Re: [PATCH for-6.2 09/25] clock: Provide builtin multiplier/divider

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 12 Aug 2021 at 13:08, Alexandre IOOSS <erdnaxe@crans.org> wrote:
>
>
> On 8/12/21 11:33 AM, Peter Maydell wrote:
> > It is quite common for a clock tree to involve possibly programmable
> > clock multipliers or dividers, where the frequency of a clock is for
> > instance divided by 8 to produce a slower clock to feed to a
> > particular device.
> >
> > Currently we provide no convenient mechanism for modelling this.  You
> > can implement it by having an input Clock and an output Clock, and
> > manually setting the period of the output clock in the period-changed
> > callback of the input clock, but that's quite clunky.
> >
> > This patch adds support in the Clock objects themselves for setting a
> > multiplier or divider.  The effect of setting this on a clock is that
> > when the clock's period is changed, all the children of the clock are
> > set to period * multiplier / divider, rather than being set to the
> > same period as the parent clock.
> >
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> >   docs/devel/clocks.rst   | 23 +++++++++++++++++++++++
> >   include/hw/clock.h      | 29 +++++++++++++++++++++++++++++
> >   hw/core/clock-vmstate.c | 24 +++++++++++++++++++++++-
> >   hw/core/clock.c         | 29 +++++++++++++++++++++++++----
> >   4 files changed, 100 insertions(+), 5 deletions(-)
> >
> > diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst
> > index 956bd147ea0..430fbd842e5 100644
> > --- a/docs/devel/clocks.rst
> > +++ b/docs/devel/clocks.rst
> > @@ -260,6 +260,29 @@ clocks get the new clock period value: *Clock 2*, *Clock 3* and *Clock 4*.
> >   It is not possible to disconnect a clock or to change the clock connection
> >   after it is connected.
> >
> > +Clock multiplier and divider settings
> > +-------------------------------------
> > +
> > +By default, when clocks are connected together, the child
> > +clocks run with the same period as their source (parent) clock.
> > +The Clock API supports a built-in period multiplier/divider
> > +mechanism so you can configure a clock to make its children
> > +run at a different period from its own. If you call the
> > +``clock_set_mul_div()`` function you can specify the clock's
> > +multiplier and divider values. The children of that clock
> > +will all run with a period of ``parent_period * multiplier / divider``.
> > +For instance, if the clock has a frequency of 8MHz and you set its
> > +multiplier to 2 and its divider to 3, the child clocks will run
> > +at 12MHz.
> > +
> > +You can change the multiplier and divider of a clock at runtime,
> > +so you can use this to model clock controller devices which
> > +have guest-programmable frequency multipliers or dividers.
>
> This looks nice!
> Does this imply that if I am going to implement the STM32 RCC (Reset and
> Clock Controller) device, then I should use this new feature?

I haven't looked at the RCC spec in detail, but yeah, I'd expect
that you'd have a device with one (or perhaps more) input clocks
and a lot of output clocks, and as the guest writes to registers
that set divider/multiplier settings you'd use clock_set_mul_div()
to configure the appropriate output clocks.

-- PMM