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Message-ID:
<SEZPR06MB69597E78EA08D5BDF66DBEBC965F2@SEZPR06MB6959.apcprd06.prod.outlook.com>
Date: Thu, 29 Feb 2024 10:13:04 +0800
From: Yang Xiwen <forbidden405@...look.com>
To: Stephen Boyd <sboyd@...nel.org>,
Michael Turquette <mturquette@...libre.com>,
Yang Xiwen via B4 Relay <devnull+forbidden405.outlook.com@...nel.org>
Cc: linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] clk: set initial best mux parent to current parent
when determining rate
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@...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.
>
> 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@...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.
>
> 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
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