linux-kernel - [PATCH] clk: set initial best mux parent to current parent when determining rate

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Message-Id: <20240215-mux-v1-1-ebb2fba31d49@outlook.com>
Date: Thu, 15 Feb 2024 23:09:11 +0800
From: Yang Xiwen via B4 Relay <devnull+forbidden405.outlook.com@...nel.org>
To: Michael Turquette <mturquette@...libre.com>, 
 Stephen Boyd <sboyd@...nel.org>
Cc: linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Yang Xiwen <forbidden405@...look.com>
Subject: [PATCH] clk: set initial best mux parent to current parent when
 determining rate

From: Yang Xiwen <forbidden405@...look.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_NEAREST.

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

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

Signed-off-by: Yang Xiwen <forbidden405@...look.com>
---
This is actually a v2 of [1], but seems too simple to have a unittest.
It's tested in a mmc host driver.

[1]: https://lore.kernel.org/linux-clk/20230421-clk-v3-1-9ff79e7e7fed@outlook.com/
---
 drivers/clk/clk.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 2253c154a824..d98cebd7ff03 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -649,6 +649,10 @@ int clk_mux_determine_rate_flags(struct clk_hw *hw,
 
 	/* find the parent that can provide the fastest rate <= rate */
 	num_parents = core->num_parents;
+	if (core->parent) {
+		best_parent = core->parent;
+		best = clk_core_get_rate_nolock(best_parent);
+	}
 	for (i = 0; i < num_parents; i++) {
 		unsigned long parent_rate;
 

---
base-commit: 8d3dea210042f54b952b481838c1e7dfc4ec751d
change-id: 20240215-mux-6db8b3714590

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