lib/tests: add KUnit test for bitops

Add a KUnit test suite for the bitops API.

The existing 'lib/test_bitops.c' is preserved as-is because it contains
ad-hoc micro-benchmarks 'test_fns' and is intended to ensure no compiler
warnings from C=1 sparse checker or -Wextra compilations.

Introduce 'lib/tests/bitops_kunit.c' for functional regression testing. It
ports the test logic and data patterns from 'lib/test_bitops.c' to KUnit,
verifying correct behavior across various input patterns and
architecture-specific edge cases using isolated stack-allocated bitmaps.

The following test logic has been ported from test_bitops_startup() in
lib/test_bitops.c:
- set_bit() / clear_bit() / find_first_bit() validation ->
  test_set_bit_clear_bit()
- get_count_order() validation -> test_get_count_order()
- get_count_order_long() validation -> test_get_count_order_long()

Also improve the find_first_bit() test to check the full bitmap length
(BITOPS_LENGTH) instead of omitting the last bit, ensuring the bitmap is
completely empty after cleanup.

Verified on x86_64, i386, and arm64 architectures.

Sample KUnit output:

    KTAP version 1
    # Subtest: bitops
    # module: bitops_kunit
    1..3
        KTAP version 1
        # Subtest: test_set_bit_clear_bit
        ok 1 BITOPS_4
        ok 2 BITOPS_7
        ok 3 BITOPS_11
        ok 4 BITOPS_31
        ok 5 BITOPS_88
    # test_set_bit_clear_bit: pass:5 fail:0 skip:0 total:5
    ok 1 test_set_bit_clear_bit
        KTAP version 1
        # Subtest: test_get_count_order
        ok 1 0x00000003
        ok 2 0x00000004
        ok 3 0x00001fff
        ok 4 0x00002000
        ok 5 0x50000000
        ok 6 0x80000000
        ok 7 0x80003000
    # test_get_count_order: pass:7 fail:0 skip:0 total:7
    ok 2 test_get_count_order
        KTAP version 1
        # Subtest: test_get_count_order_long
        ok 1 0x0000000300000000
        ok 2 0x0000000400000000
        ok 3 0x00001fff00000000
        ok 4 0x0000200000000000
        ok 5 0x5000000000000000
        ok 6 0x8000000000000000
        ok 7 0x8000300000000000
    # test_get_count_order_long: pass:7 fail:0 skip:0 total:7
    ok 3 test_get_count_order_long

[Yury: trim Kconfig help message]

CC: Jesse Brandeburg <jesse.brandeburg@intel.com>
CC: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Ryota Sakamoto <sakamo.ryota@gmail.com>
Signed-off-by: Yury Norov <ynorov@nvidia.com>

MAINTAINERS

@@ -4468,6 +4468,7 @@ F:	include/asm-generic/bitops.h
 F:	include/linux/bitops.h
 F:	lib/hweight.c
 F:	lib/test_bitops.c
+F:	lib/tests/bitops_kunit.c
 F:	tools/*/bitops*
 
 BITOPS API BINDINGS [RUST]

lib/Kconfig.debug

@@ -2652,6 +2652,19 @@ config TEST_SYSCTL
 
 	  If unsure, say N.
 
+config BITOPS_KUNIT
+	tristate "KUnit test for bitops" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  This option enables the KUnit test for the bitops library
+	  which provides functions for bit operations.
+
+	  Note that this is a partial copy of the original test_bitops module.
+	  For the full coverage, enable TEST_BITOPS.
+
+	  If unsure, say N.
+
 config BITFIELD_KUNIT
 	tristate "KUnit test bitfield functions at runtime" if !KUNIT_ALL_TESTS
 	depends on KUNIT

lib/tests/Makefile

@@ -5,6 +5,7 @@
 # KUnit tests
 CFLAGS_bitfield_kunit.o := $(DISABLE_STRUCTLEAK_PLUGIN)
 obj-$(CONFIG_BASE64_KUNIT) += base64_kunit.o
+obj-$(CONFIG_BITOPS_KUNIT) += bitops_kunit.o
 obj-$(CONFIG_BITFIELD_KUNIT) += bitfield_kunit.o
 obj-$(CONFIG_BITS_TEST) += test_bits.o
 obj-$(CONFIG_BLACKHOLE_DEV_KUNIT_TEST) += blackhole_dev_kunit.o

lib/tests/bitops_kunit.c

@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2020 Intel Corporation
+ * Copyright (C) 2026 Ryota Sakamoto <sakamo.ryota@gmail.com>
+ */
+
+#include <linux/bitops.h>
+#include <linux/module.h>
+#include <kunit/test.h>
+
+/* use an enum because that's the most common BITMAP usage */
+enum bitops_fun {
+	BITOPS_4 = 4,
+	BITOPS_7 = 7,
+	BITOPS_11 = 11,
+	BITOPS_31 = 31,
+	BITOPS_88 = 88,
+	BITOPS_LENGTH = 256
+};
+
+struct bitops_test_case {
+	const char *str;
+	const long nr;
+};
+
+static struct bitops_test_case bitops_cases[] = {
+	{
+		.str = "BITOPS_4",
+		.nr = BITOPS_4,
+	},
+	{
+		.str = "BITOPS_7",
+		.nr = BITOPS_7,
+	},
+	{
+		.str = "BITOPS_11",
+		.nr = BITOPS_11,
+	},
+	{
+		.str = "BITOPS_31",
+		.nr = BITOPS_31,
+	},
+	{
+		.str = "BITOPS_88",
+		.nr = BITOPS_88,
+	},
+};
+
+KUNIT_ARRAY_PARAM_DESC(bitops, bitops_cases, str);
+
+static void test_set_bit_clear_bit(struct kunit *test)
+{
+	const struct bitops_test_case *params = test->param_value;
+	DECLARE_BITMAP(bitmap, BITOPS_LENGTH);
+	int bit_set;
+
+	bitmap_zero(bitmap, BITOPS_LENGTH);
+
+	set_bit(params->nr, bitmap);
+	KUNIT_EXPECT_TRUE(test, test_bit(params->nr, bitmap));
+
+	clear_bit(params->nr, bitmap);
+	KUNIT_EXPECT_FALSE(test, test_bit(params->nr, bitmap));
+
+	bit_set = find_first_bit(bitmap, BITOPS_LENGTH);
+	KUNIT_EXPECT_EQ(test, bit_set, BITOPS_LENGTH);
+}
+
+struct order_test_case {
+	const char *str;
+	const unsigned int count;
+	const int expected;
+};
+
+static struct order_test_case order_test_cases[] = {
+	{"0x00000003", 0x00000003,  2},
+	{"0x00000004", 0x00000004,  2},
+	{"0x00001fff", 0x00001fff, 13},
+	{"0x00002000", 0x00002000, 13},
+	{"0x50000000", 0x50000000, 31},
+	{"0x80000000", 0x80000000, 31},
+	{"0x80003000", 0x80003000, 32},
+};
+
+KUNIT_ARRAY_PARAM_DESC(order, order_test_cases, str);
+
+static void test_get_count_order(struct kunit *test)
+{
+	const struct order_test_case *params = test->param_value;
+
+	KUNIT_EXPECT_EQ(test, get_count_order(params->count), params->expected);
+	KUNIT_EXPECT_EQ(test, get_count_order_long(params->count), params->expected);
+}
+
+#ifdef CONFIG_64BIT
+struct order_long_test_case {
+	const char *str;
+	const unsigned long count;
+	const int expected;
+};
+
+static struct order_long_test_case order_long_test_cases[] = {
+	{"0x0000000300000000", 0x0000000300000000, 34},
+	{"0x0000000400000000", 0x0000000400000000, 34},
+	{"0x00001fff00000000", 0x00001fff00000000, 45},
+	{"0x0000200000000000", 0x0000200000000000, 45},
+	{"0x5000000000000000", 0x5000000000000000, 63},
+	{"0x8000000000000000", 0x8000000000000000, 63},
+	{"0x8000300000000000", 0x8000300000000000, 64},
+};
+
+KUNIT_ARRAY_PARAM_DESC(order_long, order_long_test_cases, str);
+
+static void test_get_count_order_long(struct kunit *test)
+{
+	const struct order_long_test_case *params = test->param_value;
+
+	KUNIT_EXPECT_EQ(test, get_count_order_long(params->count), params->expected);
+}
+#endif
+
+static struct kunit_case bitops_test_cases[] = {
+	KUNIT_CASE_PARAM(test_set_bit_clear_bit, bitops_gen_params),
+	KUNIT_CASE_PARAM(test_get_count_order, order_gen_params),
+#ifdef CONFIG_64BIT
+	KUNIT_CASE_PARAM(test_get_count_order_long, order_long_gen_params),
+#endif
+	{},
+};
+
+static struct kunit_suite bitops_test_suite = {
+	.name = "bitops",
+	.test_cases = bitops_test_cases,
+};
+
+kunit_test_suite(bitops_test_suite);
+
+MODULE_AUTHOR("Jesse Brandeburg <jesse.brandeburg@intel.com>");
+MODULE_AUTHOR("Wei Yang <richard.weiyang@gmail.com>");
+MODULE_AUTHOR("Ryota Sakamoto <sakamo.ryota@gmail.com>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Bit testing module");