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 the function under test and then sets expectations for what should happen. For example:

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, 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 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:

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 fails, the test case will fail.

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 KUnit expectations, see Test API.

Note

A single test case should be short, easy to understand, and focused on a single behavior.

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:

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));
}

Assertions

An assertion is like an expectation, except that the assertion immediately terminates the test case if the condition is not satisfied. For example:

static void test_sort(struct kunit *test)
{
        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, we need to be able to allocate an array to test the sort() function. So we use KUNIT_ASSERT_NOT_ERR_OR_NULL() to abort the test if there’s an allocation error.

Note

In other test frameworks, ASSERT macros are often implemented by calling return so they only work from the test function. In KUnit, we stop the current kthread on failure, so you can call them from anywhere.

Note

Warning: There is an exception to the above rule. You shouldn’t use assertions in the suite’s exit() function, or in the free function for a resource. These run when a test is shutting down, and an assertion here prevents further cleanup code from running, potentially leading to a memory leak.

Customizing error messages

Each of the KUNIT_EXPECT and KUNIT_ASSERT macros have a _MSG variant. These take a format string and arguments to provide additional context to the automatically generated error messages.

char some_str[41];
generate_sha1_hex_string(some_str);

/* Before. Not easy to tell why the test failed. */
KUNIT_EXPECT_EQ(test, strlen(some_str), 40);

/* After. Now we see the offending string. */
KUNIT_EXPECT_EQ_MSG(test, strlen(some_str), 40, "some_str='%s'", some_str);

Alternatively, one can take full control over the error message by using KUNIT_FAIL(), e.g.

/* Before */
KUNIT_EXPECT_EQ(test, some_setup_function(), 0);

/* After: full control over the failure message. */
if (some_setup_function())
        KUNIT_FAIL(test, "Failed to setup thing for testing");

Test Suites

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 optional setup and teardown functions that run before/after the whole suite and/or every test case.

Note

A test case will only run if it is associated with a test suite.

For example:

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,
        .suite_init = example_suite_init,
        .suite_exit = example_suite_exit,
        .test_cases = example_test_cases,
};
kunit_test_suite(example_test_suite);

In the above example, the test suite example_test_suite would first run example_suite_init, then 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. Finally, example_suite_exit would be called after everything else. kunit_test_suite(example_test_suite) registers the test suite with the KUnit test framework.

Note

The exit and suite_exit functions will run even if init or suite_init fail. Make sure that they can handle any inconsistent state which may result from init or suite_init encountering errors or exiting early.

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 Test API.

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.

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.

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.

Common Patterns

Isolating Behavior

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, 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. 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:

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 rectangle, 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, 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 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:

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 code that buffers writes to the EEPROM:

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 test this code by faking out the underlying EEPROM:

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 *parent, 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:

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);
}

Testing Against Multiple Inputs

Testing just a few inputs is not enough to ensure that the code works correctly, for example: testing a hash function.

We can write a helper macro or function. The function is called for each input. For example, to test sha1sum(1), we can write:

#define TEST_SHA1(in, want) \
        sha1sum(in, out); \
        KUNIT_EXPECT_STREQ_MSG(test, out, want, "sha1sum(%s)", in);

char out[40];
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.

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.

In complicated cases, we recommend using a table-driven test compared to the helper macro variation, for example:

int i;
char out[40];

struct sha1_test_case {
        const char *str;
        const char *sha1;
};

struct sha1_test_case cases[] = {
        {
                .str = "hello world",
                .sha1 = "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed",
        },
        {
                .str = "hello world!",
                .sha1 = "430ce34d020724ed75a196dfc2ad67c77772d169",
        },
};
for (i = 0; i < ARRAY_SIZE(cases); ++i) {
        sha1sum(cases[i].str, out);
        KUNIT_EXPECT_STREQ_MSG(test, out, cases[i].sha1,
                              "sha1sum(%s)", cases[i].str);
}

There is more boilerplate code involved, but it can:

• be more readable when there are multiple inputs/outputs (due to field names).

  • For example, see fs/ext4/inode-test.c.

• reduce duplication if test cases are shared across multiple tests.

  • For example: if we want to test sha256sum, we could add a sha256 field and reuse cases.

• be converted to a “parameterized test”.

Parameterized Testing

The table-driven testing pattern is common enough that KUnit has special support for it.

By reusing the same cases array from above, we can write the test as a “parameterized test” with the following.

// This is copy-pasted from above.
struct sha1_test_case {
        const char *str;
        const char *sha1;
};
const struct sha1_test_case cases[] = {
        {
                .str = "hello world",
                .sha1 = "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed",
        },
        {
                .str = "hello world!",
                .sha1 = "430ce34d020724ed75a196dfc2ad67c77772d169",
        },
};

// Creates `sha1_gen_params()` to iterate over `cases` while using
// the struct member `str` for the case description.
KUNIT_ARRAY_PARAM_DESC(sha1, cases, str);

// Looks no different from a normal test.
static void sha1_test(struct kunit *test)
{
        // This function can just contain the body of the for-loop.
        // The former `cases[i]` is accessible under test->param_value.
        char out[40];
        struct sha1_test_case *test_param = (struct sha1_test_case *)(test->param_value);

        sha1sum(test_param->str, out);
        KUNIT_EXPECT_STREQ_MSG(test, out, test_param->sha1,
                              "sha1sum(%s)", test_param->str);
}

// Instead of KUNIT_CASE, we use KUNIT_CASE_PARAM and pass in the
// function declared by KUNIT_ARRAY_PARAM or KUNIT_ARRAY_PARAM_DESC.
static struct kunit_case sha1_test_cases[] = {
        KUNIT_CASE_PARAM(sha1_test, sha1_gen_params),
        {}
};

Allocating Memory

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.

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:

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);

        KUNIT_ASSERT_STREQ(test, buffer, "");
}

Registering Cleanup Actions

If you need to perform some cleanup beyond simple use of kunit_kzalloc, you can register a custom “deferred action”, which is a cleanup function run when the test exits (whether cleanly, or via a failed assertion).

Actions are simple functions with no return value, and a single void* context argument, and fulfill the same role as “cleanup” functions in Python and Go tests, “defer” statements in languages which support them, and (in some cases) destructors in RAII languages.

These are very useful for unregistering things from global lists, closing files or other resources, or freeing resources.

For example:

static void cleanup_device(void *ctx)
{
        struct device *dev = (struct device *)ctx;

        device_unregister(dev);
}

void example_device_test(struct kunit *test)
{
        struct my_device dev;

        device_register(&dev);

        kunit_add_action(test, &cleanup_device, &dev);
}

Note that, for functions like device_unregister which only accept a single pointer-sized argument, it’s possible to automatically generate a wrapper with the KUNIT_DEFINE_ACTION_WRAPPER() macro, for example:

KUNIT_DEFINE_ACTION_WRAPPER(device_unregister, device_unregister_wrapper, struct device *);
kunit_add_action(test, &device_unregister_wrapper, &dev);

You should do this in preference to manually casting to the kunit_action_t type, as casting function pointers will break Control Flow Integrity (CFI).

kunit_add_action can fail if, for example, the system is out of memory. You can use kunit_add_action_or_reset instead which runs the action immediately if it cannot be deferred.

If you need more control over when the cleanup function is called, you can trigger it early using kunit_release_action, or cancel it entirely with kunit_remove_action.

Testing Static Functions

If you want to test static functions without exposing those functions outside of testing, one option is conditionally export the symbol. When KUnit is enabled, the symbol is exposed but remains static otherwise. To use this method, follow the template below.

/* In the file containing functions to test "my_file.c" */

#include <kunit/visibility.h>
#include <my_file.h>
...
VISIBLE_IF_KUNIT int do_interesting_thing()
{
...
}
EXPORT_SYMBOL_IF_KUNIT(do_interesting_thing);

/* In the header file "my_file.h" */

#if IS_ENABLED(CONFIG_KUNIT)
        int do_interesting_thing(void);
#endif

/* In the KUnit test file "my_file_test.c" */

#include <kunit/visibility.h>
#include <my_file.h>
...
MODULE_IMPORT_NS("EXPORTED_FOR_KUNIT_TESTING");
...
// Use do_interesting_thing() in tests

For a full example, see this patch where a test is modified to conditionally expose static functions for testing using the macros above.

As an alternative to the method above, you could conditionally #include the test file at the end of your .c file. This is not recommended but works if needed. For example:

/* In "my_file.c" */

static int do_interesting_thing();

#ifdef CONFIG_MY_KUNIT_TEST
#include "my_kunit_test.c"
#endif

Injecting Test-Only Code

Similar to as shown above, we can add test-specific logic. For example:

/* In my_file.h */

#ifdef CONFIG_MY_KUNIT_TEST
/* Defined in my_kunit_test.c */
void test_only_hook(void);
#else
void test_only_hook(void) { }
#endif

This test-only code can be made more useful by accessing the current kunit_test as shown in next section: Accessing The Current Test.

Accessing The Current Test

In some cases, we need to call test-only code from outside the test file. This is helpful, for example, when providing a fake implementation of a function, or to fail any current test from within an error handler. We can do this via the kunit_test field in task_struct, which we can access using the kunit_get_current_test() function in kunit/test-bug.h.

kunit_get_current_test() is safe to call even if KUnit is not enabled. If KUnit is not enabled, or if no test is running in the current task, it will return NULL. This compiles down to either a no-op or a static key check, so will have a negligible performance impact when no test is running.

The example below uses this to implement a “mock” implementation of a function, foo:

#include <kunit/test-bug.h> /* for kunit_get_current_test */

struct test_data {
        int foo_result;
        int want_foo_called_with;
};

static int fake_foo(int arg)
{
        struct kunit *test = kunit_get_current_test();
        struct test_data *test_data = test->priv;

        KUNIT_EXPECT_EQ(test, test_data->want_foo_called_with, arg);
        return test_data->foo_result;
}

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;

        test_data->foo_result = 42;
        test_data->want_foo_called_with = 1;

        /* 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);
}

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.

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 Resource API.

Failing The Current Test

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:

#include <kunit/test-bug.h>

#ifdef CONFIG_EXTRA_DEBUG_CHECKS
static void validate_my_data(struct data *data)
{
        if (is_valid(data))
                return;

        kunit_fail_current_test("data %p is invalid", data);

        /* Normal, non-KUnit, error reporting code here. */
}
#else
static void my_debug_function(void) { }
#endif

kunit_fail_current_test() is safe to call even if KUnit is not enabled. If KUnit is not enabled, or if no test is running in the current task, it will do nothing. This compiles down to either a no-op or a static key check, so will have a negligible performance impact when no test is running.

Managing Fake Devices and Drivers

When testing drivers or code which interacts with drivers, many functions will require a struct device or struct device_driver. In many cases, setting up a real device is not required to test any given function, so a fake device can be used instead.

KUnit provides helper functions to create and manage these fake devices, which are internally of type struct kunit_device, and are attached to a special kunit_bus. These devices support managed device resources (devres), as described in Devres - Managed Device Resource

To create a KUnit-managed struct device_driver, use kunit_driver_create(), which will create a driver with the given name, on the kunit_bus. This driver will automatically be destroyed when the corresponding test finishes, but can also be manually destroyed with driver_unregister().

To create a fake device, use the kunit_device_register(), which will create and register a device, using a new KUnit-managed driver created with kunit_driver_create(). To provide a specific, non-KUnit-managed driver, use kunit_device_register_with_driver() instead. Like with managed drivers, KUnit-managed fake devices are automatically cleaned up when the test finishes, but can be manually cleaned up early with kunit_device_unregister().

The KUnit devices should be used in preference to root_device_register(), and instead of platform_device_register() in cases where the device is not otherwise a platform device.

For example:

#include <kunit/device.h>

static void test_my_device(struct kunit *test)
{
        struct device *fake_device;
        const char *dev_managed_string;

        // Create a fake device.
        fake_device = kunit_device_register(test, "my_device");
        KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_device)

        // Pass it to functions which need a device.
        dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");

        // Everything is cleaned up automatically when the test ends.
}