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

[Peter Maydell]

On 10 June 2011 13:51, Anthony Liguori <address@hidden> wrote:
> On 06/10/2011 03:13 AM, Markus Armbruster wrote:
>>
>> Jan Kiszka<address@hidden>  writes:
>>>
>>> Resource management, e.g. IRQs. That will be useful for other types of
>>> buses as well.
>>
>> A device should be able to say "I need to be connected to an IRQ line".
>> Feels generic to me.
>
> More specifically, a device has input IRQs.  A device has no idea what
> number the IRQ is tied to.
>
> Devices may also have output IRQs.  At the qdev layer, we should be able to
> connect an arbitrary output IRQ to an arbitrary input IRQ.

Actually, devices have input and output I/O signals (GPIOs, if you like).
A subset of these are IRQs. We already have some APIs in QEMU which
claim to be dealing with 'irq's but actually are just for wiring
up generic signals; I'd rather we didn't proliferate that terminology
confusion if possible. (And a single GPIO wire is just one kind of
thing you might want to link between two devices, obviously. MMIO is
another.)

> So the crux of the problem is that:
>
>  -device isa-serial,id=serial,irq=3
>
> Is very wrong.  It ought to look something more like
>
>  -device piix3,id=piix3 -device isa-serial,id=serial,irq=piix3.irq[3]

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 some mythical stitching language, which I think makes much
more sense than command line switches anyway.)

-- PMM