Re: [PATCH] hw/core/clock: always iterate through childs in clock_propagate_period

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 19/4/24 18:08, Raphael Poggi wrote:
> Hi Peter,
> 
> Le ven. 19 avr. 2024 à 16:08, Peter Maydell <peter.maydell@linaro.org> a écrit :
>>
>> On Thu, 18 Apr 2024 at 21:39, Raphael Poggi
>> <raphael.poggi@lynxleap.co.uk> wrote:
>>>
>>> Hi Philippe,
>>>
>>> Le jeu. 18 avr. 2024 à 20:43, Philippe Mathieu-Daudé
>>> <philmd@linaro.org> a écrit :
>>>>
>>>> Hi Raphael,
>>>>
>>>> On 18/4/24 21:16, Raphael Poggi wrote:
>>>>> When dealing with few clocks depending with each others, sometimes
>>>>> we might only want to update the multiplier/diviser on a specific clock
>>>>> (cf clockB in drawing below) and call "clock_propagate(clockA)" to
>>>>> update the childs period according to the potential new multiplier/diviser values.
>>>>>
>>>>> +--------+     +--------+      +--------+
>>>>> | clockA | --> | clockB |  --> | clockC |
>>>>> +--------+     +--------+      +--------+
>>>>>
>>>>> The actual code would not allow that because, since we cannot call
>>>>> "clock_propagate" directly on a child, it would exit on the
>>>>> first child has the period has not changed for clockB, only clockC is
>>>>
>>>> Typo "as the period has not changed"?
>>>
>>> That's a typo indeed, thanks!
>>>
>>>>
>>>> Why can't you call clock_propagate() on a child?
>>>
>>> There is an assert "assert(clk->source == NULL);" in clock_propagate().
>>> If I am not wrong, clk->source is set when the clock has a parent.
>>
>> I think that assertion is probably there because we didn't
>> originally have the idea of a clock having a multiplier/divider
>> setting. So the idea was that calling clock_propagate() on a
>> clock with a parent would always be wrong, because the only
>> reason for its period to change would be if the parent had
>> changed, and if the parent changes then clock_propagate()
>> should be called on the parent.
>>
>> We added mul/div later, and we (I) didn't think through all
>> the consequences. If you change the mul/div settings on
>> clockB in this example then you need to call clock_propagate()
>> on it, so we should remove that assert(). Then when you change
>> the mul/div on clockB you can directly clock_propagate(clockB),
>> and I don't think you need this patch at that point.
> 
> Alright, that makes sense, is that OK if I send a patch removing the assert ?

Sure, that is welcomed :)

Regards,

Phil.