linux-kernel - Re: [PATCH v3] clk: set initial best mux parent to current parent with CLK_MUX_ROUND

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Message-ID:
 <SEZPR06MB69597C834C90FE1FBA1F1D83962B2@SEZPR06MB6959.apcprd06.prod.outlook.com>
Date: Tue, 12 Mar 2024 16:52:29 +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, Sean Anderson <seanga2@...il.com>,
 Lukasz Majewski <lukma@...x.de>, u-boot@...ts.denx.de
Subject: Re: [PATCH v3] clk: set initial best mux parent to current parent
 with CLK_MUX_ROUND_CLOSEST

On 3/11/2024 5:34 PM, Maxime Ripard wrote:
> On Thu, Mar 07, 2024 at 07:18:05PM +0800, Yang Xiwen wrote:
>> On 3/7/2024 4:48 PM, Maxime Ripard wrote:
>>> Hi,
>>>
>>> On Thu, Mar 07, 2024 at 10:03:50AM +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>
>>> I don't think it would be the way to go. The biggest issue to me is that
>>> it's inconsistent, and only changing the behaviour for a given flag
>>> doesn't solve that.
>>
>> I think the current behavior is odd but conforms to the document if
>> CLK_MUX_ROUND_CLOSEST is not specified.
> clk_mux_determine_rate_flags isn't documented, and the determine_rate
> clk_ops documentation doesn't mention it can return an error.
>
>> If i understand correctly, the default behavior of mux clocks is to
>> select the closest rate lower than requested rate, and
>> CLK_MUX_ROUND_CLOSEST removes the "lower than" limitation, which is
>> what this version tries to accomplish.
> The situation is not as clear-cut as you make it to be, unfortunately.
> The determine_rate clk_ops implementation states:
>
>    Given a target rate as input, returns the closest rate actually
>    supported by the clock, and optionally the parent clock that should be
>    used to provide the clock rate.
>
> So CLK_MUX_ROUND_CLOSEST shouldn't exist, because that's what
> determine_rate expects so it should always be there.
>
> Now, the "actually supported by the clock" can be interpreted in
> multiple ways, and most importantly, doesn't state what the behaviour is
> if we can't find a rate actually supported by the clock.
>
> But now, this situation has been ambiguous for a while and thus drivers
> kind of relied on that ambiguity.
>
> So the way to fix it up is:
>
>    - Assess what drivers are relying on
>    - Document the current behaviour in clk_ops determine_rate


 From my investigation, it's totally a mess, especially for platform clk 
drivers (PLL). Some drivers always round down, the others round to 
nearest, with or without a specific flag to switch between them, depend 
on the division functions they choose. Fixing all of them seems needs 
quite a lot of time and would probably introduce some regressions.

We'd probably only have to say both rounding to nearest and rounding 
down are acceptable, though either one is preferred.


>    - Make clk_mux_determine_rate_flags consistent with that


I think we must keep existing flags and document the current behavior 
correctly because of the massive existing users of clk_mux.


That's why i'm going to only fix CLK_MUX_ROUND_CLOSEST users. Hopefully 
it won't cause too many regressions.


>    - Run that through kernelci to make sure we don't have any regression


We don't. I run 'tools/testing/kunit/kunit.py run --kunitconfig 
drivers/clk/.kunitconfig' each time before i send patches.


Over all, it seems quite a lot of work here.


>
> Maxime


The situation here becomes even more complex when it comes to U-Boot clk 
framework. They chose slightly different prototypes and stated 
clk_set_rate() can fail with -ve. It's a great burden for clk driver 
authors and maintainers when they try to port their drivers to U-Boot. 
Let's Cc U-Boot clk maintainers as well, and see how we can resolve the 
mess here.

-- 
Regards,
Yang Xiwen