PATCH v2 1/5] docs: Add documentation about tests and SBIUnit

[Ivan Orlov <ivan.orlov0322@gmail.com>]

This patch contains the documentation for SBIUnit. It describes:

- What is SBIUnit
- Simple test writing scenario
- How we can cover static functions
- How we can "mock" structures in order to test the functions which
operate on them
- SBIUnit API Reference

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
---
V1 -> V2:
- Use more appropriate language
- Update the documentation in accordance with SBIUnit API and
structure changes
- Remove the redundant documentation for every SBIUNIT_* macro.
- Make the documentation follow the 80-characters rule.

 docs/writing_tests.md | 131 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 131 insertions(+)
 create mode 100644 docs/writing_tests.md

diff --git a/docs/writing_tests.md b/docs/writing_tests.md
new file mode 100644
index 0000000..279d35b
--- /dev/null
+++ b/docs/writing_tests.md
@@ -0,0 +1,131 @@
+Writing tests for OpenSBI
+=========================
+
+SBIUnit
+-------
+SBIUnit is a set of macros and functions which simplify the test development and
+automate the test execution and evaluation. All of the SBIUnit definitions are
+in the `include/sbi/sbi_unit_test.h` header file, and implementations are
+available in `lib/sbi/sbi_unit_test.c`.
+
+Simple SBIUnit test
+-------------------
+
+For instance, we would like to test the following function from
+`lib/sbi/sbi_string.c`:
+
+```c
+size_t sbi_strlen(const char *str)
+{
+	unsigned long ret = 0;
+
+	while (*str != '\0') {
+		ret++;
+		str++;
+	}
+
+	return ret;
+}
+```
+
+Which calculates the string length.
+
+Create the file `lib/sbi/sbi_string_test.c` with the following content:
+
+```c
+#include <sbi/sbi_unit_test.h>
+#include <sbi/sbi_string.h>
+
+static void strlen_test(struct sbiunit_test_case *test)
+{
+	SBIUNIT_EXPECT_EQ(test, sbi_strlen("Hello"), 5);
+	SBIUNIT_EXPECT_EQ(test, sbi_strlen("Hell\0o"), 4);
+}
+
+static struct sbiunit_test_case string_test_cases[] = {
+	SBIUNIT_TEST_CASE(strlen_test),
+	{},
+};
+
+SBIUNIT_TEST_SUITE(string_test_suite, string_test_cases);
+```
+
+Then, add the corresponding Makefile entries to `lib/sbi/objects.mk`:
+```lang-makefile
+...
+libsbi-objs-$(CONFIG_SBIUNIT) += sbi_string_test.o
+carray-sbi_unit_tests-$(CONFIG_SBIUNIT) += string_test_suite
+```
+
+Run `make clean` in order to regenerate the carray-related files, recompile
+OpenSBI with CONFIG_SBIUNIT option enabled and run it in the QEMU.
+You will see something like this:
+```
+# make PLATFORM=generic run
+...
+# Running SBIUNIT tests #
+...
+## Running test suite: string_test_suite
+[PASSED] strlen_test
+1 PASSED / 0 FAILED / 1 TOTAL
+```
+
+Now let's try to change this test in the way that it will fail:
+
+```c
+- SBIUNIT_EXPECT_EQ(test, sbi_strlen("Hello"), 5);
++ SBIUNIT_EXPECT_EQ(test, sbi_strlen("Hello"), 100);
+```
+
+`make all` and `make run` it again:
+```
+...
+# Running SBIUNIT tests #
+...
+## Running test suite: string_test_suite
+[SBIUnit] [.../opensbi/lib/sbi/sbi_string_test.c:6]: strlen_test: Condition
+"(sbi_strlen("Hello")) == (100)" expected to be true!
+[FAILED] strlen_test
+0 PASSED / 1 FAILED / 1 TOTAL
+```
+Covering the static functions / using the static definitions
+------------------------------------------------------------
+
+SBIUnit also allows you to test static functions. In order to do so, simply
+include your test source in the file you would like to test. Complementing the
+example above, just add this to the
+`lib/sbi/sbi_string.c` file:
+
+```c
+#ifdef CONFIG_SBIUNIT
+#include "sbi_string_test.c"
+#endif
+```
+
+In this case you should only add a new carray entry pointing to the test suite
+to `lib/sbi/objects.mk`:
+```lang-makefile
+...
+carray-sbi_unit_tests-$(CONFIG_SBIUNIT) += string_test_suite
+```
+
+You don't have to compile the `sbi_string_test.o` separately, because the
+test code will be included into the `sbi_string` object file.
+
+See example in `lib/sbi/sbi_console_test.c`, where statically declared
+`console_dev` variable is used to mock the `sbi_console_device` structure.
+
+"Mocking" the structures
+------------------------
+See the example of structure "mocking" in the `lib/sbi/sbi_console_test.c`,
+where the sbi_console_device structure was mocked to be used in various
+console-related functions in order to test them.
+
+API Reference
+-------------
+All of the `SBIUNIT_EXPECT_*` macros will cause a test case to fail if the
+corresponding conditions are not met, however, the execution of a particular
+test case will not be stopped.
+
+All of the `SBIUNIT_ASSERT_*` macros will cause a test case to fail and stop
+immediately, triggering the panic.
-- 
2.34.1