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Subject: Re: [PATCH RESEND] clk: set initial best mux parent to current parent when determining rate
From: Stephen Boyd <sboyd@kernel.org>
Cc: linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org, Yang Xiwen <forbidden405@outlook.com>
To: Michael Turquette <mturquette@baylibre.com>, Yang Xiwen via B4 Relay <devnull+forbidden405.outlook.com@kernel.org>, forbidden405@outlook.com
Date: Wed, 28 Feb 2024 17:58:18 -0800
User-Agent: alot/0.10

Quoting Yang Xiwen via B4 Relay (2024-02-23 09:18:52)
> From: Yang Xiwen <forbidden405@outlook.com>
>=20
> 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.

Did you mean CLK_MUX_ROUND_CLOSEST?

>=20
> For example, if the lowest possible rate privided by the mux is 1000Hz,

s/privided/provided/

> 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.
>=20
> Setting the initial best parent to current parent could solve this bug
> very well.
>=20
> Signed-off-by: Yang Xiwen <forbidden405@outlook.com>
> ---
> This is actually a v2 of [1], but seems too simple to have a unittest.
> It's tested in a mmc host driver.

It's not too simple for a unittest.

>=20
> [1]: https://lore.kernel.org/linux-clk/20230421-clk-v3-1-9ff79e7e7fed@out=
look.com/

In that thread I asked you to please Cc Maxime. Please do that.

> ---
>  drivers/clk/clk.c | 4 ++++
>  1 file changed, 4 insertions(+)
>=20
> 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,
> =20
>         /* find the parent that can provide the fastest rate <=3D rate */
>         num_parents =3D core->num_parents;
> +       if (core->parent) {
> +               best_parent =3D core->parent;
> +               best =3D clk_core_get_rate_nolock(best_parent);
> +       }

Is the problem that we're not using abs_diff()?

----8<----
diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index a3bc7fb90d0f..91023345595f 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -542,7 +542,7 @@ static bool mux_is_better_rate(unsigned long rate, unsi=
gned long now,
 			   unsigned long best, unsigned long flags)
 {
 	if (flags & CLK_MUX_ROUND_CLOSEST)
-		return abs(now - rate) < abs(best - rate);
+		return abs_diff(now, rate) < abs_diff(best, rate);
=20
 	return now <=3D rate && now > best;
 }