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Date: Thu, 29 Feb 2024 10:13:04 +0800
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Subject: Re: [PATCH RESEND] clk: set initial best mux parent to current parent
 when determining rate
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To: Stephen Boyd <sboyd@kernel.org>,
 Michael Turquette <mturquette@baylibre.com>,
 Yang Xiwen via B4 Relay <devnull+forbidden405.outlook.com@kernel.org>
Cc: linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20240224-mux-v1-1-608cc704ef43@outlook.com>
 <0db752d31016823ccd3799925255a74a.sboyd@kernel.org>
From: Yang Xiwen <forbidden405@outlook.com>
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On 2/29/2024 9:58 AM, Stephen Boyd wrote:
> Quoting Yang Xiwen via B4 Relay (2024-02-23 09:18:52)
>> 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_NEAREST.
> 
> Did you mean CLK_MUX_ROUND_CLOSEST?

You are right :).

> 
>>
>> 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.
>>
>> Setting the initial best parent to current parent could solve this bug
>> very well.
>>
>> 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.
> 
>>
>> [1]: https://lore.kernel.org/linux-clk/20230421-clk-v3-1-9ff79e7e7fed@outlook.com/
> 
> In that thread I asked you to please Cc Maxime. Please do that.
> 
>> ---
>>  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);
>> +       }
> 
> Is the problem that we're not using abs_diff()?


No, i think. It has nothing to do with the code here. It's because of
the initial best_parent/best_parent_rate.

> 
> ----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, unsigned 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);

Without this patch, the initial `best` rate would be always 0. This is
wrong for most cases, 0Hz might (usually) be unavailable. We should use
a valid rate(i.e. current rate) initially.

>
>  	return now <= rate && now > best;
>  }

-- 
Best regards,
Yang Xiwen