From: Yang Xiwen <forbidden405@outlook.com>

Originally, the initial clock rate is hardcoded to 0, this can lead to
some problem when setting a very small rate with CLK_MUX_ROUND_CLOSEST.

For example, if the lowest possible rate provided by the mux is 1000Hz,
setting a rate below 500Hz will fail, because no clock can provide a
better rate than the non-existant 0Hz. But it should succeed with 1000Hz
being set.

Setting the initial best parent to current parent could solve this bug.

Signed-off-by: Yang Xiwen <forbidden405@outlook.com>
---
This is actually a v2 of [1]. It's tested in a mmc host driver.

[1]: https://lore.kernel.org/linux-clk/20230421-clk-v3-1-9ff79e7e7fed@outlook.com/
---
Changes in v3:
- remove the fix for clk_test.c
- limit the fix to CLK_MUX_ROUND_CLOSEST
- Link to v2: https://lore.kernel.org/r/20240306-mux-v2-0-92a5fa461fd2@outlook.com

Changes in v2:
- cover letter: remove statements about unittest
- add fix for clk_test.c
- s/privide/provide
- Cc Maxime
All suggested by Stephan Boyd
- Link to v1: https://lore.kernel.org/r/20240215-mux-v1-1-ebb2fba31d49@outlook.com
---
 drivers/clk/clk.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 2253c154a824..74dd61f7269f 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -647,6 +647,12 @@ int clk_mux_determine_rate_flags(struct clk_hw *hw,
 	if (core->flags & CLK_SET_RATE_NO_REPARENT)
 		return clk_core_determine_rate_no_reparent(hw, req);
 
+	/* if MUX_ROUND_CLOSEST is set, set the initial best parent to current parent */
+	if (flags & CLK_MUX_ROUND_CLOSEST) {
+		best_parent = core->parent;
+		best = clk_core_get_rate_nolock(core);
+	}
+
 	/* find the parent that can provide the fastest rate <= rate */
 	num_parents = core->num_parents;
 	for (i = 0; i < num_parents; i++) {

---
base-commit: 90d35da658da8cff0d4ecbb5113f5fac9d00eb72
change-id: 20240215-mux-6db8b3714590

Best regards,
-- 
Yang Xiwen <forbidden405@outlook.com>

Hi,

On Thu, Mar 07, 2024 at 10:03:50AM +0800, Yang Xiwen via B4 Relay wrote:
> From: Yang Xiwen <forbidden405@outlook.com>
> 
> Originally, the initial clock rate is hardcoded to 0, this can lead to
> some problem when setting a very small rate with CLK_MUX_ROUND_CLOSEST.
> 
> For example, if the lowest possible rate provided by the mux is 1000Hz,
> setting a rate below 500Hz will fail, because no clock can provide a
> better rate than the non-existant 0Hz. But it should succeed with 1000Hz
> being set.
> 
> Setting the initial best parent to current parent could solve this bug.
> 
> Signed-off-by: Yang Xiwen <forbidden405@outlook.com>

I don't think it would be the way to go. The biggest issue to me is that
it's inconsistent, and only changing the behaviour for a given flag
doesn't solve that.

And again, either way, we should document it. And run it through kernelci.

Maxime