linux-kernel - Re: [PATCH v2 1/2] clk: set initial best mux parent to current parent when determining rate

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Message-ID:
 <SEZPR06MB69595E580354C25A9AE9C16596212@SEZPR06MB6959.apcprd06.prod.outlook.com>
Date: Wed, 6 Mar 2024 22:30:15 +0800
From: Yang Xiwen <forbidden405@...look.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Michael Turquette <mturquette@...libre.com>,
 Stephen Boyd <sboyd@...nel.org>, linux-clk@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] clk: set initial best mux parent to current parent
 when determining rate

On 3/6/2024 10:24 PM, Maxime Ripard wrote:
> Hi,
>
> On Wed, Mar 06, 2024 at 12:22:23AM +0800, Yang Xiwen via B4 Relay wrote:
>> 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_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@...look.com>
> That patch makes sense to me, but this changes the behaviour of the function.
>
> Before, if we couldn't find a good configuration for the rate, we were
> error'ing out. Now, we keep the current configuration. We should
> document the new behaviour in the function documentation, and we should
> probably run that through kernelci to make sure we aren't breaking any
> platform (and from experience, we probably are).


We can limit the new behavior to CLK_MUX_ROUND_CLOSEST as well. The 
current behavior is okay for common muxes i think. Though probably wrong 
for CLK_MUX_ROUND_CLOSEST.


>
> Maxime


-- 
Regards,
Yang Xiwen