clk: fractional-divider: tests: Add test suite for edge cases

In light of the recent discovery that the fractional divisor
approximation does not utilize the full available range for clocks that
are flagged CLK_FRAC_DIVIDER_ZERO_BASED [1], implement tests for the
edge cases of this clock type.

Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
Link: https://lore.kernel.org/lkml/20230529133433.56215-1-frank@oltmanns.dev [1]
Link: https://lore.kernel.org/r/20230617131041.18313-3-frank@oltmanns.dev
[sboyd@kernel.org: Rename suite and tests slightly, drop unused
includes, store parent rate to compare instead of repeating equation]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/.kunitconfig

@@ -2,4 +2,5 @@ CONFIG_KUNIT=y
 CONFIG_COMMON_CLK=y
 CONFIG_CLK_KUNIT_TEST=y
 CONFIG_CLK_GATE_KUNIT_TEST=y
+CONFIG_CLK_FD_KUNIT_TEST=y
 CONFIG_UML_PCI_OVER_VIRTIO=n

drivers/clk/Kconfig

@@ -517,4 +517,11 @@ config CLK_GATE_KUNIT_TEST
 	help
 	  Kunit test for the basic clk gate type.
 
+config CLK_FD_KUNIT_TEST
+	tristate "Basic fractional divider type Kunit test" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Kunit test for the clk-fractional-divider type.
+
 endif

drivers/clk/Makefile

@@ -12,6 +12,7 @@ obj-$(CONFIG_COMMON_CLK)	+= clk-multiplier.o
 obj-$(CONFIG_COMMON_CLK)	+= clk-mux.o
 obj-$(CONFIG_COMMON_CLK)	+= clk-composite.o
 obj-$(CONFIG_COMMON_CLK)	+= clk-fractional-divider.o
+obj-$(CONFIG_CLK_FD_KUNIT_TEST) += clk-fractional-divider_test.o
 obj-$(CONFIG_COMMON_CLK)	+= clk-gpio.o
 ifeq ($(CONFIG_OF), y)
 obj-$(CONFIG_COMMON_CLK)	+= clk-conf.o

drivers/clk/clk-fractional-divider_test.c

@@ -0,0 +1,147 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Kunit test for clock fractional divider
+ */
+#include <linux/clk-provider.h>
+#include <kunit/test.h>
+
+#include "clk-fractional-divider.h"
+
+/*
+ * Test the maximum denominator case for fd clock without flags.
+ *
+ * Expect the highest possible denominator to be used in order to get as close as possible to the
+ * requested rate.
+ */
+static void clk_fd_test_approximation_max_denominator(struct kunit *test)
+{
+	struct clk_fractional_divider *fd;
+	unsigned long rate, parent_rate, parent_rate_before, m, n, max_n;
+
+	fd = kunit_kzalloc(test, sizeof(*fd), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, fd);
+
+	fd->mwidth = 3;
+	fd->nwidth = 3;
+	max_n = 7;
+
+	rate = 240000000;
+	parent_rate = (max_n + 1) * rate; /* so that it exceeds the maximum divisor */
+	parent_rate_before = parent_rate;
+
+	clk_fractional_divider_general_approximation(&fd->hw, rate, &parent_rate, &m, &n);
+	KUNIT_ASSERT_EQ(test, parent_rate, parent_rate_before);
+
+	KUNIT_EXPECT_EQ(test, m, 1);
+	KUNIT_EXPECT_EQ(test, n, max_n);
+}
+
+/*
+ * Test the maximum numerator case for fd clock without flags.
+ *
+ * Expect the highest possible numerator to be used in order to get as close as possible to the
+ * requested rate.
+ */
+static void clk_fd_test_approximation_max_numerator(struct kunit *test)
+{
+	struct clk_fractional_divider *fd;
+	unsigned long rate, parent_rate, parent_rate_before, m, n, max_m;
+
+	fd = kunit_kzalloc(test, sizeof(*fd), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, fd);
+
+	fd->mwidth = 3;
+	max_m = 7;
+	fd->nwidth = 3;
+
+	rate = 240000000;
+	parent_rate = rate / (max_m + 1); /* so that it exceeds the maximum numerator */
+	parent_rate_before = parent_rate;
+
+	clk_fractional_divider_general_approximation(&fd->hw, rate, &parent_rate, &m, &n);
+	KUNIT_ASSERT_EQ(test, parent_rate, parent_rate_before);
+
+	KUNIT_EXPECT_EQ(test, m, max_m);
+	KUNIT_EXPECT_EQ(test, n, 1);
+}
+
+/*
+ * Test the maximum denominator case for zero based fd clock.
+ *
+ * Expect the highest possible denominator to be used in order to get as close as possible to the
+ * requested rate.
+ */
+static void clk_fd_test_approximation_max_denominator_zero_based(struct kunit *test)
+{
+	struct clk_fractional_divider *fd;
+	unsigned long rate, parent_rate, parent_rate_before, m, n, max_n;
+
+	fd = kunit_kzalloc(test, sizeof(*fd), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, fd);
+
+	fd->flags = CLK_FRAC_DIVIDER_ZERO_BASED;
+	fd->mwidth = 3;
+	fd->nwidth = 3;
+	max_n = 8;
+
+	rate = 240000000;
+	parent_rate = (max_n + 1) * rate; /* so that it exceeds the maximum divisor */
+	parent_rate_before = parent_rate;
+
+	clk_fractional_divider_general_approximation(&fd->hw, rate, &parent_rate, &m, &n);
+	KUNIT_ASSERT_EQ(test, parent_rate, parent_rate_before);
+
+	KUNIT_EXPECT_EQ(test, m, 1);
+	KUNIT_EXPECT_EQ(test, n, max_n);
+}
+
+/*
+ * Test the maximum numerator case for zero based fd clock.
+ *
+ * Expect the highest possible numerator to be used in order to get as close as possible to the
+ * requested rate.
+ */
+static void clk_fd_test_approximation_max_numerator_zero_based(struct kunit *test)
+{
+	struct clk_fractional_divider *fd;
+	unsigned long rate, parent_rate, parent_rate_before, m, n, max_m;
+
+	fd = kunit_kzalloc(test, sizeof(*fd), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, fd);
+
+	fd->flags = CLK_FRAC_DIVIDER_ZERO_BASED;
+	fd->mwidth = 3;
+	max_m = 8;
+	fd->nwidth = 3;
+
+	rate = 240000000;
+	parent_rate = rate / (max_m + 1); /* so that it exceeds the maximum numerator */
+	parent_rate_before = parent_rate;
+
+	clk_fractional_divider_general_approximation(&fd->hw, rate, &parent_rate, &m, &n);
+	KUNIT_ASSERT_EQ(test, parent_rate, parent_rate_before);
+
+	KUNIT_EXPECT_EQ(test, m, max_m);
+	KUNIT_EXPECT_EQ(test, n, 1);
+}
+
+static struct kunit_case clk_fd_approximation_test_cases[] = {
+	KUNIT_CASE(clk_fd_test_approximation_max_denominator),
+	KUNIT_CASE(clk_fd_test_approximation_max_numerator),
+	KUNIT_CASE(clk_fd_test_approximation_max_denominator_zero_based),
+	KUNIT_CASE(clk_fd_test_approximation_max_numerator_zero_based),
+	{}
+};
+
+/*
+ * Test suite for clk_fractional_divider_general_approximation().
+ */
+static struct kunit_suite clk_fd_approximation_suite = {
+	.name = "clk-fd-approximation",
+	.test_cases = clk_fd_approximation_test_cases,
+};
+
+kunit_test_suites(
+	&clk_fd_approximation_suite
+);
+MODULE_LICENSE("GPL");