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

[Peter Maydell]

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.

thanks
-- PMM