lib/math: add KUnit test suite for polynomial_calc()

Add a KUnit test suite for the polynomial_calc() function, which had no
in-kernel test coverage.  The tests verify correct evaluation of constant,
linear, quadratic, and cubic polynomials, including negative coefficients,
negative input data, zero-coefficient terms.

The Kconfig entry uses 'select POLYNOMIAL' rather than 'depends on
POLYNOMIAL' because POLYNOMIAL is a promptless tristate that cannot
be manually enabled on UML without an explicit selector.

Link: https://lore.kernel.org/20260606030319.316752-1-adinata.softwareengineer@gmail.com
Signed-off-by: Adi Nata <adinata.softwareengineer@gmail.com>
Cc: Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
Cc: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <david@davidgow.net>
Cc: Rae Moar <raemoar63@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 271 insertions, 0 deletions

diff --git a/lib/math/tests/Makefile b/lib/math/tests/Makefile
index 13dc96e48408..85e1ad59f29d 100644
--- a/lib/math/tests/Makefile
+++ b/lib/math/tests/Makefile
@@ -4,5 +4,6 @@ obj-$(CONFIG_GCD_KUNIT_TEST)		+= gcd_kunit.o
 obj-$(CONFIG_INT_LOG_KUNIT_TEST)	+= int_log_kunit.o
 obj-$(CONFIG_INT_POW_KUNIT_TEST)	+= int_pow_kunit.o
 obj-$(CONFIG_INT_SQRT_KUNIT_TEST)	+= int_sqrt_kunit.o
+obj-$(CONFIG_POLYNOMIAL_KUNIT_TEST)	+= polynomial_kunit.o
 obj-$(CONFIG_PRIME_NUMBERS_KUNIT_TEST)	+= prime_numbers_kunit.o
 obj-$(CONFIG_RATIONAL_KUNIT_TEST)	+= rational_kunit.o
diff --git a/lib/math/tests/polynomial_kunit.c b/lib/math/tests/polynomial_kunit.c
new file mode 100644
index 000000000000..ef443b57fc12
--- /dev/null
+++ b/lib/math/tests/polynomial_kunit.c
@@ -0,0 +1,270 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <kunit/test.h>
+#include <linux/polynomial.h>
+
+struct polynomial_test_param {
+	const struct polynomial *poly;
+	long data;
+	long expected;
+	const char *name;
+};
+
+/* f(x) = 5 */
+static const struct polynomial poly_constant = {
+	.total_divider = 1,
+	.terms = {
+		{0, 5, 1, 1},
+	}
+};
+
+/* f(x) = 2x^2 + 3x + 5 */
+static const struct polynomial poly_simple = {
+	.total_divider = 1,
+	.terms = {
+		{2, 2, 1, 1},
+		{1, 3, 1, 1},
+		{0, 5, 1, 1},
+	}
+};
+
+/* f(x) = -5x + 100 */
+static const struct polynomial poly_negative_coef = {
+	.total_divider = 1,
+	.terms = {
+		{1, -5, 1, 1},
+		{0, 100, 1, 1},
+	}
+};
+
+/* f(x) = (150x + 50) / 10 */
+static const struct polynomial poly_total_divider = {
+	.total_divider = 10,
+	.terms = {
+		{1, 150, 1, 1},
+		{0,  50, 1, 1},
+	}
+};
+
+/*
+ * f(x) = x / 2
+ * divider=2 applied once per multiply: mult_frac(coef, data, 2) = coef*data/2
+ */
+static const struct polynomial poly_step_divider = {
+	.total_divider = 1,
+	.terms = {
+		{1, 1, 2, 1},
+		{0, 0, 1, 1},
+	}
+};
+
+/*
+ * f(x) = (100/500) * x^2 = 0.2 * x^2
+ * Encoded as coef=100, divider=10, divider_leftover=5:
+ *   denom = 10^2 * 5 = 500
+ */
+static const struct polynomial poly_leftover = {
+	.total_divider = 1,
+	.terms = {
+		{2, 100, 10, 5},
+		{0,   0,  1, 1},
+	}
+};
+
+/*
+ * f(x) = 2x^3  (single high-degree term, no constant)
+ * Used to exercise the power loop alone.
+ */
+static const struct polynomial poly_cubic = {
+	.total_divider = 1,
+	.terms = {
+		{3, 2, 1, 1},
+		{0, 0, 1, 1},
+	}
+};
+
+/*
+ * f(x) = 4x + 1  with a zero-coefficient quadratic term.
+ * The deg-2 term contributes nothing regardless of input.
+ */
+static const struct polynomial poly_zero_coef = {
+	.total_divider = 1,
+	.terms = {
+		{2, 0, 1, 1},
+		{1, 4, 1, 1},
+		{0, 1, 1, 1},
+	}
+};
+
+/*
+ * f(x) = 9  with total_divider = 0.
+ * The implementation treats 0 as 1 via `total_divider ?: 1`, so the
+ * result must equal the constant term unchanged.
+ */
+static const struct polynomial poly_zero_total_divider = {
+	.total_divider = 0,
+	.terms = {
+		{0, 9, 1, 1},
+	}
+};
+
+
+static const struct polynomial_test_param test_params[] = {
+	{
+		.poly     = &poly_constant,
+		.data     = 0,
+		.expected = 5,
+		.name     = "Constant polynomial at x=0",
+	},
+	{
+		.poly     = &poly_constant,
+		.data     = 42,
+		.expected = 5,
+		.name     = "Constant polynomial is independent of input",
+	},
+	{
+		.poly     = &poly_simple,
+		.data     = 0,
+		.expected = 5,	/* zero input collapses all power terms */
+		.name     = "Zero input yields constant term only",
+	},
+	{
+		.poly     = &poly_simple,
+		.data     = 10,
+		.expected = 235,	/* 2*100 + 3*10 + 5 */
+		.name     = "Simple quadratic at x=10",
+	},
+	{
+		.poly     = &poly_negative_coef,
+		.data     = 10,
+		.expected = 50,		/* -5*10 + 100 */
+		.name     = "Negative coefficient at x=10",
+	},
+	{
+		.poly     = &poly_negative_coef,
+		.data     = 20,
+		.expected = 0,		/* -5*20 + 100 = 0 */
+		.name     = "Negative coefficient result is zero",
+	},
+	{
+		.poly     = &poly_total_divider,
+		.data     = 3,
+		.expected = 50,		/* (150*3 + 50) / 10 = 500/10 */
+		.name     = "total_divider scales the final sum",
+	},
+	{
+		.poly     = &poly_step_divider,
+		.data     = 100,
+		.expected = 50,		/* 1*100/2 */
+		.name     = "Per-step divider halves input",
+	},
+	{
+		.poly     = &poly_leftover,
+		.data     = 30,
+		.expected = 180,	/* 100*30^2 / (10^2 * 5) = 90000/500 */
+		.name     = "divider_leftover with quadratic term",
+	},
+	/* Boundary: unit and negative-unit input */
+	{
+		/*
+		 * data=1: each mult_frac(tmp, 1, divider) strips one factor of
+		 * divider from coef per degree, so coef is left-shifted right
+		 * until intermediate precision is exhausted.
+		 * 2*1 + 3*1 + 5 = 10
+		 */
+		.poly     = &poly_simple,
+		.data     = 1,
+		.expected = 10,
+		.name     = "Boundary: data=1 (unit input)",
+	},
+	{
+		/*
+		 * data=-1: even degrees produce positive contributions,
+		 * odd degrees produce negative ones.
+		 * 2*(-1)^2 + 3*(-1) + 5 = 2 - 3 + 5 = 4
+		 */
+		.poly     = &poly_simple,
+		.data     = -1,
+		.expected = 4,
+		.name     = "Boundary: data=-1 (negative unit input)",
+	},
+
+	/* Boundary: negative non-trivial input */
+	{
+		/*
+		 * 2*(-3)^2 + 3*(-3) + 5 = 18 - 9 + 5 = 14
+		 * Verifies sign handling for negative data across all degrees.
+		 */
+		.poly     = &poly_simple,
+		.data     = -3,
+		.expected = 14,
+		.name     = "Boundary: negative data with quadratic",
+	},
+
+	/* Boundary: total_divider = 0 is treated as 1 */
+	{
+		.poly     = &poly_zero_total_divider,
+		.data     = 42,
+		.expected = 9,
+		.name     = "Boundary: total_divider=0 defaults to 1",
+	},
+
+	/* Boundary: zero-coefficient high-degree term */
+	{
+		/*
+		 * The deg-2 term has coef=0, so it contributes 0 regardless
+		 * of data. Result: 0 + 4*10 + 1 = 41
+		 */
+		.poly     = &poly_zero_coef,
+		.data     = 10,
+		.expected = 41,
+		.name     = "Boundary: zero-coefficient term is inert",
+	},
+
+	/* Boundary: single high-degree term, no constant */
+	{
+		/* 2 * 5^3 = 250; also verifies the loop terminates on deg-0 */
+		.poly     = &poly_cubic,
+		.data     = 5,
+		.expected = 250,
+		.name     = "Boundary: single cubic term",
+	},
+	{
+		/* 2 * (-2)^3 = -16; odd power preserves sign of negative data */
+		.poly     = &poly_cubic,
+		.data     = -2,
+		.expected = -16,
+		.name     = "Boundary: single cubic term, negative data",
+	},
+
+};
+
+static void get_desc(const struct polynomial_test_param *param, char *desc)
+{
+	strscpy(desc, param->name, KUNIT_PARAM_DESC_SIZE);
+}
+
+KUNIT_ARRAY_PARAM(polynomial, test_params, get_desc);
+
+static void polynomial_calc_test(struct kunit *test)
+{
+	const struct polynomial_test_param *param = test->param_value;
+
+	KUNIT_EXPECT_EQ(test, polynomial_calc(param->poly, param->data),
+			param->expected);
+}
+
+static struct kunit_case polynomial_test_cases[] = {
+	KUNIT_CASE_PARAM(polynomial_calc_test, polynomial_gen_params),
+	{}
+};
+
+static struct kunit_suite polynomial_test_suite = {
+	.name = "math-polynomial",
+	.test_cases = polynomial_test_cases,
+};
+
+kunit_test_suites(&polynomial_test_suite);
+
+MODULE_DESCRIPTION("math.polynomial_calc KUnit test suite");
+MODULE_LICENSE("GPL");