[PATCH v3 0/2] Fix clk_divider_bestdiv() to get max clk rate supported and add some kunit test suites

[PATCH v3 0/2] Fix clk_divider_bestdiv() to get max clk rate supported and add some kunit test suites

[Prabhakar]

From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>

Hi all,

This patch series includes two patches. The first patch fixes the
clk_divider_bestdiv() function in clk-divider driver to return the
maximum clock rate supported by the divider when the requested rate
is larger than the parent clock rate. The second patch adds some
kunit test suites for clk-divider driver to verify the fix.

v2->v3:
- Added Rb tag
- Swapped the order of the patches
- Added the expected value in the fixup patch

v1->v2:
- Updated the patch#1 fixing review comments from Sashiko
- Added a new patch#2 to add some kunit test suites for clk-divider driver

# case 1: Without the fix
-------------------------
root@rzn2h-evk:~# modprobe clk-divider_test;modprobe -r clk-divider_test
[   28.584792] KTAP version 1
[   28.591070] 1..1
[   28.595258]     KTAP version 1
[   28.602327]     # Subtest: clk_divider_bestdiv
[   28.612609]     # module: clk_divider_test
[   28.612618]     1..2
[   28.627986]     ok 1 clk_divider_bestdiv_ulong_max_returns_max_rate​
[   28.628793]     ok 2 clk_divider_bestdiv_mux_ulong_max_returns_max_rate
[   28.643341] # clk_divider_bestdiv: pass:2 fail:0 skip:0 total:2
[   28.658389] # Totals: pass:2 fail:0 skip:0 total:2
[   28.672085] ok 1 clk_divider_bestdiv

# case 2: With the fix and no changes to Kunit test case
--------------------------------------------------------
root@rzn2h-evk:~# modprobe clk-divider_test;modprobe -r clk-divider_test
[   43.105680] KTAP version 1
[   43.108854] 1..1
[   43.110998]     KTAP version 1
[   43.114449]     # Subtest: clk_divider_bestdiv
[   43.119601]     # module: clk_divider_test
[   43.119609]     1..2
[   43.127192]     # clk_divider_bestdiv_ulong_max_returns_max_rate: EXPECTATION FAILED at drivers/clk/clk-divider_test.c:71
[   43.127192]     Expected rate == 1000000000UL / 8, but
[   43.127192]         rate == 500000000 (0x1dcd6500)
[   43.127192]         1000000000UL / 8 == 125000000 (0x7735940)
[   43.127578]     not ok 1 clk_divider_bestdiv_ulong_max_returns_max_rate
[   43.158547]     # clk_divider_bestdiv_mux_ulong_max_returns_max_rate: EXPECTATION FAILED at drivers/clk/clk-divider_test.c:141
[   43.158547]     Expected rate == 0, but
[   43.158547]         rate == 2000000000 (0x77359400)
[   43.189942]     not ok 2 clk_divider_bestdiv_mux_ulong_max_returns_max_rate
[   43.189953] # clk_divider_bestdiv: pass:0 fail:2 skip:0 total:2
[   43.197828] # Totals: pass:0 fail:2 skip:0 total:2
[   43.204592] not ok 1 clk_divider_bestdiv
root@rzn2h-evk:~#

# Case 3: With the fixes to core and Kunit test case
----------------------------------------------------
root@rzn2h-evk:~# modprobe clk-divider_test;modprobe -r clk-divider_test
[   21.006247] KTAP version 1
[   21.009423] 1..1
[   21.011565]     KTAP version 1
[   21.015135]     # Subtest: clk_divider_bestdiv
[   21.020189]     # module: clk_divider_test
[   21.020197]     1..2
[   21.028241]     ok 1 clk_divider_bestdiv_ulong_max_returns_max_rate
[   21.029020]     ok 2 clk_divider_bestdiv_mux_ulong_max_returns_max_rate
[   21.036313] # clk_divider_bestdiv: pass:2 fail:0 skip:0 total:2
[   21.043976] # Totals: pass:2 fail:0 skip:0 total:2
[   21.050732] ok 1 clk_divider_bestdiv

Cheers,
Prabhakar

Lad Prabhakar (2):
  clk: divider: Add KUnit tests for clk_divider_bestdiv() ULONG_MAX
    handling
  clk: divider: Fix clk_divider_bestdiv() returning min rate for large
    rate requests

 drivers/clk/Kconfig            |   8 ++
 drivers/clk/Makefile           |   1 +
 drivers/clk/clk-divider.c      |  25 ++++--
 drivers/clk/clk-divider_test.c | 158 +++++++++++++++++++++++++++++++++
 4 files changed, 184 insertions(+), 8 deletions(-)
 create mode 100644 drivers/clk/clk-divider_test.c

-- 
2.53.0

[PATCH v3 1/2] clk: divider: Add KUnit tests for clk_divider_bestdiv() ULONG_MAX handling

[Prabhakar]

From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>

Add KUnit tests to verify the behaviour of clk_divider_bestdiv() when
clk_round_rate() is called with ULONG_MAX, which is the canonical way
to probe the maximum rate a clock can produce.

Two test cases are introduced:

- clk_divider_bestdiv_ulong_max_returns_max_rate: registers a 1 GHz
  fixed-rate parent driving a table-based divider whose smallest entry
  is div=2 (entries: 2, 4, 8). Calls clk_hw_round_rate(div_hw, ULONG_MAX)
  and checks the result.

- clk_divider_bestdiv_mux_ulong_max_returns_max_rate: places a two-input
  mux (4 GHz and 2 GHz fixed-rate parents, CLK_SET_RATE_PARENT) ahead of
  the same table-based divider to verify correct parent selection under
  ULONG_MAX.

Both tests use an explicit clk_div_table with a minimum divider of 2 so
that the pre-loop maxdiv clamping in clk_divider_bestdiv():

    maxdiv = min(ULONG_MAX / rate, maxdiv);

clamps maxdiv to 1, causing _next_div() to return 2 on the first
iteration and skip the loop body entirely. This makes bestdiv fall back
to the maximum divider, returning the minimum rate rather than the
maximum.

The expected values intentionally reflect the buggy output:
  - test 1: PARENT_RATE_1GHZ / 8  (minimum rate, not maximum)
  - test 2: 0                      (invalid, loop never populated bestdiv)

These will be corrected to PARENT_RATE_1GHZ / 2 and PARENT_RATE_4GHZ / 2
respectively once the fix to clk_divider_bestdiv() is applied.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
---
v2->v3:
- Added false positive expected values
- Updated the commit message
- Added dependency on !S390 in Kconfig
---
 drivers/clk/Kconfig            |   8 ++
 drivers/clk/Makefile           |   1 +
 drivers/clk/clk-divider_test.c | 158 +++++++++++++++++++++++++++++++++
 3 files changed, 167 insertions(+)
 create mode 100644 drivers/clk/clk-divider_test.c

diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
index b2efbe9f6acb..fdeb87ff8fd9 100644
--- a/drivers/clk/Kconfig
+++ b/drivers/clk/Kconfig
@@ -573,4 +573,12 @@ config CLK_FD_KUNIT_TEST
 	help
 	  Kunit test for the clk-fractional-divider type.
 
+config CLK_DIVIDER_KUNIT_TEST
+	tristate "KUnit tests for clk divider bestdiv" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	depends on !S390
+	default KUNIT_ALL_TESTS
+	help
+	  Kunit test for the clk-divider type.
+
 endif
diff --git a/drivers/clk/Makefile b/drivers/clk/Makefile
index a3e2862ebd7e..dc653b458f56 100644
--- a/drivers/clk/Makefile
+++ b/drivers/clk/Makefile
@@ -21,6 +21,7 @@ clk-test-y			:= clk_test.o \
 				   kunit_clk_hw_get_dev_of_node.dtbo.o \
 				   kunit_clk_parent_data_test.dtbo.o
 obj-$(CONFIG_COMMON_CLK)	+= clk-divider.o
+obj-$(CONFIG_CLK_DIVIDER_KUNIT_TEST) += clk-divider_test.o
 obj-$(CONFIG_COMMON_CLK)	+= clk-fixed-factor.o
 obj-$(CONFIG_COMMON_CLK)	+= clk-fixed-rate.o
 obj-$(CONFIG_CLK_FIXED_RATE_KUNIT_TEST)	+= clk-fixed-rate-test.o
diff --git a/drivers/clk/clk-divider_test.c b/drivers/clk/clk-divider_test.c
new file mode 100644
index 000000000000..3087c77fb157
--- /dev/null
+++ b/drivers/clk/clk-divider_test.c
@@ -0,0 +1,158 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit tests for clk_divider_bestdiv()
+ */
+#include <kunit/test.h>
+#include <linux/clk.h>
+#include <linux/clk-provider.h>
+#include <linux/limits.h>
+#include <linux/units.h>
+
+#define PARENT_RATE_1GHZ	GIGA
+#define PARENT_RATE_2GHZ	(2 * GIGA)
+#define PARENT_RATE_4GHZ	(4 * GIGA)
+
+static const struct clk_div_table bestdiv_table[] = {
+	{ .val = 0, .div = 2 },
+	{ .val = 1, .div = 4 },
+	{ .val = 2, .div = 8 },
+	{ /* sentinel */ }
+};
+
+static void unregister_fixed_rate(void *hw)
+{
+	clk_hw_unregister_fixed_rate(hw);
+}
+
+static void unregister_divider(void *hw)
+{
+	clk_hw_unregister_divider(hw);
+}
+
+static void unregister_mux(void *hw)
+{
+	clk_hw_unregister_mux(hw);
+}
+
+/*
+ * Test that clk_round_rate(clk, ULONG_MAX) returns the maximum achievable
+ * rate for a divider clock.
+ */
+static void clk_divider_bestdiv_ulong_max_returns_max_rate(struct kunit *test)
+{
+	struct clk_hw *parent_hw, *div_hw;
+	u32 *fake_reg;
+	unsigned long rate;
+
+	fake_reg = kunit_kzalloc(test, sizeof(*fake_reg), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg);
+
+	parent_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-parent",
+					       NULL, 0, PARENT_RATE_1GHZ);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_hw);
+	KUNIT_ASSERT_EQ(test, 0,
+			kunit_add_action_or_reset(test, unregister_fixed_rate,
+						  parent_hw));
+
+	div_hw = clk_hw_register_divider_table(NULL, "bestdiv-div",
+					       "bestdiv-parent",
+					       CLK_SET_RATE_PARENT,
+					       (void __iomem __force *)fake_reg,
+					       0, 2, 0, bestdiv_table, NULL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, div_hw);
+	KUNIT_ASSERT_EQ(test, 0,
+			kunit_add_action_or_reset(test, unregister_divider, div_hw));
+
+	/*
+	 * ULONG_MAX is the canonical way to probe the maximum rate a clock
+	 * can produce.
+	 */
+	rate = clk_hw_round_rate(div_hw, ULONG_MAX);
+	KUNIT_EXPECT_EQ(test, rate, PARENT_RATE_1GHZ / 8);
+}
+
+/*
+ * Test that clk_round_rate(clk, ULONG_MAX) returns the correct maximum rate
+ * when a mux clock sits between a divider and its parent candidates.
+ *
+ * Topology:
+ *
+ *   [fixed 4 GHz] --\
+ *                    +--> [mux CLK_SET_RATE_PARENT] --> [div {2,4,8} CLK_SET_RATE_PARENT]
+ *   [fixed 2 GHz] --/
+ *
+ */
+static void clk_divider_bestdiv_mux_ulong_max_returns_max_rate(struct kunit *test)
+{
+	static const char * const mux_parents[] = {
+		"bestdiv-mux-parent-a",
+		"bestdiv-mux-parent-b",
+	};
+	struct clk_hw *parent_a_hw, *parent_b_hw, *mux_hw, *div_hw;
+	u32 *fake_reg_mux, *fake_reg_div;
+	unsigned long rate;
+
+	fake_reg_mux = kunit_kzalloc(test, sizeof(*fake_reg_mux), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg_mux);
+
+	fake_reg_div = kunit_kzalloc(test, sizeof(*fake_reg_div), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg_div);
+
+	/* Higher-rate parent: the mux should select this for ULONG_MAX. */
+	parent_a_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-mux-parent-a",
+						 NULL, 0, PARENT_RATE_4GHZ);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_a_hw);
+	KUNIT_ASSERT_EQ(test, 0,
+			kunit_add_action_or_reset(test, unregister_fixed_rate,
+						  parent_a_hw));
+
+	/* Lower-rate parent: should not be selected. */
+	parent_b_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-mux-parent-b",
+						 NULL, 0, PARENT_RATE_2GHZ);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_b_hw);
+	KUNIT_ASSERT_EQ(test, 0,
+			kunit_add_action_or_reset(test, unregister_fixed_rate,
+						  parent_b_hw));
+
+	/*
+	 * 1-bit mux register selects between the two parents.
+	 * CLK_SET_RATE_PARENT allows the divider's rate request to
+	 * propagate into clk_mux_determine_rate().
+	 */
+	mux_hw = clk_hw_register_mux(NULL, "bestdiv-mux",
+				     mux_parents, ARRAY_SIZE(mux_parents),
+				     CLK_SET_RATE_PARENT,
+				     (void __iomem __force *)fake_reg_mux,
+				     0, 1, 0, NULL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, mux_hw);
+	KUNIT_ASSERT_EQ(test, 0,
+			kunit_add_action_or_reset(test, unregister_mux, mux_hw));
+
+	div_hw = clk_hw_register_divider_table(NULL, "bestdiv-mux-div",
+					       "bestdiv-mux",
+					       CLK_SET_RATE_PARENT,
+					       (void __iomem __force *)fake_reg_div,
+					       0, 2, 0, bestdiv_table, NULL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, div_hw);
+	KUNIT_ASSERT_EQ(test, 0,
+			kunit_add_action_or_reset(test, unregister_divider, div_hw));
+
+	rate = clk_hw_round_rate(div_hw, ULONG_MAX);
+	KUNIT_EXPECT_EQ(test, rate, 0);
+}
+
+static struct kunit_case clk_divider_bestdiv_test_cases[] = {
+	KUNIT_CASE(clk_divider_bestdiv_ulong_max_returns_max_rate),
+	KUNIT_CASE(clk_divider_bestdiv_mux_ulong_max_returns_max_rate),
+	{}
+};
+
+static struct kunit_suite clk_divider_bestdiv_test_suite = {
+	.name = "clk_divider_bestdiv",
+	.test_cases = clk_divider_bestdiv_test_cases,
+};
+
+kunit_test_suite(clk_divider_bestdiv_test_suite);
+
+MODULE_DESCRIPTION("KUnit tests for clk_divider_bestdiv()");
+MODULE_LICENSE("GPL");
-- 
2.53.0

Re: [PATCH v3 1/2] clk: divider: Add KUnit tests for clk_divider_bestdiv() ULONG_MAX handling

[Brian Masney]

Hi Lad,

On Tue, Apr 21, 2026 at 07:25:25PM +0100, Prabhakar wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> 
> Add KUnit tests to verify the behaviour of clk_divider_bestdiv() when
> clk_round_rate() is called with ULONG_MAX, which is the canonical way
> to probe the maximum rate a clock can produce.
> 
> Two test cases are introduced:
> 
> - clk_divider_bestdiv_ulong_max_returns_max_rate: registers a 1 GHz
>   fixed-rate parent driving a table-based divider whose smallest entry
>   is div=2 (entries: 2, 4, 8). Calls clk_hw_round_rate(div_hw, ULONG_MAX)
>   and checks the result.
> 
> - clk_divider_bestdiv_mux_ulong_max_returns_max_rate: places a two-input
>   mux (4 GHz and 2 GHz fixed-rate parents, CLK_SET_RATE_PARENT) ahead of
>   the same table-based divider to verify correct parent selection under
>   ULONG_MAX.
> 
> Both tests use an explicit clk_div_table with a minimum divider of 2 so
> that the pre-loop maxdiv clamping in clk_divider_bestdiv():
> 
>     maxdiv = min(ULONG_MAX / rate, maxdiv);
> 
> clamps maxdiv to 1, causing _next_div() to return 2 on the first
> iteration and skip the loop body entirely. This makes bestdiv fall back
> to the maximum divider, returning the minimum rate rather than the
> maximum.
> 
> The expected values intentionally reflect the buggy output:
>   - test 1: PARENT_RATE_1GHZ / 8  (minimum rate, not maximum)
>   - test 2: 0                      (invalid, loop never populated bestdiv)
> 
> These will be corrected to PARENT_RATE_1GHZ / 2 and PARENT_RATE_4GHZ / 2
> respectively once the fix to clk_divider_bestdiv() is applied.
> 
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> ---
> v2->v3:
> - Added false positive expected values
> - Updated the commit message
> - Added dependency on !S390 in Kconfig
> ---
>  drivers/clk/Kconfig            |   8 ++
>  drivers/clk/Makefile           |   1 +
>  drivers/clk/clk-divider_test.c | 158 +++++++++++++++++++++++++++++++++
>  3 files changed, 167 insertions(+)
>  create mode 100644 drivers/clk/clk-divider_test.c

CONFIG_CLK_DIVIDER_KUNIT_TEST=y needs to be added to
drivers/clk/.kunitconfig.

> 
> diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
> index b2efbe9f6acb..fdeb87ff8fd9 100644
> --- a/drivers/clk/Kconfig
> +++ b/drivers/clk/Kconfig
> @@ -573,4 +573,12 @@ config CLK_FD_KUNIT_TEST
>  	help
>  	  Kunit test for the clk-fractional-divider type.
>  
> +config CLK_DIVIDER_KUNIT_TEST
> +	tristate "KUnit tests for clk divider bestdiv" if !KUNIT_ALL_TESTS
> +	depends on KUNIT
> +	depends on !S390
> +	default KUNIT_ALL_TESTS
> +	help
> +	  Kunit test for the clk-divider type.
> +
>  endif
> diff --git a/drivers/clk/Makefile b/drivers/clk/Makefile
> index a3e2862ebd7e..dc653b458f56 100644
> --- a/drivers/clk/Makefile
> +++ b/drivers/clk/Makefile
> @@ -21,6 +21,7 @@ clk-test-y			:= clk_test.o \
>  				   kunit_clk_hw_get_dev_of_node.dtbo.o \
>  				   kunit_clk_parent_data_test.dtbo.o
>  obj-$(CONFIG_COMMON_CLK)	+= clk-divider.o
> +obj-$(CONFIG_CLK_DIVIDER_KUNIT_TEST) += clk-divider_test.o
>  obj-$(CONFIG_COMMON_CLK)	+= clk-fixed-factor.o
>  obj-$(CONFIG_COMMON_CLK)	+= clk-fixed-rate.o
>  obj-$(CONFIG_CLK_FIXED_RATE_KUNIT_TEST)	+= clk-fixed-rate-test.o
> diff --git a/drivers/clk/clk-divider_test.c b/drivers/clk/clk-divider_test.c
> new file mode 100644
> index 000000000000..3087c77fb157
> --- /dev/null
> +++ b/drivers/clk/clk-divider_test.c
> @@ -0,0 +1,158 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * KUnit tests for clk_divider_bestdiv()
> + */
> +#include <kunit/test.h>
> +#include <linux/clk.h>
> +#include <linux/clk-provider.h>
> +#include <linux/limits.h>
> +#include <linux/units.h>
> +
> +#define PARENT_RATE_1GHZ	GIGA
> +#define PARENT_RATE_2GHZ	(2 * GIGA)
> +#define PARENT_RATE_4GHZ	(4 * GIGA)
> +
> +static const struct clk_div_table bestdiv_table[] = {
> +	{ .val = 0, .div = 2 },
> +	{ .val = 1, .div = 4 },
> +	{ .val = 2, .div = 8 },
> +	{ /* sentinel */ }
> +};

Is it possible to drop the table like I suggested in v2? You can use
CLK_DIVIDER_ONE_BASED. (See docs in include/linux/clk-provider.h).
You can also use CLK_DIVIDER_POWER_OF_TWO if you want the divider to be
larger.

> +
> +static void unregister_fixed_rate(void *hw)
> +{
> +	clk_hw_unregister_fixed_rate(hw);
> +}
> +
> +static void unregister_divider(void *hw)
> +{
> +	clk_hw_unregister_divider(hw);
> +}
> +
> +static void unregister_mux(void *hw)
> +{
> +	clk_hw_unregister_mux(hw);
> +}

Can these 3 wrapper functions be dropped, and you just call the inner
function in the call to kunit_add_action_or_reset()? For example,
instead of:

    static void unregister_fixed_rate(void *hw)
    {
        clk_hw_unregister_fixed_rate(hw);
    }
    ...
    KUNIT_ASSERT_EQ(test, 0,
                    kunit_add_action_or_reset(test, unregister_fixed_rate,
                                              parent_hw));

You should be able to do this:

    KUNIT_ASSERT_EQ(test, 0,
                    kunit_add_action_or_reset(test, clk_hw_unregister_fixed_rate,
                                              parent_hw));

I think we should just drop the wrappers completely if they aren't
needed. If more functionality is needed in the future, then the wrappers
can be added then.

> +
> +/*
> + * Test that clk_round_rate(clk, ULONG_MAX) returns the maximum achievable
> + * rate for a divider clock.
> + */
> +static void clk_divider_bestdiv_ulong_max_returns_max_rate(struct kunit *test)
> +{
> +	struct clk_hw *parent_hw, *div_hw;
> +	u32 *fake_reg;
> +	unsigned long rate;

Reverse Christmas tree order.

> +
> +	fake_reg = kunit_kzalloc(test, sizeof(*fake_reg), GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg);
> +
> +	parent_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-parent",
> +					       NULL, 0, PARENT_RATE_1GHZ);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_hw);
> +	KUNIT_ASSERT_EQ(test, 0,
> +			kunit_add_action_or_reset(test, unregister_fixed_rate,
> +						  parent_hw));
> +
> +	div_hw = clk_hw_register_divider_table(NULL, "bestdiv-div",
> +					       "bestdiv-parent",
> +					       CLK_SET_RATE_PARENT,
> +					       (void __iomem __force *)fake_reg,
> +					       0, 2, 0, bestdiv_table, NULL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, div_hw);
> +	KUNIT_ASSERT_EQ(test, 0,
> +			kunit_add_action_or_reset(test, unregister_divider, div_hw));
> +
> +	/*
> +	 * ULONG_MAX is the canonical way to probe the maximum rate a clock
> +	 * can produce.
> +	 */
> +	rate = clk_hw_round_rate(div_hw, ULONG_MAX);
> +	KUNIT_EXPECT_EQ(test, rate, PARENT_RATE_1GHZ / 8);
> +}
> +
> +/*
> + * Test that clk_round_rate(clk, ULONG_MAX) returns the correct maximum rate
> + * when a mux clock sits between a divider and its parent candidates.
> + *
> + * Topology:
> + *
> + *   [fixed 4 GHz] --\
> + *                    +--> [mux CLK_SET_RATE_PARENT] --> [div {2,4,8} CLK_SET_RATE_PARENT]
> + *   [fixed 2 GHz] --/
> + *
> + */
> +static void clk_divider_bestdiv_mux_ulong_max_returns_max_rate(struct kunit *test)
> +{
> +	static const char * const mux_parents[] = {
> +		"bestdiv-mux-parent-a",
> +		"bestdiv-mux-parent-b",
> +	};
> +	struct clk_hw *parent_a_hw, *parent_b_hw, *mux_hw, *div_hw;
> +	u32 *fake_reg_mux, *fake_reg_div;
> +	unsigned long rate;
> +
> +	fake_reg_mux = kunit_kzalloc(test, sizeof(*fake_reg_mux), GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg_mux);
> +
> +	fake_reg_div = kunit_kzalloc(test, sizeof(*fake_reg_div), GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg_div);
> +
> +	/* Higher-rate parent: the mux should select this for ULONG_MAX. */
> +	parent_a_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-mux-parent-a",
> +						 NULL, 0, PARENT_RATE_4GHZ);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_a_hw);
> +	KUNIT_ASSERT_EQ(test, 0,
> +			kunit_add_action_or_reset(test, unregister_fixed_rate,
> +						  parent_a_hw));
> +
> +	/* Lower-rate parent: should not be selected. */
> +	parent_b_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-mux-parent-b",
> +						 NULL, 0, PARENT_RATE_2GHZ);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_b_hw);
> +	KUNIT_ASSERT_EQ(test, 0,
> +			kunit_add_action_or_reset(test, unregister_fixed_rate,
> +						  parent_b_hw));
> +
> +	/*
> +	 * 1-bit mux register selects between the two parents.
> +	 * CLK_SET_RATE_PARENT allows the divider's rate request to
> +	 * propagate into clk_mux_determine_rate().
> +	 */
> +	mux_hw = clk_hw_register_mux(NULL, "bestdiv-mux",
> +				     mux_parents, ARRAY_SIZE(mux_parents),
> +				     CLK_SET_RATE_PARENT,
> +				     (void __iomem __force *)fake_reg_mux,
> +				     0, 1, 0, NULL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, mux_hw);
> +	KUNIT_ASSERT_EQ(test, 0,
> +			kunit_add_action_or_reset(test, unregister_mux, mux_hw));
> +
> +	div_hw = clk_hw_register_divider_table(NULL, "bestdiv-mux-div",
> +					       "bestdiv-mux",
> +					       CLK_SET_RATE_PARENT,
> +					       (void __iomem __force *)fake_reg_div,
> +					       0, 2, 0, bestdiv_table, NULL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, div_hw);
> +	KUNIT_ASSERT_EQ(test, 0,
> +			kunit_add_action_or_reset(test, unregister_divider, div_hw));
> +
> +	rate = clk_hw_round_rate(div_hw, ULONG_MAX);
> +	KUNIT_EXPECT_EQ(test, rate, 0);
> +}
> +
> +static struct kunit_case clk_divider_bestdiv_test_cases[] = {
> +	KUNIT_CASE(clk_divider_bestdiv_ulong_max_returns_max_rate),
> +	KUNIT_CASE(clk_divider_bestdiv_mux_ulong_max_returns_max_rate),
> +	{}
> +};
> +
> +static struct kunit_suite clk_divider_bestdiv_test_suite = {
> +	.name = "clk_divider_bestdiv",
> +	.test_cases = clk_divider_bestdiv_test_cases,
> +};
> +
> +kunit_test_suite(clk_divider_bestdiv_test_suite);
> +
> +MODULE_DESCRIPTION("KUnit tests for clk_divider_bestdiv()");

KUnit tests for clk divider.

I have another tests that will eventually be added to this file.

Brian

Re: [PATCH v3 1/2] clk: divider: Add KUnit tests for clk_divider_bestdiv() ULONG_MAX handling

[Lad, Prabhakar]

Hi Brian,

Thank you for the review.

On Fri, Apr 24, 2026 at 8:35 PM Brian Masney <bmasney@redhat.com> wrote:
>
> Hi Lad,
>
> On Tue, Apr 21, 2026 at 07:25:25PM +0100, Prabhakar wrote:
> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> >
> > Add KUnit tests to verify the behaviour of clk_divider_bestdiv() when
> > clk_round_rate() is called with ULONG_MAX, which is the canonical way
> > to probe the maximum rate a clock can produce.
> >
> > Two test cases are introduced:
> >
> > - clk_divider_bestdiv_ulong_max_returns_max_rate: registers a 1 GHz
> >   fixed-rate parent driving a table-based divider whose smallest entry
> >   is div=2 (entries: 2, 4, 8). Calls clk_hw_round_rate(div_hw, ULONG_MAX)
> >   and checks the result.
> >
> > - clk_divider_bestdiv_mux_ulong_max_returns_max_rate: places a two-input
> >   mux (4 GHz and 2 GHz fixed-rate parents, CLK_SET_RATE_PARENT) ahead of
> >   the same table-based divider to verify correct parent selection under
> >   ULONG_MAX.
> >
> > Both tests use an explicit clk_div_table with a minimum divider of 2 so
> > that the pre-loop maxdiv clamping in clk_divider_bestdiv():
> >
> >     maxdiv = min(ULONG_MAX / rate, maxdiv);
> >
> > clamps maxdiv to 1, causing _next_div() to return 2 on the first
> > iteration and skip the loop body entirely. This makes bestdiv fall back
> > to the maximum divider, returning the minimum rate rather than the
> > maximum.
> >
> > The expected values intentionally reflect the buggy output:
> >   - test 1: PARENT_RATE_1GHZ / 8  (minimum rate, not maximum)
> >   - test 2: 0                      (invalid, loop never populated bestdiv)
> >
> > These will be corrected to PARENT_RATE_1GHZ / 2 and PARENT_RATE_4GHZ / 2
> > respectively once the fix to clk_divider_bestdiv() is applied.
> >
> > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> > ---
> > v2->v3:
> > - Added false positive expected values
> > - Updated the commit message
> > - Added dependency on !S390 in Kconfig
> > ---
> >  drivers/clk/Kconfig            |   8 ++
> >  drivers/clk/Makefile           |   1 +
> >  drivers/clk/clk-divider_test.c | 158 +++++++++++++++++++++++++++++++++
> >  3 files changed, 167 insertions(+)
> >  create mode 100644 drivers/clk/clk-divider_test.c
>
> CONFIG_CLK_DIVIDER_KUNIT_TEST=y needs to be added to
> drivers/clk/.kunitconfig.
>
> >
> > diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
> > index b2efbe9f6acb..fdeb87ff8fd9 100644
> > --- a/drivers/clk/Kconfig
> > +++ b/drivers/clk/Kconfig
> > @@ -573,4 +573,12 @@ config CLK_FD_KUNIT_TEST
> >       help
> >         Kunit test for the clk-fractional-divider type.
> >
> > +config CLK_DIVIDER_KUNIT_TEST
> > +     tristate "KUnit tests for clk divider bestdiv" if !KUNIT_ALL_TESTS
> > +     depends on KUNIT
> > +     depends on !S390
> > +     default KUNIT_ALL_TESTS
> > +     help
> > +       Kunit test for the clk-divider type.
> > +
> >  endif
> > diff --git a/drivers/clk/Makefile b/drivers/clk/Makefile
> > index a3e2862ebd7e..dc653b458f56 100644
> > --- a/drivers/clk/Makefile
> > +++ b/drivers/clk/Makefile
> > @@ -21,6 +21,7 @@ clk-test-y                  := clk_test.o \
> >                                  kunit_clk_hw_get_dev_of_node.dtbo.o \
> >                                  kunit_clk_parent_data_test.dtbo.o
> >  obj-$(CONFIG_COMMON_CLK)     += clk-divider.o
> > +obj-$(CONFIG_CLK_DIVIDER_KUNIT_TEST) += clk-divider_test.o
> >  obj-$(CONFIG_COMMON_CLK)     += clk-fixed-factor.o
> >  obj-$(CONFIG_COMMON_CLK)     += clk-fixed-rate.o
> >  obj-$(CONFIG_CLK_FIXED_RATE_KUNIT_TEST)      += clk-fixed-rate-test.o
> > diff --git a/drivers/clk/clk-divider_test.c b/drivers/clk/clk-divider_test.c
> > new file mode 100644
> > index 000000000000..3087c77fb157
> > --- /dev/null
> > +++ b/drivers/clk/clk-divider_test.c
> > @@ -0,0 +1,158 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * KUnit tests for clk_divider_bestdiv()
> > + */
> > +#include <kunit/test.h>
> > +#include <linux/clk.h>
> > +#include <linux/clk-provider.h>
> > +#include <linux/limits.h>
> > +#include <linux/units.h>
> > +
> > +#define PARENT_RATE_1GHZ     GIGA
> > +#define PARENT_RATE_2GHZ     (2 * GIGA)
> > +#define PARENT_RATE_4GHZ     (4 * GIGA)
> > +
> > +static const struct clk_div_table bestdiv_table[] = {
> > +     { .val = 0, .div = 2 },
> > +     { .val = 1, .div = 4 },
> > +     { .val = 2, .div = 8 },
> > +     { /* sentinel */ }
> > +};
>
> Is it possible to drop the table like I suggested in v2? You can use
> CLK_DIVIDER_ONE_BASED. (See docs in include/linux/clk-provider.h).
> You can also use CLK_DIVIDER_POWER_OF_TWO if you want the divider to be
> larger.
>
When using CLK_DIVIDER_ONE_BASED/CLK_DIVIDER_POWER_OF_TWO I dont get the issue.

For example for a parent of 1GHz and when using
CLK_DIVIDER_POWER_OF_TWO, Im getting expected result of 1GHz for
`clk_hw_round_rate(div_hw, ULONG_MAX)`

Note with CLK_DIVIDER_POWER_OF_TWO flag Im using
    div_hw = clk_hw_register_divider(NULL, "bestdiv-div",
                     "bestdiv-parent",
                     CLK_SET_RATE_PARENT,
                     (void __iomem __force *)fake_reg,
                     0, 2, CLK_DIVIDER_POWER_OF_TWO, NULL);

Case #1 With bestdiv_table
_get_maxdiv() --> walks table, finds max div = 8
maxdiv = min(ULONG_MAX / ULONG_MAX, 8) = min(1, 8) = 1

_next_div(table, 0) --> first table entry --> i = 2

Loop: 2 <= 1 --> FALSE, never enters

Fallback (!bestdiv):
    bestdiv = _get_maxdiv() = 8
    *best_parent_rate = clk_hw_round_rate(parent, 1) = 1GHz  <-- fixed
rate ignores request

return 8 --> 1GHz / 8 = 125MHz

Case #2 With CLK_DIVIDER_POWER_OF_TWO

_get_maxdiv() --> 1 << clk_div_mask(2) = 1 << 3 = 8
maxdiv = min(ULONG_MAX / ULONG_MAX, 8) = min(1, 8) = 1

_next_div(NULL, 0, CLK_DIVIDER_POWER_OF_TWO) --> 1 << 0 = i = 1

Loop: 1 <= 1 --> TRUE  <-- enters because min div is 1, not 2

    rate * i = ULONG_MAX * 1 = ULONG_MAX (no overflow)
    parent_rate = clk_hw_round_rate(parent, ULONG_MAX) = 1GHz
    now = 1GHz / 1 = 1GHz
    bestdiv = 1, best = 1GHz

    next i = 2 > 1 --> loop ends

return 1 --> 1GHz / 1 = 1GHz

> > +
> > +static void unregister_fixed_rate(void *hw)
> > +{
> > +     clk_hw_unregister_fixed_rate(hw);
> > +}
> > +
> > +static void unregister_divider(void *hw)
> > +{
> > +     clk_hw_unregister_divider(hw);
> > +}
> > +
> > +static void unregister_mux(void *hw)
> > +{
> > +     clk_hw_unregister_mux(hw);
> > +}
>
> Can these 3 wrapper functions be dropped, and you just call the inner
> function in the call to kunit_add_action_or_reset()? For example,
> instead of:
>
>     static void unregister_fixed_rate(void *hw)
>     {
>         clk_hw_unregister_fixed_rate(hw);
>     }
>     ...
>     KUNIT_ASSERT_EQ(test, 0,
>                     kunit_add_action_or_reset(test, unregister_fixed_rate,
>                                               parent_hw));
>
> You should be able to do this:
>
>     KUNIT_ASSERT_EQ(test, 0,
>                     kunit_add_action_or_reset(test, clk_hw_unregister_fixed_rate,
>                                               parent_hw));
>
> I think we should just drop the wrappers completely if they aren't
> needed. If more functionality is needed in the future, then the wrappers
> can be added then.
>
Ok, I willl need to cast and use it like below:

+       KUNIT_ASSERT_EQ(test, 0,
+                       kunit_add_action_or_reset(test,
+                                                 (kunit_action_t
*)clk_hw_unregister_divider,
+                                                 div_hw));

> > +
> > +/*
> > + * Test that clk_round_rate(clk, ULONG_MAX) returns the maximum achievable
> > + * rate for a divider clock.
> > + */
> > +static void clk_divider_bestdiv_ulong_max_returns_max_rate(struct kunit *test)
> > +{
> > +     struct clk_hw *parent_hw, *div_hw;
> > +     u32 *fake_reg;
> > +     unsigned long rate;
>
> Reverse Christmas tree order.
>
OK.

> > +
> > +     fake_reg = kunit_kzalloc(test, sizeof(*fake_reg), GFP_KERNEL);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg);
> > +
> > +     parent_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-parent",
> > +                                            NULL, 0, PARENT_RATE_1GHZ);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_hw);
> > +     KUNIT_ASSERT_EQ(test, 0,
> > +                     kunit_add_action_or_reset(test, unregister_fixed_rate,
> > +                                               parent_hw));
> > +
> > +     div_hw = clk_hw_register_divider_table(NULL, "bestdiv-div",
> > +                                            "bestdiv-parent",
> > +                                            CLK_SET_RATE_PARENT,
> > +                                            (void __iomem __force *)fake_reg,
> > +                                            0, 2, 0, bestdiv_table, NULL);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, div_hw);
> > +     KUNIT_ASSERT_EQ(test, 0,
> > +                     kunit_add_action_or_reset(test, unregister_divider, div_hw));
> > +
> > +     /*
> > +      * ULONG_MAX is the canonical way to probe the maximum rate a clock
> > +      * can produce.
> > +      */
> > +     rate = clk_hw_round_rate(div_hw, ULONG_MAX);
> > +     KUNIT_EXPECT_EQ(test, rate, PARENT_RATE_1GHZ / 8);
> > +}
> > +
> > +/*
> > + * Test that clk_round_rate(clk, ULONG_MAX) returns the correct maximum rate
> > + * when a mux clock sits between a divider and its parent candidates.
> > + *
> > + * Topology:
> > + *
> > + *   [fixed 4 GHz] --\
> > + *                    +--> [mux CLK_SET_RATE_PARENT] --> [div {2,4,8} CLK_SET_RATE_PARENT]
> > + *   [fixed 2 GHz] --/
> > + *
> > + */
> > +static void clk_divider_bestdiv_mux_ulong_max_returns_max_rate(struct kunit *test)
> > +{
> > +     static const char * const mux_parents[] = {
> > +             "bestdiv-mux-parent-a",
> > +             "bestdiv-mux-parent-b",
> > +     };
> > +     struct clk_hw *parent_a_hw, *parent_b_hw, *mux_hw, *div_hw;
> > +     u32 *fake_reg_mux, *fake_reg_div;
> > +     unsigned long rate;
> > +
> > +     fake_reg_mux = kunit_kzalloc(test, sizeof(*fake_reg_mux), GFP_KERNEL);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg_mux);
> > +
> > +     fake_reg_div = kunit_kzalloc(test, sizeof(*fake_reg_div), GFP_KERNEL);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fake_reg_div);
> > +
> > +     /* Higher-rate parent: the mux should select this for ULONG_MAX. */
> > +     parent_a_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-mux-parent-a",
> > +                                              NULL, 0, PARENT_RATE_4GHZ);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_a_hw);
> > +     KUNIT_ASSERT_EQ(test, 0,
> > +                     kunit_add_action_or_reset(test, unregister_fixed_rate,
> > +                                               parent_a_hw));
> > +
> > +     /* Lower-rate parent: should not be selected. */
> > +     parent_b_hw = clk_hw_register_fixed_rate(NULL, "bestdiv-mux-parent-b",
> > +                                              NULL, 0, PARENT_RATE_2GHZ);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, parent_b_hw);
> > +     KUNIT_ASSERT_EQ(test, 0,
> > +                     kunit_add_action_or_reset(test, unregister_fixed_rate,
> > +                                               parent_b_hw));
> > +
> > +     /*
> > +      * 1-bit mux register selects between the two parents.
> > +      * CLK_SET_RATE_PARENT allows the divider's rate request to
> > +      * propagate into clk_mux_determine_rate().
> > +      */
> > +     mux_hw = clk_hw_register_mux(NULL, "bestdiv-mux",
> > +                                  mux_parents, ARRAY_SIZE(mux_parents),
> > +                                  CLK_SET_RATE_PARENT,
> > +                                  (void __iomem __force *)fake_reg_mux,
> > +                                  0, 1, 0, NULL);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, mux_hw);
> > +     KUNIT_ASSERT_EQ(test, 0,
> > +                     kunit_add_action_or_reset(test, unregister_mux, mux_hw));
> > +
> > +     div_hw = clk_hw_register_divider_table(NULL, "bestdiv-mux-div",
> > +                                            "bestdiv-mux",
> > +                                            CLK_SET_RATE_PARENT,
> > +                                            (void __iomem __force *)fake_reg_div,
> > +                                            0, 2, 0, bestdiv_table, NULL);
> > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, div_hw);
> > +     KUNIT_ASSERT_EQ(test, 0,
> > +                     kunit_add_action_or_reset(test, unregister_divider, div_hw));
> > +
> > +     rate = clk_hw_round_rate(div_hw, ULONG_MAX);
> > +     KUNIT_EXPECT_EQ(test, rate, 0);
> > +}
> > +
> > +static struct kunit_case clk_divider_bestdiv_test_cases[] = {
> > +     KUNIT_CASE(clk_divider_bestdiv_ulong_max_returns_max_rate),
> > +     KUNIT_CASE(clk_divider_bestdiv_mux_ulong_max_returns_max_rate),
> > +     {}
> > +};
> > +
> > +static struct kunit_suite clk_divider_bestdiv_test_suite = {
> > +     .name = "clk_divider_bestdiv",
> > +     .test_cases = clk_divider_bestdiv_test_cases,
> > +};
> > +
> > +kunit_test_suite(clk_divider_bestdiv_test_suite);
> > +
> > +MODULE_DESCRIPTION("KUnit tests for clk_divider_bestdiv()");
>
> KUnit tests for clk divider.
>
> I have another tests that will eventually be added to this file.
>
Ok.

Cheers,
Prabhakar

[PATCH v3 2/2] clk: divider: Fix clk_divider_bestdiv() returning min rate for large rate requests

[Prabhakar]

From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>

clk_divider_bestdiv() clamps maxdiv using:

    maxdiv = min(ULONG_MAX / rate, maxdiv);

to avoid overflow in rate * i. However, requests like
clk_round_rate(clk, ULONG_MAX), which are used to determine the maximum
supported rate of a clock, result in maxdiv being clamped to 1. If no
valid divider of 1 exists in the table the loop is never entered and
bestdiv falls back to the maximum divider with the minimum parent rate,
causing clk_round_rate(clk, ULONG_MAX) to incorrectly return the minimum
supported rate instead of the maximum.

Fix this by removing the pre-loop maxdiv clamping and replacing the
unprotected rate * i multiplication with check_mul_overflow(). Guard
the exact-match short-circuit with !overflow to prevent a clamped
target_parent_rate of ULONG_MAX from falsely matching parent_rate_saved
and causing premature loop exit. Break out of the loop after evaluating
the first overflowing divider since clk_hw_round_rate(parent, ULONG_MAX)
returns a constant for all subsequent iterations, meaning no better
candidate can be found, and continuing would cause exponential recursive
calls in chained divider clocks.

Update the KUnit test expected values to reflect the corrected behaviour:
  - clk_divider_bestdiv_ulong_max_returns_max_rate: PARENT_RATE_1GHZ / 8
    (minimum rate, pre-fix) -> PARENT_RATE_1GHZ / 2 (maximum rate)
  - clk_divider_bestdiv_mux_ulong_max_returns_max_rate: 0 (invalid,
    pre-fix) -> PARENT_RATE_4GHZ / 2 (maximum rate with mux selecting
    the 4 GHz parent and applying the smallest table divider of 2)

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Brian Masney <bmasney@redhat.com>
---
v2->v3:
- Added Rb tag
- Added the expected value for the tests
- updated the commit message
---
 drivers/clk/clk-divider.c      | 25 +++++++++++++++++--------
 drivers/clk/clk-divider_test.c |  4 ++--
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index c3804bbc06f9..36bfaf7200e2 100644
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -15,6 +15,7 @@
 #include <linux/err.h>
 #include <linux/string.h>
 #include <linux/log2.h>
+#include <linux/overflow.h>
 
 /*
  * DOC: basic adjustable divider clock that cannot gate
@@ -301,6 +302,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, struct clk_hw *parent,
 	int i, bestdiv = 0;
 	unsigned long parent_rate, best = 0, now, maxdiv;
 	unsigned long parent_rate_saved = *best_parent_rate;
+	unsigned long target_parent_rate;
 
 	if (!rate)
 		rate = 1;
@@ -315,15 +317,11 @@ static int clk_divider_bestdiv(struct clk_hw *hw, struct clk_hw *parent,
 		return bestdiv;
 	}
 
-	/*
-	 * The maximum divider we can use without overflowing
-	 * unsigned long in rate * i below
-	 */
-	maxdiv = min(ULONG_MAX / rate, maxdiv);
-
 	for (i = _next_div(table, 0, flags); i <= maxdiv;
 					     i = _next_div(table, i, flags)) {
-		if (rate * i == parent_rate_saved) {
+		bool overflow = check_mul_overflow(rate, (unsigned long)i, &target_parent_rate);
+
+		if (!overflow && target_parent_rate == parent_rate_saved) {
 			/*
 			 * It's the most ideal case if the requested rate can be
 			 * divided from parent clock without needing to change
@@ -332,13 +330,24 @@ static int clk_divider_bestdiv(struct clk_hw *hw, struct clk_hw *parent,
 			*best_parent_rate = parent_rate_saved;
 			return i;
 		}
-		parent_rate = clk_hw_round_rate(parent, rate * i);
+		/*
+		 * Clamp target_parent_rate to ULONG_MAX on overflow. The true
+		 * required parent rate exceeds what can be represented, so ask
+		 * the parent for the highest rate it can produce. There is no
+		 * point continuing the loop past this since larger dividers
+		 * only move further from the requested rate.
+		 */
+		if (overflow)
+			target_parent_rate = ULONG_MAX;
+		parent_rate = clk_hw_round_rate(parent, target_parent_rate);
 		now = DIV_ROUND_UP_ULL((u64)parent_rate, i);
 		if (_is_best_div(rate, now, best, flags)) {
 			bestdiv = i;
 			best = now;
 			*best_parent_rate = parent_rate;
 		}
+		if (overflow)
+			break;
 	}
 
 	if (!bestdiv) {
diff --git a/drivers/clk/clk-divider_test.c b/drivers/clk/clk-divider_test.c
index 3087c77fb157..300b8ba04279 100644
--- a/drivers/clk/clk-divider_test.c
+++ b/drivers/clk/clk-divider_test.c
@@ -68,7 +68,7 @@ static void clk_divider_bestdiv_ulong_max_returns_max_rate(struct kunit *test)
 	 * can produce.
 	 */
 	rate = clk_hw_round_rate(div_hw, ULONG_MAX);
-	KUNIT_EXPECT_EQ(test, rate, PARENT_RATE_1GHZ / 8);
+	KUNIT_EXPECT_EQ(test, rate, PARENT_RATE_1GHZ / 2);
 }
 
 /*
@@ -138,7 +138,7 @@ static void clk_divider_bestdiv_mux_ulong_max_returns_max_rate(struct kunit *tes
 			kunit_add_action_or_reset(test, unregister_divider, div_hw));
 
 	rate = clk_hw_round_rate(div_hw, ULONG_MAX);
-	KUNIT_EXPECT_EQ(test, rate, 0);
+	KUNIT_EXPECT_EQ(test, rate, PARENT_RATE_4GHZ / 2);
 }
 
 static struct kunit_case clk_divider_bestdiv_test_cases[] = {
-- 
2.53.0