Re: [Qemu-devel] [PATCH 06/15] piix: create i8254 through composition

[Anthony Liguori]

On 01/31/2012 08:51 AM, Jan Kiszka wrote:
> On 2012-01-31 15:47, Anthony Liguori wrote:
> > On 01/31/2012 08:34 AM, Jan Kiszka wrote:
> > > On 2012-01-26 20:00, Anthony Liguori wrote:
> > > > @@ -548,6 +550,13 @@ static int piix3_realize(PCIDevice *dev)
> > > >        /* Setup the RTC IRQ */
> > > >        s->rtc.irq = rtc_irq;
> > > >
> > > > +    /* Realize the PIT */
> > > > +    qdev_set_parent_bus(DEVICE(&s->pit), BUS(s->bus));
> > > > +    qdev_init_nofail(DEVICE(&s->pit));
> > > > +
> > > > +    /* FIXME this should be refactored */
> > > > +    pcspk_init(ISA_DEVICE(&s->pit));
> > >
> > > Fixing ATM, ie. converting to qdev/QOM.
> > >
> > > Q: How do I use qdev_property_add_link&   Co. to establish the relation
> > > from the speaker port device to the PIT?
> >
> > In the state structure, have:
> >
> > struct PCSpkState {
> >       ...
> >       PITState *pit;
> > };
> >
> > In the pcspk instance_init, do:
> >
> > object_property_add_link(obj, "pit", TYPE_PIT, (Object **)&s->pit, NULL);
> >
> > In the pcspk realize function (DeviceClass::init), do:
> >
> > assert(s->pit != NULL); // make sure the pit link is set
> >
> > And that's it.
> >
> > You can set the s->pit field directly.  You are not required to use any special
> > QOM function to interact with link properties.
> >
> > BTW, this is yet another benefit of making structures public.  You can take the
> > address of a child and set link fields directly without accessors.
>
> Well, that has two sides. We introduced properties to avoid this direct
> messing.
>
> Does linking also work without exposing internals?

Yes, you can set links through properties (although I haven't added those 
accessors yet).

But...  you lose type safety because now you're dealing with strings.  If we 
want to avoid exposing internals, we should provide accessors to get/set links.  So:

typedef struct PCSpkState PCSpkState;

void pcspk_set_pit(PCSpkState *s, PITState *pit);

Regards,

Anthony Liguori

> Jan