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

[Raphael Poggi]

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 ?

Thanks,
>
> thanks
> -- PMM