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

[Chuan Liu <chuan.liu@amlogic.com>]

Hi Martin:

Thanks for the detailed explanation.


On 9/29/2025 4:55 AM, Martin Blumenstingl wrote:
> [ EXTERNAL EMAIL ]
>
> Hello,
>
> On Sun, Sep 28, 2025 at 8:41 AM Chuan Liu <chuan.liu@amlogic.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:
>>
>>
>>          __    __    __    __    __    __    __    __
>> ori:  ↑  |__↑  |__↑  |__↑  |__↑  |__↑  |__↑  |__↑  |__↑
>>                     ^
>>                     1 * cycle original channel.
>>          _   _   _   _   _   _   _   _   _   _   _   _
>> new:  ↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑ |_↑
>>                                         ^
>>                                         5 * cycles new channel.
>>          __    __                        _   _   _   _
>> out:  ↑  |__↑  |______________________↑ |_↑ |_↑ |_↑ |_↑
>>                ^                        ^
>>                start switching mux.     switch to new channel.
> 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.


Yes, our glitch-free mux is designed to prevent glitches caused by
excessively short high or low levels in the clock output.


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


A V2 version was submitted later with changes based on your suggestions.
Regarding the "Fixes" tag, Jerome had proposed some modifications.

[PATCH v2 0/3] clk: Fix issues related to CLK_IGNORE_UNUSED failures and 
amlogic glitch free mux - Chuan Liu via B4 Relay 
<https://lore.kernel.org/all/20241111-fix_glitch_free-v2-0-0099fd9ad3e5@amlogic.com/>


Adding CLK_OPS_PARENT_ENABLE causes the CLK_IGNORE_UNUSED configuration
of it's parent clocks on the chain to become ineffective, so this patch
depends on fixing that issue before it can proceed.


Jerome and I have submitted patches to address the issue of
CLK_IGNORE_UNUSED becoming ineffective. I originally planned to wait
for progress on this patch and then incorporate Jerome's feedback before
sending the V3 version.

Hi Jerome, sorry if this caused any misunderstanding on your part; I
will provide timely feedback moving forward.


>
>
> Best regards,
> Martin