Re: [Qemu-devel] [PATCH RFC 0/3] basic support for composing sysbus devices

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

On 10 June 2011 14:43, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> On 2011-06-10 15:10, Peter Maydell wrote:
>> This makes the wiring of this signal look like a property of the
>> isa-serial device, which is a bit odd, since it's just as much
>> a property of the piix3. Actually it's neither, it's a property
>> of the machine model, and you might actually want a syntax a bit
>> more like:
>>
>>  piix3 = piix3(property=value, property=value...);
>>  serial = isa-serial(property=value...);
>>  connect(serial.irq, piix3.irq[3]);
>
> In fact, in the ISA case, it is a device property: The device, and only
> the device decides which IRQ to use - from the bus it is attached to. So
> attaching an ISA device to the bus of an ISA bridge like the PIIX3 and
> selecting local IRQ 3 are the steps we can already express today.

Ah, in that case Anthony's suggestion of
  -device piix3,id=piix3 -device isa-serial,id=serial,irq=piix3.irq[3]
wrong in a different way -- the isa-serial shouldn't care
what other device is providing the ISA bus it is sitting on,
it just has a property of which ISA irq line it is using
(and rely on an isa bus abstraction to wire things up at
the machine model level). [As you say, this works now.]

But I think that's a non-typical case compared to the usual one
of "these wires are just hardwired this way by the machine".

-- PMM