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Re: When is SysBusDevice's num_mmios determined? and does SYS_BUS_DEVICE

From: Peter Maydell
Subject: Re: When is SysBusDevice's num_mmios determined? and does SYS_BUS_DEVICE macro allocate memory?
Date: Sat, 5 Jun 2021 20:57:47 +0100

On Sat, 5 Jun 2021 at 16:09, <ckim@etri.re.kr> wrote:
> I have a questions for my clarification.
> In function sysbut_init_mmio, I see these two lines,
>      assert(dev->num_mmio < QDEV_MAX_MMIO);
>      n = dev->num_mmio++;
> I gathered this QDEV_MAX_MMIO means (currently defined to 32): maximum number 
> of instances a same device can be attached to the system bus. Is my 
> understanding correct?

No. It means no device can have more than 32 different MMIO regions
(ie its init or realize function mustn't call sysbus_init_mmio()
more than that many times). Most devices have just one MMIO
region ("my bank of registers"). A handful provide two or three,
for example because they have several sets of registers that
live at separated places in the system address space (eg a
PCI controller might have one for its config registers, one
for the PCI memory window and one for the PCI IO window).

There's no fixed limit on how many instances of any device
you can create.

> And I have an opinion about the comment in code for OBJECT_CHECK :
> By the way, when I see the comment on OBJECT_CHECK, it says
> /**
>  * @type: The C type to use for the return value.
>  * @obj: A derivative of @type to cast.
>  * @name: The QOM typename of @type
>  *
>   (...)
> #define OBJECT_CHECK(type, obj, name) \
>     ((type *)object_dynamic_cast_assert(OBJECT(obj), (name), \
>                                         __FILE__, __LINE__, __func__))

> static void smc91c111_reset(DeviceState *dev)
> {
>     smc91c111_state *s = SMC91C111(dev);
> Here in macro OBJECT_CHECK is used to cast DeviceState to its derivative 
> smc91c111_state.
> So in the comment above, shouldn't "@obj: A derivative of @type to cast". be 
> modified to "@obj: A derivative or a base of @type to case"?
> I understand this casting is possible from base to derivative, or from 
> derivative to base.

It is, but I think what the comment is trying to say is that
the underlying object must actually be of either type SMC91C111
or some type derived from SMC91C111, even if at the moment the
pointer that you have to it is an Object* or a DeviceState*.
If you pass it a DeviceState* which is really some other
device like a PL011, then the attempt to cast to SMC91C111
will not work (it will assert).

-- PMM

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