linux-kernel - Re: [PATCH 2/2] clk: meson: Fix glitch free mux related issues

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Message-ID: <1jfrc563wa.fsf@starbuckisacylon.baylibre.com>
Date: Mon, 29 Sep 2025 10:48:05 +0200
From: Jerome Brunet <jbrunet@...libre.com>
To: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc: Chuan Liu <chuan.liu@...ogic.com>,  Michael Turquette
 <mturquette@...libre.com>,  Stephen Boyd <sboyd@...nel.org>,  Neil
 Armstrong <neil.armstrong@...aro.org>,  Kevin Hilman
 <khilman@...libre.com>,  linux-clk@...r.kernel.org,
  linux-kernel@...r.kernel.org,  linux-amlogic@...ts.infradead.org,
  linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] clk: meson: Fix glitch free mux related issues

On Sun 28 Sep 2025 at 22:55, Martin Blumenstingl <martin.blumenstingl@...glemail.com> wrote:

> Hello,
>
> On Sun, Sep 28, 2025 at 8:41 AM Chuan Liu <chuan.liu@...ogic.com> wrote:
>>
>>
>> On 9/28/2025 2:05 PM, Chuan Liu wrote:
>> > Hi Jerome & Martin:
>> >
>> > Sorry for the imprecise description of the glitch-free mux earlier.
>> >
>> > Recently, while troubleshooting a CPU hang issue caused by glitches,
>> > I realized there was a discrepancy from our previous understanding,
>> > so I'd like to clarify it here.
> [...]
>> An example of the clock waveform is shown below:
>>
>>
                     1                  2
                     v                  v
>>         __    __    __    __    __    __    __    __
>> ori:  ↑  |__↑  |__↑  |__↑  |__↑  |__↑  |__↑  |__↑  |__↑
>>                    ^
>>                    1 * cycle original channel.
>>         _   _   _   _   _   _   _   _   _   _   _   _
>> new:  ↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑
>>                                        ^
>>                                        5 * cycles new channel.
>>         __    __                        _   _   _   _
>> out:  ↑  |__↑  |______________________↑ |_↑ |_↑ |_↑ |_↑
>>               ^                        ^
>>               start switching mux.     switch to new channel.

Ok ... but when is it safe to disable the "ori" clock ?
Can you do it at '1' already ? or do you have to wait for '2' ?


> Thank you for the detailed report!
> This is indeed problematic behavior. I guess the result is somewhat
> random: depending on load (power draw), silicon lottery (quality),
> temperature, voltage supply, ... - one may or may not see crashes
> caused by this.
>
> Based on the previous discussion on this topic, my suggestion is to
> split the original patch:
> - one to add CLK_SET_RATE_GATE where needed (I think the meson8b.c
> driver already has this where needed) to actually enable the
> glitch-free mux behavior
> - another one with the CLK_OPS_PARENT_ENABLE change (meson8b.c would
> also need to be updated) to prevent the glitch-free mux from
> temporarily outputting an electrical low signal. Jerome also asked to
> document the behavior so we don't forget why we set this flag

Yes please split the changes and visit all the controllers shipping this
type of muxes.

>
> Both patches should get the proper "Fixes" tags.

... and proper fixes tag maybe different depending on the controller so
there might more that just 2 changes.

> I think it would also be great if you could include the waveform
> example in at least the commit message as it helps understand the
> problem.
>
> Let's also give Jerome some time to comment before you send patches.
>
>
> Best regards,
> Martin

-- 
Jerome