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

[Anthony Liguori <aliguori@us.ibm.com>]

On 06/10/2011 08:43 AM, Jan Kiszka wrote:
> On 2011-06-10 15:10, Peter Maydell wrote:
>> On 10 June 2011 13:51, Anthony Liguori<aliguori@us.ibm.com>  wrote:
>>> On 06/10/2011 03:13 AM, Markus Armbruster wrote:
>>>>
>>>> Jan Kiszka<jan.kiszka@siemens.com>   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.
>
> Generally true. Two exceptions still require to make the path explorable
> / transfer information in the reverse direction: IRQ de-coalescing and
> physical device assignment. That's something a new generic API should
> encapsulate so that we can stop peaking into the machine details.
>
>>>
>>> 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 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.

If you really want to be pedantic, each ISA device has 5 input Pins that 
are supposed to correspond to IRQ 3, IRQ 4, IRQ 5, IRQ 6, and IRQ 7.

This could easily be modelled by doing the following:

  -device piix3,id=piix3 -device 
isa-serial,id=serial,irq[3]=piix3.irq[3],irq[4]=piix3.irq[4],...

But I don't think we benefit from modelling it this correctly.  The 
point is that the infrastructure could handle it though.

Regards,

Anthony Liguori

>
> Jan
>